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Advantages of Nuclear Power Plants - Big Hope in the Minute Atom
The energy demands of the world are continuously increasing. Experts are worried about the future of power generation because there are not enough supplies of coal, water and gas to fulfill the needs of mankind in the long term future. Alternative sources of energy such as nuclear energy are being developed. Nuclear energy has several advantages over other sources of energy because it is not limited by space or location. In this article we will learn about nuclear power plants and some of the basic underlying concepts.
What is a Nuclear Power Plant?
As the name itself suggests, a nuclear power plant is a facility where nuclear energy is harnessed to generated electricity. For those of us who haven’t heard about this term, it may seem like a new concept since we usually hear of atomic and hydrogen bombs which use nuclear energy for large scale destruction. But the same power is used for constructive purposes in nuclear power plants
The basic underlying principle of a nuclear power plant can be understood from the equation of mass-energy equivalence which is stated as follows
E = ∆mc2
Where E is the amount of energy released when a change in mass occurs during a nuclear reaction. This equation may not seem very complicated to you, but as you know “c” represents the speed of light which is of the order of 3 lakh kilometers per second. Just imagine the amount of energy released even if a tiny amount of mass is converted into energy.
This gives an edge to nuclear power plants over conventional sources like coal or gas because it means freedom from geographical factors and parameters. Furthermore since the amount of fuel required is much less as compared to conventional sources of power generation, there is no need to have extensive storage facilities and transportation networks for the same amount of power generated.
Basic Nuclear Reactions
Nuclear reactions fall into two major categories: fission and fusion. Fission refers to the nuclear reaction where a heavy nucleus is broken into nuclei of intermediate atomic number. Fusion refers to the nuclear reaction wherein light nuclei get combined to form a new nucleus.
Energy can be either released or absorbed during the process depending on whether the final mass of the products is greater than or less than the initial mass of the reactants.
The Chain Reaction
The above mentioned types of reactions are not of much use for generating electrical energy on their own. We require something known as a controlled chain reaction if power is to be generated in a nuclear power plant. When fission is started in a nuclear material it could die out slowly, sustain itself constantly or develop into an uncontrolled reaction. The first and the last options are not useful for generation of electricity. It is only when we have a sustained reaction, that we can utilize nuclear energy in an effective manner
There are lots of other interesting things to be learnt about nuclear power plants regarding their working, layout, processes and so forth which we shall do in later articles in this series.
How Does a Nuclear Power Plant Work?
Whenever the term nuclear power plant is mentioned, it usually brings images of the Chernobyl disaster into mind, or related images of the nuclear technology triggered device which destroyed 2 cities of Japan during the Second World War. I agree that these incidents were very unfortunate and should have never happened in the first place, but believe me when I say that nuclear power is quite safe. Though nuclear energy has devastating capabilities such incidents or accidents mainly happen due to human errors of carelessness or prejudice. Otherwise nuclear technology is as safe as any other technology used to generate electricity and possibly much more effective in several situations. You will appreciate this viewpoint better once you know how does a nuclear power plant work?
The Energy Mass Ratio
In order to give you an idea about the scale of fuel quantities involved in a nuclear power station vis-à-vis traditional power stations, I ask you to imagine that around a pound of nuclear fuel like say Uranium gives the energy equivalent to burning a million gallons of gasoline. This should not come as a surprise since we have already learned that the energy released in a nuclear
reaction is the equivalent of the mass change which takes place during the process. It is therefore huge compared to energy which is released as a result of combustion and related chemical reactions during traditional fuel burning.
How Does it All Work?
It is all very well to hear that tremendous energy lies within atomic particles, which is converted into electrical energy in a nuclear power plant. The million dollar question is- how is it achieved?
Well the nuclear energy isn’t converted directly into electricity but the heat released during the fission reaction is used to convert water into steam which in turn runs a turbine. The turbine turns the alternator which produces electricity to be fed into the power grid.
Of course the overall process is not as simple as it seems and there are several types of nuclear power plants which are classified according to different parameters, which will be discussed in separate articles on this topic.
One concept which must be well understood in context of nuclear power plants is the critical mass of the fuel used. We know that fission occurs whenever an atom splits into two or more components. Let us take the case of U 235 which splits to give 2-3 neutrons in the process which in turn strike other atoms and cause further splitting. This chain can only be sustained if the mass of U 235 is of a certain minimum value known as the critical mass. Below this critical value the reaction would ultimately die out, while if the critical value is exceeded it may result in the likes of an atomic bomb.
The above statement might have sent jitters down your spine, but just relax. Technology is quite advanced these days and so nuclear power plants simply do not blow up every other day as if they were nuclear bombs . The very few incidents that have occurred to date were mainly caused by carelessness.
Safety Measures for Nuclear Power Plants
No industrial activity or operation is without its inherent risks. In fact the same goes for all facets of life. Nevertheless certain activities and operations have the potential of great damage not only for the person handling those operations but for society in general. Nuclear technology and nuclear power generation are certainly one of those areas where the potential for damage is literally unlimited if something goes wrong. This has been observed time and again through
various accidents which happened in different parts of the world at different times such as that of the Chernobyl nuclear reactor. Hence safety measures for nuclear power plants must be followed strictly, so that nuclear power becomes an obedient servant and not a terrible master to humanity.
Nuclear Power Plant Safety
Safety should be ensured in a nuclear power plant from all aspects and during all stages: from the inception of the plant as an idea to its full fledged commissioning providing continuous source of power to the requisite purpose. It would not be possible to go into the full details of the safety aspect in this article, but we will certainly have a look at some of the most basic features relating to nuclear power plant safety.
The Building: since the nuclear power plant has exothermic nuclear reactions going on inside its core, it is very important that the structure housing this reactor should be made from relevant materials which have the appropriate capacity to shield the outside environment both during normal operations as well as minimize risk of damage in case of unfortunate accidents such as the Chernobyl blast.
The Core: this is the place where the actual reaction takes place. Fission occurs with the release of neutrons causing further fission thus sustaining a chain reaction. Appropriate measures must be taken to maintain ideal conditions via control rods and core cooling.
Monitoring: human beings working inside the power plant need to be constantly monitored for any over exposure of radiation as a result of their routine job operations. The standards laid down in this regard should be strictly adhered to and the working environment should be regularly checked for radiation levels.
Waste Disposal: one of the most challenging tasks is the proper disposal of waste materials from the nuclear power plant. These waste materials come in different forms such as solid, liquid and gaseous. All these types of wastes have their own methods of disposal and the main idea is to dispose off these wastes in a manner which is least harmful for human beings, flora, fauna and the natural environment.
Proper Emergency Response Plans: nobody wants an accident to happen but things do go out of control sometimes either due to human error or machinery failure. The best thing is to be prepared for such a situation and have properly trained personnel as well as the requisite equipment in order to deal effectively with such situations.
If the above mentioned dictums are followed properly, it would ensure that the tremendous energy which lies in the atom is harnessed in a proper manner without causing damage to men, material, or environment.
Types of Nuclear Power Plants – Pressurized Water Reactors (PWR) 4 Comments
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Types of Nuclear Power Plants – Pressurized Water Reactors (PWR)
Pressurized Water Reactor (PWR) is a type of a nuclear power reactor that uses enriched Uranium as a fuel which in turn heats the light water used for producing steam. The main feature which differentiates it from a BWR nuclear reactor is that a PWR has a separate arrangement to make steam in the form of a heat exchanger
The Arrangement of PWR
A pressurized water reactor (PWR) is a type of power plant reactor consisting of two basic circuits having light water as the working fluid. In one of the circuits water is heated to a high temperature and kept at high pressure as well, so that it does not get converted into a gaseous state. This superheated water is used as a coolant and a moderator for the nuclear reactor core hence the name PWR or pressurized water reactor.
The secondary circuit consists of water at high pressure in the gaseous state i.e. steam which is used to run the turbine-alternator arrangement. The point of interaction between these two circuits is the heat exchanger or the boiler wherein heat from the superheated high pressure water converts the water in the secondary circuit to steam.
Advantages of PWR
Much fewer control rods are required in a PWR. In fact for a typical 1000 MW plant just around 5 dozen control rods are sufficient.
Since the two circuits are independent of each other, it makes it very easy for the maintenance staff to inspect the components of the secondary circuit without having to shut down the power plant entirely.
A PWR has got a high power density and this, combined with the fact that enriched Uranium is used as fuel instead of normal Uranium, leads to the construction of very compact core size for a given power output.
One feature which makes a PWR reactor very suitable for practical applications is its positive demand coefficient which serves to increase the output as a direct proportion to demand of power.
The water used in the primary circuit is different from that used in the secondary circuit and there is no intermixing between the two, except for heat transfer which takes place in the boiler or heat exchanger. This means that the water used in the turbine side is free
from radioactive steam hence the piping on that side is not required to be clad with special shielding materials.
Drawbacks of PWR
The primary circuit consists of high temperature, high pressure water which accelerates corrosion. This means that the vessel should be constructed of very strong material such as stainless steel which adds to construction costs of PWR.
PWR fuel charging requires the plant to be shut down and this certainly requires a long time period of the order of at least a couple of months.
The pressure in the secondary circuit is relatively quite low as compared to the primary circuit hence the thermodynamic efficiency of PWR reactors is quite low of the order of 20
One important point to note here is that despite the changing loads the pressure in the primary circuit needs to be maintained at a constant value. This is achieved by installing a device known as pressure equalizer in the primary circuit. It basically consists of a dome shaped structure which has heating coils which are used to increase or decrease pressure as and when required depending on varied load conditions
Types of Nuclear Power Plants – Boiling Water Reactors (BWR) Boiling Water Reactors (BWR) are types of power plants that work similar to a pressure cooker where steam is generated from heat within the reactor core which in turn is used to drive the turbine blades that turns the generator. Read on for some interesting information about BWR.
Steam possesses immense power and we see that in a lot of applications in every day life, right from the pressure cooker which makes those stubborn pulses soft, to driving the heavy steam engine (although they are hardly seen these days). This steam can also be used to generate electricity by driving a turbine alternator arrangement with it. Nothing new, you might say, but then here water is not boiled by traditional heat sources but using the heat of the atom, in the form of exothermic heat from a nuclear fission reaction. BWR or boiling water reactor plants form an important variety amongst the commonly used types of power plants.
The BWR Reactor: 3 in 1 Functionality
The main beauty of a BWR lies in the fact that the same water is used for all three purposes as a moderator, coolant, and the source for steam which drives the turbine blades. Water passes over
the reactor core absorbing heat and turning into steam which is fed to the turbine blades. The used steam from the turbines is then fed into the condenser which coverts it into liquid state to be fed again into the reactor core, thus completing the closed loop cycle. The Fuel used in this type of reactor is enriched Uranium.
Benefits and Drawbacks
Since there is no need for a separate boiler arrangement to produce steam unlike in some other types of nuclear reactors, it reduces the complexity of the arrangement and related costs. Moreover this leads to a reduced size of the reactor for a given amount of output.
A BWR has self controlling characteristic which stems from the fact that an increase in reactivity causes increase of steam formation in the reactor. This increase of steam is accompanied by a decrease of density of the coolant which also acts as the moderator hence pushing the fuel towards sub-critical zone.
One of the biggest drawbacks of a BWR reactor is its inefficiency to deal with sudden increase in load. This is due to the very fact of self balancing as explained in the previous point which is good for a fixed load but makes it a drawback when dealing with sudden increases of load.
Since the same water is used for all purposes including moderation and cooling, it tends to be slightly radioactive. Hence when this water is fed to the turbines, they need to be shielded for this very reason. However the good news is that the half life of the radioactive steam is just of the order of 15 minutes which is manageable.
The chances of fuel getting "burnt out" are significant in a BWR reactor mainly because water is in direct contact with fuel and when the same water gets converted to steam it may blank parts of the fuel surface from coming in contact with water thus leading to such a situation.
Hence we see that a boiling water reactor is pretty useful due to its good thermal efficiency, smaller size per unit power output and its characteristic to use water as coolant, moderator and steam generator.
Gas Cooled Reactors in Nuclear Plants
As the name itself suggests, a gas cooled reactor is cooled using a gas, and the heat extracted by the gas during the process of cooling the reactor is used either indirectly to generate steam which in turn is used for turbine propulsion, or this heated coolant could be used directly as the working fluid of the gas turbine thus eliminating the need for a separate steam circuit. Of course both these approaches have their own set of features and limitations. The moderator used in these types of reactors is Graphite which offers the advantages of being stable under conditions of high radiation as well as high temperatures.
Gases used for Cooling
There are several options available to choose for the coolant including gases but mainly carbon dioxide and helium are used as coolants apart from hydrogen in certain situations. Thermodynamically speaking Helium offers the best alternative since it has a high specific heat and low capture cross section for thermal neutrons but it is much expensive as compared to carbon-dioxide. The advantages that a gaseous coolant offers over light or heavy water are as follows
1. The gases are less prone to react chemically with the structural material of the reactor unlike water which has higher affinity for chemical reactions with these elements.
2. Gases are more flexible in terms of the temperatures and pressure ranges to which they can be subject to as compared to water. Of course certain practical considerations do limit these ranges but certainly they are more than those available for water.
3. Gas cooled reactors are more stable and safe because the reactivity of the reactor is not a function of the quantity of gas present in the core. Hence if a gas leak occurs accidentally the reactor would be much safer than a similar leak developed in water cooled reactor
As described above the direct system uses the same gas which is used as a coolant to act as the working fluid rotating the turbine blades from the enthalpy generated during heat absorption during the reactor cooling process. The main feature of such a system is that it gives the highest thermal efficiency of all types of nuclear reactors which are currently being used in the industry and typically gives efficiency of the order of nearly 42% within moderate operating temperatures.
And indirect circuit consists of cooling gas in the primary circuit while the secondary circuit consists of water as the working fluid which is converted into steam using a heat exchanger.
Features of Gas Cooled Reactors
1. If CO2 is used as the cooling gas it eliminates the possibility of explosion which is always present in water cooled reactors
2. There is no need for cladding the metallic fuel which leads to simple fuel processing techniques as compared to other types of reactors where cladding is necessary
3. The main drawback of these plants is there low power density which requires large size of the reactor for relatively smaller power requirements
4. Although Helium is an excellent cooling medium from the thermodynamic point of view, its low neutron absorbing capacity makes it unsuitable for load control
Components of Nuclear Power Plant – FuelA nuclear power plant is not much different from a conventional power plant except for the manner in which heat is generated using nuclear reactions. We will study one very important component of nuclear power plants here, namely the fuel rods used in the reactor core, and the nuclear fuel cycle.
Fuel is needed for any energy producing process and refers to the material which is either burned or altered in order to produce energy. Burning takes place in case of chemical reactions, whilst alternation takes place in the nuclear reactions. Both these processes are exothermic but the latter leads to much more release of thermal energy as compared to chemical reactions for similar quantities of fuel. No doubt fuel rods top the list in the components of nuclear power plants for there would be no "fire" without the fuel.
What is a Fissile Nuclide?
As you must have surely guessed, fissile has something to do with fission and you are right about this. A fissile material is that which attains fission when hit by a neutron of any energy level. The commonly known fissile nuclides are isotopes of Uranium and Plutonium namely U-233, U-235, Pu-239 and Pu-241.
What is a Fertile Nuclide?
Whereas a fissile nuclide can achieve fission with any neutron, a fertile nuclide is one which requires neutrons of more than a certain level of energy to achieve the same usually in the range of 1-MeV. If you are wondering why the name fertile is given to a seemingly infertile nucleus, let me explain the reason to you. Actually a fertile nuclide becomes fissile upon absorption of the appropriate neutron hence the name fertile. We all know fertile nuclide include the U-235 isotope of Uranium and Thorium Th-232.
Shape of Fuel Used
Fuel is usually placed within the reactor core in the form of fuel rods which are fabricated and placed within the reactor in such a manner so that it leads to a uniform production of heat within the reactor. There are two types of reactors based on the manner in which the fuel and moderator are placed within the core as follows.
1. The homogenous reactor is one in which the fuel and moderator are mixed to form a uniform mixture which is then placed in the form of rods and plates inside the reactor core.
2. A heterogeneous reactor on the other hands has pure fuel in the form of rods or plates while the moderator surrounds the fuel elements separately. In this case the fuel rods are often clad with different materials including Aluminium, Stainless Steel or Zirconium which help to prevent oxidation of Uranium.
The Fuel Cycle
The fuel cycle with regards to the nuclear power plant refers to the total process of preparation of fuel, burning of fuel and final disposal. If the fuel from the last stage is recycled to be used again in the nuclear reactor, it is known as a closed fuel cycle otherwise it is known as open fuel cycle. Of course in the former case, fuel is not thrown or dumped away at any random place but is placed and packaged properly in order to prevent contamination of the biosphere.
We learnt a few basic things about the fuels used in nuclear power plants in this article. These fuel materials certainly act as the backbone of nuclear industry and will help to achieve a powerful source of viable alternative energy for our future energy requirements.
Components of Nuclear Power Plant – Moderator
Moderation is necessary in all aspects of life if one has to achieve success. Usually extreme of anything is bad, no matter whether it is good or bad. It is no wonder the same principle applies to nuclear reactions as well. Just learn few basic concepts about moderation and moderators in context of nuclear power plants
The nuclear fission reaction consists of bombarding fuels such as Uranium with energetic neutrons. This makes the target unstable and makes it split into two parts accompanied with the release of energy which is utilized to generate electricity. There is a certain threshold below which the neutron will not be absorbed by the target nucleus, but that does not mean that above that threshold any neutron can cause fission. Infact there is a range of energy within which they can cause fission. Neutrons which fall above that range are known as fast neutrons and they are not readily absorbed by the target nucleus and hence not useful in sustaining a chain reaction. A moderator is one of the important components of nuclear power plant helping to maintain neutron population in the thermal energy range.
The problem lies in the fact that whenever a thermal neutron causes fission it also leads to the release of fast neutrons. Now these fast neutrons have to be slowed down and brought to lower energy levels if they have to cause successful fission in turn. It is here that the concept of a moderator comes in the picture.
As you must have understood above, a moderator is a medium which is used to absorb a portion of the kinetic energy of fast neutrons so that they come in the category of thermal neutrons which help to sustain a controlled chain reaction. The mechanism of speed control works in such a way that fast moving neutrons strike the nuclei of moderator material which is not efficient at absorbing them but simply slows them down with repeated collisions thus bringing them into the thermal zone.
Materials for Moderator
There are several materials which are used for the purpose including the following
Normal or Light Water is used in majority of the reactors simply because of its cheap and abundant availability. The only flipside of using light-water is that the fuel has to be enriched to use with water
Deuterium - also known as heavy water in common terminology, Deuterium is costly to manufacture as compared to light water but gives the option of using un-enriched fuel in the reactor which is a big advantage
Miscellaneous - Several materials such as Graphite, Beryllium, Lithium are used in different types of reactors as moderators
Is it always necessary?
Although moderators are necessary in most nuclear reactors this does not mean to say that all reactors require moderators. There is a special class of reactors known as fast reactors which do not use moderators but depend on the use of fast moving neutrons for causing fission. Even otherwise it must be remembered that fast moving neutrons have lesser probability of getting absorbed and causing fission but it does not mean that they are incapable of causing the fission reaction. Just to give you a relative idea a fast moving neutron travels with a speed which is nearly in the region of 10% of the speed of light, while a thermal neutron travels with a speed which is typically of the order of a few kilometers per second.
There are also other categories of neutrons based on their energy levels such as slow neutrons, cold neutrons, ultra cold neutrons and so forth.
Components of Nuclear Power Plant – Reflector
The chain reaction inside a nuclear reactor is what sustains “combustion” of the fuel which in turn depends on ample supply of thermal energy neutrons within the core. A reflector material is used to ensure that neutrons do not simply fly off the reactor leaving little room for the chain reaction to continue
The principle of reflection is fairly simple and we come across it in our everyday lives. It is the same principle of reflection which lets you see how you look in a mirror by reflecting the light waves. In fact the term reflection refers to any wave or particle being thrown back after hitting a reflecting surface. This principle is extremely useful in the reactor core and helps to maintain an ample amount of thermal energy neutrons, the lack of which could simply extinguish the fission process, rendering the device useless for producing power. Hence a reflector holds an important position amongst the components of nuclear power plant.
The Reflection Process
As we know the reactor consists of the fission process which occurs when a thermal energy neutron is absorbed by the target nucleus leading to its division into two nuclei and emission of 2 or 3 neutrons apart from the heat energy. These neutrons fly randomly in all directions and are usually in the region of fast moving energy neutrons. The moderator is used to control the speed of these neutrons so that they act usefully in creating more fission, but many of these neutrons may simply get lost by flying off the reactor core and thus serving no useful purpose. This might hinder the progression of a chain reaction which is very necessary for the nuclear reactor.
In order to reduce this process of neutron loss the inner surface of the reactor core is surrounded by a material which helps to reflect these escaping neutrons back towards the core of the reactor and these materials are known as reflecting materials.
Materials used as Reflectors
There are a variety of materials which are used as a reflecting medium for neutrons and whatever material is used for the process, it must possess these properties.
1. Low absorption - this is necessary since if the reflecting material itself starts to absorb the very neutrons it is supposed to reflect back, then the purpose of installing the reflector material would itself be defeated and it would be better not to install any reflector at all.
2. High reflection - this is an obvious property and does not need any explanation for that is the very purpose for which the reflector exists in the core
3. Radiation stability - since the reflector material will be exposed to high levels of radiation, it is but natural to assume that it should have a high stability towards radiation
4. Resistance to Oxidation - the material should not get oxidized otherwise it will fail to serve the requisite purpose
In actual practice there may not be a different material for moderator and reflector for the simple reason that most of the moderators also possess the above mentioned properties of a good reflector as well. Hence they serve the dual purpose of a reflector and a moderator as well. There light water, heavy water and carbon are mostly used as reflectors since they possess the above mentioned characteristics.
The use of a proper reflector helps to reduce the size of the reactor core for a given power output since the number of neutrons leaking are lesser and help to propagate the fission process instead. It also reduces the consumption of the fissile material.
Components of Nuclear Power Plant – Coolant
A nuclear reactor is a source of intense heat which is generated through the exothermic fission reactions taking place inside the core. Therefore a coolant is necessary to ensure that this heat is taken away and utilized in a proper manner.
The immense amount of heat energy present in the nuclear reactor core needs to be transferred in some manner so that it is converted into electrical energy. This also helps to keep the working temperature of the core within safe limits for the materials used in the construction of the reactor. Hence a coolant plays an important role in components of nuclear power plant and serves the dual purpose of removing the heat from the reactor as well as transferring it to the electricity generation circuit either directly or indirectly depending on the type of nuclear reactor being used for the purpose.
Properties of an Ideal Coolant
There are some properties of the coolant which are necessary to ensure safety of the reactor and well as proper performance of the coolant for the intended purpose. Some of the desired properties of an ideal coolant are as follows
A coolant should not absorb neutrons or should have a minimum neutron absorption cross section. The reason for this is obvious since this function should be left to the moderator and not the coolant.
Since a coolant is exposed to high temperatures and well as severe levels of radiation, it is obvious that it should posses excellent resistance to both high temperatures as well as high levels of radiation.
A coolant should be non-corrosive in nature otherwise it might tend to damage and corrode the very core which is meant to be protected by it through proper removal of heat.
Coolants used in nuclear reactors could be either in the liquid state or in the solid state. In case the coolant is a liquid it should have a high boiling point so that it does not get evaporated due to the high heat inside the reactor. But in case it is a solid it should have a relatively low melting point due to obvious reasons.
Since a coolant needs to circulate using a pump it should be capable of being pumped easily so that least amount of energy is spent in pumping the coolant.
It can be well imagined that the above list is quite extensive and therefore there is hardly any material which satisfies all the above criteria to the maximum possible extent. Therefore different types of coolants are used in different types of reactors depending on various factors and parameters.
Commonly Used Coolants
Since no single material qualifies as an ideal coolant, different coolants are used in different circumstances and some of the commonly used coolants are light water, heavy water, carbondioxide, helium, nitrogen, sodium, sodium-potassium mixture and so on. It can be seen that the coolants used vary from solids, liquids and gases and depending on the type of the reactor, the appropriate coolant is preferred.
It must be also kept in mind that sometimes a coolant is used to transfer heat to the working fluid in the secondary circuit through a heat exchanger while in other cases it is directly used in the turbine to rotate the blades and then fed back to the reactor after cooling in the condenser.
Components of Nuclear Power Plant – Control RodsWhat do control rods in a nuclear power plant do? It has been often said that power corrupts and absolute power corrupts absolutely. This maxim is usually associated with politics but applies equally to a nuclear reactor as well. Unless there is something to control the immense power that a nuclear reaction wields, the reactor would simply go haywire.
Nuclear fission is a source of tremendous energy which could be either used for destructive purposes such as nuclear weapons or constructive purposes such as a nuclear reactor for producing electrical energy. Even though a nuclear reactor in a power plant has got peaceful intentions, the tremendous power, heat and energy which is associated with nuclear fission cannot be left on its own but needs to be controlled in a predictable manner. It is here that controls rods come in the picture and form an important part of the components of nuclear power plant.
Why the Need to Control?
It does not require much reflection to imagine why proper control is necessary with in nuclear reactor. Some of the basic reasons are as follows.
A nuclear chain reaction should be started when a reactor fires from the cold condition. In the absence of such a reaction the process would soon die out.
It is not only necessary and sufficient to start the chain reaction but it is equally necessary to ensure that the reaction is sustained in the long run as long as the power requirements are present.
In case of emergency situations such as a sudden mechanical or structural damage, the reactor needs to be shut down quickly in order to prevent any major disaster like say Chernobyl which could be very costly in terms of loss to life and environment.
Fuel rods inside the reactor should be prevented from melting or getting disintegrated and therefore a control mechanism is absolutely necessary.
We have seen the reasons for controlling and taming the wild nuclear power and the best method to achieve this is through the use of control rods which can be inserted or withdrawn from the core and help to control the nuclear reactions taking place inside the reactor.
What do Control Rods Do?
One property which is a must for control rod material is the heavy absorption capacity for neutrons so that they can carry out the control function effectively. The commonly used materials which satisfy these criteria include cadmium, boron, iridium, silver and hafnium. Another property of control rods is that the material should not start a fission reaction despite the heavy absorption of neutrons. Infact you can imagine the function of a control rod just like a blotting paper which sucks the extra ink that has spilled somewhere but doesn't let it spread in a wider region.
The mechanism of control consists of arranging control rods in assembles which are usually mounted vertically within the reactor core and are inserted into the guide tubes with the fuel elements. For purposes of safety of a reactor in case the lifting mechanism also suffers a failure, the control are arranged in such a way that they will get into the stop position and shut down the reactor completely in such a case.
Hence we see that control rods tend to provide a mechanism wherein the immense nuclear energy can be tamed within reasonable limits and ensure safety and security of the reactor as well the outside environment.
Components of Nuclear Power Plant – ShieldingA nuclear reaction is a source of intense radiation apart from the heat generated in the exothermic process. Because of the risk, radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting the outside men and materials.
As you know when a nucleus gets split into two parts during the fission process it results in the production of large amounts of heat energy since the reaction is exothermic in nature. But this is not the only product of nuclear fuel "combustion" but there are several other by-products such as alpha rays, beta rays, gamma rays and of course the fast moving neutrons. The fast moving neutrons are controlled, moderation and reflected in order to contain them within the reactor core so that a sustained and controlled chain reaction takes place but what do you think happens to the other by-products? Just read on to find out why a shielding is one of the important components of nuclear power plant.
Yes, you guessed it right. These by-products in the form of different kinds of radiation would simply leak out into the atmosphere in the absence of proper arrangements to prevent this. Radiation leakage would be very harmful for the personnel working in the nuclear plant as well as the nearby flora and fauna.
This makes clear the case for having a proper shield so that these radiations get absorbed within the reactor without having a chance to escape into open air. This is done by using materials which are good absorbents of the same. Concrete and steel are very good at absorbing radiation and they are equally strong as well, hence used in forming the shielding material.
The question now arises that how much thickness of these materials should be used to prevent radiation from leaking out into the atmosphere? If you just compare it with the amount of thickness of typical steel plate required for preventing a powerful bullet from going across it you are in for a surprise. Although I am not a weapons expert but I know for sure that for stopping ordinary bullets a few mm of steel plate should be sufficient and a few cm of plate should be sufficient to stop even the most powerful of guns.
You might wonder that if such a thickness is required for a bullet which is quite bulky and dangerous, then only a couple of mm should be sufficient for humble intangible rays and neutrons but if you think so you are utterly wrong. A typical reactor core would require an inner lining which is of the order of nearly half a meter thickness of steel (don't gasp for breath).
The icing on the cake is that even this much thick steel is not considered entire safe. It is further reinforced by using a few meters of concrete to make it safer. This should give you an idea about how powerful these radiations are and their penetrating capability.
It is also interesting to note that the amount of radiation to which human beings could be exposed safely without causing any harm to the body is expressed in units of rad and rem which give the amount of absorbed radiation from different perspectives.
Components of Nuclear Power Plant – Reactor VesselA nuclear reactor consists of various parts which carry out different functions related to heat generation by “burning” of nuclear fuel, but a housing is needed to contain all these parts and act as a covering for all these paraphernalia
Just imagine if your beautiful body did not have the cover of the skin, and when you met any individual you could simply see through their various organs and into their "dirty" workings. This would certainly be not a very pleasing sight and would take out the very charm of human personality. This is not much different in the case of nuclear reactors as well. I cannot imagine going to a nuclear power plant just to find that the reactor core, fuel rods, control rods etc are all lying bare bones without any proper cover of enclosure. Hence the outside component of nuclear power plant is very important and is known as the reactor vessel.
Vessels are often used to cook food, and though a nuclear reactor may not be cooking food directly for you, it certainly provides a source of an equally valuable food for the society: electrical energy. But apart from the cooking business there are a lot of functions which a nuclear reactor vessel has to perform and some of these are as follows.
It acts to enclose the various parts inside the reactor including the core, shield, reflector etc.
The coolant needs a passage to flow through the reactor so that it can be used to transfer the heat to the working fluid or the turbine directly, as the case may be, and this passage is provided by the reactor vessel.
To withstand the high pressure with exists inside the reactor and could be of the order of 200 kgf/cm2, to provide a safe working environment for all concerned.
Control of the nuclear reaction is absolutely necessary and this is done with the help of control rods. The reactor vessel provides a place to insert these control rods in the nuclear reactor and move them in or out of the reactor core depending on the requirements of power.
The Pressure Vessel
Although the reactor vessel has been compared to a cookery vessel in the common usage of the term, technically speaking it is more of a pressure vessel. There are legal implications associated with defining a pressure vessel and these vary with the country in which it is being used or manufactured. Different countries have different authorities which govern rules and regulations regarding pressure vessels and in the US this is done by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. The material used for the construction of a nuclear vessel is usually steel which would be expected as the material has to be very strong and resilient.
Pressure vessels of all kinds are subject to various tests to check for their strength against laid down standards which is very important to ensure safety of these vessels. This is more so important in the case of nuclear reactor vessels which house source of intense raditaions and heat energy.
Hence we see that though a nuclear reactor vessel may not be performing any useful function directly in the generation of electrical energy, it acts to hold together all major components of the power plant.
Nuclear Waste Disposal MethodsEvery useful process does leave behind some waste in one form or the other. When the process is as sensitive as the nuclear reaction, the waste obviously has to be handled very carefully. Read here how nuclear power plant waste is treated!
It is all very well to eat the fruits (electricity) of labour (nuclear reaction) in a nuclear power plant setup but the reaction also leaves behind some waste materials which should be disposed off and discarded in a proper manner, simply because of the reason that it is a radioactive waste hence cannot be dumped like some ordinary waste material
A nuclear reaction leads to the production of various types of wastes during different stages of the reaction. These waste materials could include all three phases of matter namely solid, liquid as well as gaseous. Needless to say these wastes are radioactive in nature and need to be disposed off in such a manner that human society, flora, fauna and the environment in general are not harmed by them.
Different methods are used to dispose off each of these wastes and these are disposed off in ground, air or water as the case may be.
Ground is a resource which is available in plenty and offers one of the cheapest methods of disposal of nuclear waste. Earth has got good radio activity absorption capacity but the problem is that if the waste is buried at the ground water level, the water would get poisoned from "radioactivity" and hence they have to be buried at a lesser depth. Sometimes used coal mines which are not mined for coal anymore and the solid wastes are buried in them within heaps of sodium chloride. Usually solid radioactive waste is buried few meters deep in water for nearly 3 months. This leads to disappearance of a major proportion of their radioactivity after which it is buried in the ground.
The gaseous wastes can be left freely in the air but the problem here is that these gases will get absorbed by the plants and finally will get into the human food chain thus entering the human body and causing serious health hazards in the long run as the number of nuclear reactors in the world increases. Hence another safe method is to collect the gases in solid containers and keep them buried in ground; then disperse them off in the air when their radioactivity levels fall to considerably lower level.
Liquid wastes of highly radioactive nature are first enclosed in concrete containers and buried inside the ground just like solid wastes till the decay of their radioactivity. Other lesser toxic wastes are disposed off directly into the oceans but care should be taken to see that the sea life is not affected in a harmful manner.
Whatever be the method of disposal of radioactive waste materials, it must be always kept in mind that the safety of all forms of life is of utmost importance and should not be compromised even if it means taking some extra pains or incurring some extra costs to properly dispose off the waste material. Only then the nuclear energy would be useful for the human race in the long run and our future generations will also benefit from it.
Uranium 235 Vs. Thorium 90 Compared - Nuclear Power Generation Perspective
Uranium – 235 is currently used as fuel in the world’s nuclear reactors, producing High Level Wastes such as Plutonium-238 (half life 87 yrs). However, when irradiated Thorium-232 is used as a fuel, it is projected to produce up to 10,000 times less long-lived radioactive waste than uranium-235.
Uranium and Thorium Nuclear Power
Nuclear power has been producing thermal energy since the mid-fifties, when the first Russian nuclear plant generated power for their national grid.
Since then many types of nuclear reactors have been designed and are operating world-wide.
Enriched uranium is used as thq fuel in reactors, but it has the ever present inherent trait of producing high level waste and actinides in the form of plutonium and curium. These remain at very high temperatures and must be kept under water in ponds, normally at the nuclear plant, for five or six years.
Alternative fuels have been investigated to power nuclear reactors, with Thorium being one of these options.
This is another article on Nuclear Energy, and in particular the use of uranium and thorium as nuclear fuel and subsequent radioactive waste produced. We begin with a look at how the different fuels can be extracted and processed.
The Extraction and Processing of fuels used in Nuclear Power Generation.
Uranium Extraction and Enrichment.
The top three countries mining uranium ore are Kazakhstan, Canada, and Australia in that order.
Mining the Uranium Ore
There are several methods currently in use to mine uranium ore, traditional open pit and underground mining being the most popular techniques.
Underground mining for uranium ore used to be a very hazardous occupation with many miners suffering from effects of radon gas. Nowadays however, underground workings are well ventilated with large air ducts expelling the radon gas.
The uranium ore contains about 99.25% U238, 0.7% U235, and trace amounts of U234.
The U235 is extracted from the ore through crushing, milling, and leaching in acid; it is further processed to yellow cake that contains many uranium oxides. (U3O8)
The yellow cake is calcined and purified; converting U3O8 into UO3 in the process, then being further processed into 3-4% enriched uranium dioxide (UO2) discs or pellets.
The fuel is now ready to be loaded into fuel cells and installed into the reactor core.
The sodium hydroxide is heated in a vessel to around 140⁰C (284⁰F) for 3 hours which produces a mud-like solution. This is then fed into a dilution vessel where water at 100C is injected to dissolve the valuable sodium phosphates that are later extracted.
Following this, the solution is then filtered to remove mixed hydrous oxides that are heated to 150⁰C, crushed, and nitric acid added forming an aqueous solution.
This is further processed using various screening/filtration techniques from which Thorium Oxide (ThO2) is produced which can now be further processed to nuclear fissile material.
Basically, this is carried out in a fast or thermal reactor where the thorium-232 is irradiated with uranium 233 and absorbs a neutron converting it to thorium–233U; a fertile material which can now be used as a fuel in a thorium nuclear reactor.
Countries with Nuclear Power PlantsWhich country generates most of it electricity from nuclear energy power plants? Which country has the most number of nuclear energy power plants? How many countries have nuclear energy power plants?
Countries that Use Nuclear Energy Power Plants
Generating electricity from nuclear energy causes little pollution when compared to the pollution caused when the same amount of electricity is generated using thermal energy or other non-renewable energy resources. Even with such an advantage, it is not popular among the common people because the advantage comes at a huge cost. Disposing of the nuclear waste is very difficult and needs to be done after a lot of planning by the experts. The radioactive waste takes years to be no longer hazardous. In addition there is a risk of a nuclear accident like the one at Chernobyl. In spite of that, with the help of modern technology many countries use nuclear power plants to generate electricity and the number of nuclear power plants is increasing every year. Let us look look at some statistics that throw more light on the countries that use nuclear energy power plants.
Image Source - Wikipedia
Top 10 Countries that use Nuclear Energy Power Plants in Terms of Power Generated in Megawatts
The energy produced by nuclear energy power plants is measured in megawatts. The United States tops the list with more than 101 megawatts of power produced. This is roughly one-fourth of the total nuclear energy produced in the world. France follows at a far second with energy output of 63 megawatts. Japan takes the third place followed by Russia and Germany, which completes the list of the top five countries that produce electricity from nuclear energy power plants.
The Top 10 Countries that use Nuclear Energy Power Plants in Terms of Megawatts of Energy
1) United States of America
6) South Korea
9) United Kingdom
Top 10 Countries that use Nuclear Energy Power Plants in Terms of Percentage of Electricity Generated from Nuclear Energy
Although the United States ranks first in the amount of energy produced (in megawatts) from nuclear power plants, it is France that leads the pack when the percentage of electricity generated from nuclear energy is concerned. France generates more than 76% of its total electricity from nuclear energy. The U.S. comes far behind at 16th as only 19.7% of the total electricity produced is produced from nuclear energy. 14% of electricity produced in the world is generated from nuclear energy. Surprisingly smaller countries like Slovakia, Belgium, Ukraine and Armenia complete the top five list. All these countries generate an average of more than 50% of the total electricity generated from nuclear energy.
The Top 10 Countries that Use Nuclear Energy Power Plants in Terms of Percentage of Electricity Generated from Nuclear Energy
1) France - 76.2%
2) Slovakia - 56.4%
3) Belgium - 53.8%
4) Ukraine - 47.4%
5) Armenia - 43.5%
6) Sweden - 42%
7) Slovenia and Croatia share the same place - 41.7%
8) Switzerland - 39.2%
9) Hungary - 37.2%
10) South Korea - 35.6%
The Future of Nuclear Energy
The future of generation energy from nuclear energy looks strong as countries like China, Russia, South Korea, and India have 24, 10, 6 and 4 nuclear power plants under construction. Many more countries are planning to build one and many have already built and are undergoing final checks before the nuclear energy power plants are up and running. Until there is enormous progress in generating power from renewable energy resources like solar power, wind power,
tidal energy, etc., power generation from nuclear energy will be more popular than power generation using other conventional methods.
Is Nuclear Energy a Renewable Resource?
Defining Nuclear and Renewable Energy
Nuclear power originates from controlled nuclear reactions (basically fission) taking place inside nuclear reactors. Nuclear reactors use radioactive fuel (uranium), in order to heat up water and produce steam. The steam is then used for the production of electric power. The processing of uranium results in radioactive waste which consists of unconverted uranium, plutonium, and curium.
The definition of renewable energy entails the long term availability of the energy source, the ability to replenish over time, and minimum environmental impact.
The question is whether nuclear energy is actually renewable or not.
Facts Suggesting that Nuclear Power is not Renewable
The most widespread point of view states that nuclear power is not a renewable form of energy, and this is based on a number of clues:
Uranium is not a renewable fuel. The resources are limited and the process of mining and refining it is hazardous for the environment. The secure transport of uranium can raise the cost and consumption of energy significantly.
After processing, large quantities of radioactive waste are produced. The elements produced have extreme storage requirements and may stay radioactive and dangerous for thousands of years. Their recycling is costly and ineffective, and they cannot be safely stored. According to the National Academy of Sciences, the nuclear storage sites can always become a target of terrorists.
Facts Suggesting That Nuclear Power Could Become Renewable
Although uranium supplies are limited, their conversion to plutonium can considerably extend the available resources. Light water reactors use uranium-235 (0.7% of all natural uranium); fast breeder reactors or IFRs use uranium-238 (99.3% of all natural uranium). Fast-breeder nuclear reactors have the capability to produce large amounts of fissionable plutonium that could sustain nuclear reactions (the splitting of uranium atoms), much longer than the conventional fuel. This process of obtaining more reactor fuel than the original could provide the characterization of "semi-renewable" to nuclear energy.
Another benefit is that the remaining waste becomes less hazardous. After a few hundred years, the waste stops being radioactive
Nuclear waste can be reprocessed in reprocessing units so that 95% of the spent fuel can be recycled and be returned to usage in a power plant. This procedure reduces the radio-toxicity and volume of high-level nuclear waste, allowing separate handling (destruction or storage) of the nuclear waste components. The reprocessing of nuclear waste is a politically controversial issue and the anti-technology lobby strives to stop nuclear proliferation by emphasizing the high cost of reprocessing and the possibility of terrorism.
For more information on nuclear waste reprocessing, check the following article: "Is Nuclear Waste Recycling Possible?"
Is Nuclear Energy Renewable or Not?
Despite the contradiction, nuclear energy cannot be characterized as renewable, at least not if we take into account the current conditions of production and waste disposal. Uranium is still a finite fuel source and the breeder reactors processing can become unstable and dangerous.
The reprocessing of nuclear waste is still a procedure that doesn't take place the way it should.
However if the adjustments of the new reactors and processability of waste would take place after all, nuclear energy could be redefined. The overcoming of practical difficulties regarding nuclear fusion - that uses deuterium as a fuel extracted from water (which is regarded as infinite source) - could also redefine nuclear energy under a completely new scope.
Japan Nuclear Meltdown: How it Happened? It is hard not to have come across about what happened in Japan on March 11th. The earthquake of magnitude 9.0 triggered another incident that would have been as disastrous as the damage caused by the earthquake - The Fukushima I nuclear accident raising questions about the future nuclear energy.
Japan and Earthquakes
The entire land mass that Japan rests is present in a region that experiences lots of earthquakes each year. To the Japanese, earthquakes are nothing new as they experience a few every year. This has led them to adopt building architectures that are capable of withstanding earthquakes all the way to 6.0 on the Richter scale. But an earthquake of the magnitude 9.0, that Japan saw on March 11th, was something out of the blue. It was the worst earthquake that the world has ever seen.
This earthquake not only triggered a tsunami but also damaged the Fukushima I reactor to such an extent that many were afraid of a possible Chernobyl-like incident or something even worse.
A Brief About the Japan Nuclear Incident
For those who came in late, here is a brief about the series of events that has now left all the countries with nuclear power plants, in deep thoughts about the future of nuclear energy.
On March 11th of 2011, an earthquake, of magnitude 9.0 on the Richter scale, hit Japan. The earthquake caused damage to the Fukushima nuclear reactors. The damage stopped the power source to the reactors before they could shutdown. As designed, the backup generators kicked in and the reactor proceeded with its shutdown process.
However this earthquake triggered a tsunami that ravaged a huge area of land and made its way to the Fukushima nuclear reactors. The tsunami reached the reactor and damaged the backup power supply before the nuclear reactor shutdown totally.
This was something that was not expected when the reactor was designed, although it was designed to withstand earthquakes up to the magnitude of 8.2. The incomplete shutdown led to the explosion of two reactors (not a nuclear explosion but an explosion due to the build up of gases inside the reactor).
The damage caused to the nuclear reactor was so bad that the core of the nuclear reactor was exposed in the days that followed. This led to the increase in nuclear radiation level around the surrounding areas.
Image Credit: Flickr - Official U.S Navy Imagery
Lessons Learned After the Japan Nuclear Incident
Tapping energy from the nuclear energy resources is, so far, the best way to tap energy among the non-renewable energy resources but in the event of natural disasters like the one at Japan, things can go wrong and cause damage of a very large degree.
In case of damages to thermal power stations, the damage may happen just to the building, which can be constructed again in a matter of months but when a nuclear reactor is damaged and the radiation leaks out of the reactor, a few kilometers of land in the surrounding area can become so highly contaminated with radiation that the entire region may be rendered useless for decades to come.
After this incident, various countries that have and are building nuclear power reactors have been exposed to the fact that when it comes to mother nature, anything can happen. Therefore these countries have started to spend more on research and development on making the nuclear reactors safer when disasters like above occur.
Also there is a rise in switching over to renewable energy resources that causes no pollution and in the wake of disasters, only the property is damaged.
Nuclear Power Reactors
(updated March 2011)
Most nuclear electricity is generated using just two kinds of reactors which were developed in the 1950s and improved since.
New designs are coming forward and some are in operation as the first generation reactors come to the end of their operating lives.
Over 16% of the world's electricity is produced from nuclear energy, more than from all sources worldwide in 1960.
A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. (In a research reactor the main purpose is to utilise the actual neutrons produced in the core. In most naval reactors, steam drives a turbine directly for propulsion.)
The principles for using nuclear power to produce electricity are the same for most types of reactor. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants).
In the world's first nuclear reactors about two billion years ago, the energy was not harnessed since these operated in rich uranium orebodies for a couple of million of years, moderated by percolating rainwater. Those at Oklo in west Africa, each less than 100 kWt, consumed about six tonnes of that uranium.
Components of a nuclear reactor
There are several components common to most types of reactors:
Fuel. Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.*
* In a new reactor with new fuel a neutron source is needed to get the reaction going. Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with some used
fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.
Moderator. This is material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.* In some PWR reactors, special control rods are used to enable the core to sustain a low level of power efficiently. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)
* In fission, most of the neutrons are released promptly, but some are delayed. These are crucial in enabling a chain reacting system (or reactor) to be
controllable and to be able to be held precisely critical.
Coolant. A liquid or gas circulating through the core so as to transfer the heat from it. . In light water reactors the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the steam is made. (see also later section on primary coolant characteristics)
Pressure vessel or pressure tubes. Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.
Steam generator. (not in BWR) Part of the cooling system where the primary coolant bringing heat from the reactor is used to make steam for the turbine. Reactors may have up to four "loops", each with a steam generator.
Containment. The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a metre-thick concrete and steel structure.
There are several different types of reactors as indicated in the following Table.
Nuclear power plants in commercial operation
Reactor type Main Countries Number GWe Fuel Coolant Moderator
Pressurised Water Reactor (PWR)
US, France, Japan, Russia, China
Boiling Water Reactor (BWR) US, Japan, Sweden 94 86.4enriched
UO2 water water
Pressurised Heavy Water Reactor 'CANDU' (PHWR)
Canada 44 24.3 natural UO2 heavy water
Gas-cooled Reactor (AGR & Magnox)
UK 18 10.8
natural U (metal),enriched
Light Water Graphite Reactor (RBMK)
Russia 12 12.3enriched
UO2 water graphite
Fast Neutron Reactor (FBR) Japan, Russia 2 1.0 PuO2 and UO2
Reactor type Main Countries Number GWe Fuel Coolant Moderator
Other Russia 4 0.05enriched
UO2 water graphite
TOTAL 439 386.5
GWe = capacity in thousands of megawatts (gross)Source: Nuclear Engineering International Handbook 2010
For reactors under construction: see paper Plans for New Reactors Worldwide.
Fuelling a nuclear power reactor
Most reactors need to be shut down for refuelling, so that the pressure vessel can be opened up. In this case refuelling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual pressure tubes.
If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium. Natural uranium has the same elemental composition as when it was mined (0.7% U-235, over 99.2% U-238), enriched uranium has had the proportion of the fissile isotope (U-235) increased by a process called enrichment, commonly to 3.5 - 5.0%. In this case the moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.
During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up providing about one third of the energy from the fuel.
In most reactors the fuel is ceramic uranium oxide (UO2 with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons.* Numerous rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5 to 4 metres long.
*Zirconium is an important mineral for nuclear power, where it finds its main use. It is therefore subject to controls on trading. It is normally contaminated with hafnium, a neutron absorber, so very pure 'nuclear grade' Zr is used to make the zircaloy, which is about 98% Zr plus tin, iron, chromium and
sometimes nickel to enhance its strength.
Burnable poisons are often used (especially in BWR) in fuel or coolant to even out the performance of the reactor over time from fresh fuel being loaded to refuelling. These are neutron absorbers which decay under neutron exposure, compensating for the progressive build up of neutron absorbers in the fuel as it is burned. The best known is gadolinium, which is a vital
ingredient of fuel in naval reactors where installing fresh fuel is very inconvenient, so reactors are designed to run more than a decade between refuellings.
The power rating of a nuclear power reactor
Nuclear power plant reactor power outputs are quoted in three ways:
Thermal MWt, which depends on the design of the actual nuclear reactor itself, and relates to the quantity and quality of the steam it produces.
Gross electrical MWe indicates the power produced by the attached steam turbine and generator, and also takes into account the ambient temperature for the condenser circuit (cooler means more electric power, warmer means less). Rated gross power assumes certain conditions with both.
Net electrical MWe, which is the power available to be sent out from the plant to the grid, after deducting the electrical power needed to run the reactor (cooling and feed-water pumps, etc.) and the rest of the plant.*
* footnote: This (as also actual gross MWe) varies slightly from summer to winter, so normally the lower summer figure, or an average figure, is used. If the summer figure is quoted plants may show a capacity factor greater than 100% in cooler times. Some design options, such as powering the main large feed-water pumps with electric motors (as in EPR) rather than steam turbines (taking steam before it gets to the main turbine-generator), explains some gross to net differences between different reactor types. The EPR has a relatively large drop from gross to net MWe for this reason.
The relationship between these is expressed in two ways:
Thermal efficiency %, the ratio of gross MWe to thermal MW. This relates to the difference in temperature between the steam from the reactor and the cooling water. It is often 33-37%.
Net efficiency %, the ratio of net MWe achieved to thermal MW. This is a little lower, and allows for plant usage.
In WNA papers and figures and WNN items, generally net MWe is used for operating plants, and gross MWe for those under construction or planned/proposed.
Pressurised Water Reactor (PWR)
This is the most common type, with over 230 in use for power generation and several hundred more employed for naval propulsion. The design of PWRs originated as a submarine power plant. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine. In Russia these are known as VVER types - water-moderated and -cooled.
A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium.
Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.
The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.
Boiling Water Reactor (BWR)
This design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there. BWR units can operate in load-following mode more readily then PWRs.
The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the
savings due to the simpler design. Most of the radioactivity in the water is very short-lived*, so the turbine hall can be entered soon after the reactor is shut down.
* mostly N-16, with a 7 second half-life
A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that more steam in the top part reduces moderation.
Pressurised Heavy Water Reactor (PHWR or CANDU)
The PHWR reactor design has been developed since the 1950s in Canada as the CANDU, and more recently also in India. It uses natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O).**
** with the CANDU system, the moderator is enriched (ie water) rather than the fuel, - a cost trade-off.
The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.
A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown on the diagram above).
Newer PHWR designs such as the Advanced Candu Reactor (ACR) have light water cooling and slightly-enriched fuel.
CANDU reactors can readily be run on recycled uranium from reprocessing LWR used fuel, or a blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of PWR can then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium.
Advanced Gas-cooled Reactor (AGR)
These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel. Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant.
The AGR was developed from the Magnox reactor, also graphite moderated and CO2 cooled, and two of these are still operating in UK. They use natural uranium fuel in metal form. Secondary coolant is water.
Light water graphite-moderated reactor (RBMK)
This is a Soviet design, developed from plutonium production reactors. It employs long (7 metre) vertical pressure tubes running through graphite moderator, and is cooled by water, which is allowed to boil in the core at 290°C, much as in a BWR. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 metres long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorbtion without inhibiting the fission reaction, and a positive feedback problem can arise, which is why they have never been built outside the Soviet Union.
Several generations of reactors are commonly distinguished. Generation I reactors were developed in 1950-60s and very few are still running today. They mostly used natural uranium fuel and used graphite as moderator. Generation II reactors are typified by the present US fleet and most in operation elsewhere. They typically use enriched uranium fuel and are mostly cooled and moderated by water. Generation III are the Advanced Reactors, the first few of which are in operation in Japan and others are under construction and ready to be ordered. They are developments of the second generation with enhanced safety.
Generation IV designs are still on the drawing board and will not be operational before 2020 at the earliest, probably later. They will tend to have closed fuel cycles and burn the long-lived actinides now forming part of spent fuel, so that fission products are the only high-level waste. Many will be fast neutron reactors.
More than a dozen (Generation III) advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor, a few of which are now operating with others under construction. The best-known radical new design is the Pebble Bed Modular Reactor, using helium as coolant, at very high temperature, to drive a turbine directly.
Considering the closed fuel cycle, Generation 1-3 reactors recycle plutonium (and possibly uranium), while Generation IV are expected to have full actinide recycle.
Fast neutron reactors (FNR)
Some reactors (only one in commercial service) do not have a moderator and utilise fast neutrons, generating power from plutonium while making more of it from the U-238 isotope in or around the fuel. While they get more than 60 times as much energy from the original uranium compared with the normal reactors, they are expensive to build. Further development of them is likely in the next decade, and the main designs expected to be built in two decades are FNRs. If they are configure to produce more fissile material (plutonium) than they consume they are called Fast Breeder Reactors (FBR). See also Fast Neutron Reactors and Small Reactors papers.
Floating nuclear power plants
Apart from over 200 nuclear reactors powering various kinds of ships, Rosatom in Russia has set up a subsidiary to supply floating nuclear power plants ranging in size from 70 to 600 MWe. These will be mounted in pairs on a large barge, which will be permanently moored where it is needed to supply power and possibly some desalination to a shore settlement or industrial complex. The first has two 40 MWe reactors based on those in icebreakers and will operate at Vilyuchinsk, Kamchatka peninsula, to ensure sustainable electricity and heat supplies to the naval base there from 2013. The second plant of this size is planned for Pevek on the Chukotka peninsula in the Chaun district of the far northeast, near Bilibino. Electricity cost is expected to be much lower than from present alternatives.
The Russian KLT-40S is a reactor well proven in icebreakers and now proposed for wider use in desalination and, on barges, for remote area power supply. Here a 150 MWt unit produces 35 MWe (gross) as well as up to 35 MW of heat for desalination or district heating. These are designed to run 3-4 years between refuelling and it is envisaged that they will be operated in pairs to allow for outages, with on-board refuelling capability and used fuel storage. At the end of a 12-year operating cycle the whole plant is taken to a central facility for 2-year overhaul and removal of used fuel, before being returned to service. Two units will be mounted on a 21,000 tonne barge. A larger Russian factory-built and barge-mounted reactor is the VBER-150, of 350 MW thermal, 110 MWe. The larger VBER-300 PWR is a 325 MWe unit, originally envisaged in pairs as a floating nuclear power plant, displacing 49,000 tonnes. As a cogeneration plant it is rated at 200 MWe and 1900 GJ/hr.
Lifetime of nuclear reactors.
Most of today's nuclear plants which were originally designed for 30 or 40-year operating lives. However, with major investments in systems, structures and components lives can be extended, and in several countries there are active programs to extend operating lives. In the USA most of the more than one hundred reactors are expected to be granted licence extensions from 40 to 60 years. This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins.
Some components simply wear out, corrode or degrade to a low level of efficiency. These need to be replaced. Steam generators are the most prominent and expensive of these, and many have been replaced after about 30 years where the reactor otherwise has the prospect of running for 60 years. This is essentially an economic decision. Lesser components are more straightforward to replace as they age. In Candu reactors, pressure tube replacement has been undertaken on some plants after about 30 years operation.
A second issue is that of obsolescence. For instance, older reactors have analogue instrument and control systems. Thirdly, the properties of materials may degrade with age, particularly with heat and neutron irradiation. In respect to all these aspects, investment is needed to maintain reliability and safety. Also, periodic safety reviews are undertaken on older plants in line with international safety conventions and principles to ensure that safety margins are maintained.
See also section on Ageing, in Safety of Nuclear Power Reactors paper.
Nuclear power plants are essentially base-load generators, running continuously. This is because their power output cannot readily be ramped up and down on a daily and weekly basis, and in this respect they are similar to most coal-fired plants. (It is also uneconomic to run them at less than full capacity, since they are expensive to build but cheap to run.) However, in some situations it is necessary to vary the output according to daily and weekly load cycles on a regular basis, for instance in France, where there is a very high reliance on nuclear power.
While BWRs can be made to follow loads reasonably easily without burning the core unevenly, this is not as readily achieved in a PWR. The ability of a PWR to run at less than full power for much of the time depends on whether it is in the early part of its 18 to 24-month refueling cycle or late in it, and whether it is designed with special control rods which diminish power levels throughout the core without shutting it down. Thus, though the ability on any individual PWR reactor to run on a sustained basis at low power decreases markedly as it progresses through the refueling cycle, there is considerable scope for running a fleet of reactors in load-following mode. See further information in the Nuclear Power in France paper.
The advent of some of the designs mentioned above provides opportunity to review the various primary coolants used in nuclear reactors. There is a wide variety - gas, water, light metal, heavy
metal and salt:
Water or heavy water must be maintained at very high pressure (1000-2200 psi, 7-15 MPa) to enable it to function above 100°C, as in present reactors. This has a major influence on reactor engineering. However, supercritical water around 25 MPa can give 45% thermal efficiency - as at some fossil-fuel power plants today with outlet temperatures of 600°C, and at ultra supercritical levels (30+ MPa) 50% may be attained.
Helium must be used at similar pressure (1000-2000 psi, 7-14 MPa) to maintain sufficient density for efficient operation. Again, there are engineering implications, but it can be used in the Brayton cycle to drive a turbine directly.
Carbon dioxide was used in early British reactors and their AGRs. It is denser than helium and thus likely to give better thermal conversion efficiency. There is now interest in supercritical CO2
for the Brayton cycle.
Sodium, as normally used in fast neutron reactors, melts at 98°C and boils at 883°C at atmospheric pressure, so despite the need to keep it dry the engineering required to contain it is relatively modest. However, normally water/steam is used in the secondary circuit to drive a turbine (Rankine cycle) at lower thermal efficiency than the Brayton cycle.
Lead or lead-bismuth eutectic in fast neutron reactors are capable of higher temperature operation. They are transparent to neutrons, aiding efficiency, and since they do not react with water the heat exchanger interface is safer. They do not burn when exposed to air. However, they are corrosive of fuel cladding and steels, which originally limited temperatures to 550°C. With today's materials 650°C can be reached, and in future 700°C is in sight, using oxide dispersion-strengthened steels. A problem is that Pb-Bi yields toxic polonium (Po-210) activation products. Pb-Bi melts at a relatively low 125°C (hence eutectic) and boils at 1670°C, Pb melts at 327°C and boils at 1737°C but is very much more abundant and cheaper to produce than bismuth, hence is envisaged for large-scale use in the future, though freezing must be prevented. The development of nuclear power based on Pb-Bi cooled fast neutron reactors is likely to be limited to a total of 50-100 GWe, basically for small reactors in remote places. In 1998 Russia declassified a lot of research information derived from its experience with submarine reactors, and US interest in using Pb or Pb-Bi for small reactors has increased subsequently. The Hyperion reactor will use lead-bismuth eutectic which is 45% Pb, 55% Bi.
Molten fluoride salt boils at 1400°C at atmospheric pressure, so allows several options for use of the heat, including using helium in a secondary Brayton cycle with thermal efficiencies of 48% at 750°C to 59% at 1000°C, or manufacture of hydrogen.
Low-pressure liquid coolants allow all their heat to be delivered at high temperatures, since the temperature drop in heat exchangers is less than with gas coolants. Also, with a good margin between operating and boiling temperatures, passive cooling for decay heat is readily achieved.
The removal of passive decay heat is a vital feature of primary cooling systems, beyond heat transfer to do work. When the fission process stops, fission product decay continues and a substantial amount of heat is added to the core. At the moment of shutdown, this is about 6% of the full power level, but it quickly drops to about 1% as the short-lived fission products decay. This heat could melt the core of a light water reactor unless it is reliably dissipated. Typically some kind of convection flow is relied upon.
See also paper on Cooling Power Plants.
Nuclear reactors for process heat
Producing steam to drive a turbine and generator is relatively easy, and a light water reactor running at 350°C does this readily. As the above section and Figure show, other types of reactor are required for higher temperatures. A 2010 US Department of Energy document quotes 500°C for a liquid metal cooled reactor (FNR), 860°C for a molten salt reactor (MSR), and 950°C for a high temperature gas-cooled reactor (HTR). Lower-temperature reactors can be used with supplemental gas heating to reach higher temperatures, though employing an LWR would not be practical or economic. The DOE said that high reactor outlet temperatures in the range 750 to 950°C were required to satisfy all end user requirements evaluated to date for the Next Generation Nuclear Plant.
The world's oldest known nuclear reactors operated at what is now Oklo in Gabon, West Africa. About 2 billion years ago, at least 17 natural nuclear reactors achieved criticality in a rich deposit of uranium ore. Each operated at about 20 kW thermal. At that time the concentration of U-235 in all natural uranium was 3.7 percent instead of 0.7 percent as at present. (U-235 decays much faster than U-238, whose half-life is about the same as the age of the Earth.) These natural chain reactions, started spontaneously by the presence of water acting as a moderator, continued for about 2 million years before finally dying away.
During this long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the orebody. The initial radioactive products have long since decayed into stable elements but close study of the amount and location of these has shown that there was little movement of radioactive wastes during and after the nuclear reactions. Plutonium and the other transuranics remained immobile
Chemistry & Nuclear Power
What are the most important elements to humans? Other than the basic organic elements of Carbon, Nitrogen, Hydrogen and Oxygen, many elements play an important part of energy conversion and production. As Silicon, Boron, Phosphorus, Arsenic and Aluminum are important elements of solar electricity, Uranium, Plutonium, and play crucial roles in nuclear power.
Uranium plays a very important role in the production of energy through nuclear power. A heavy element located in the Actinide series it has 92 protons as indicated by its atomic number. Look at Uranium on the periodic table. Uranium has many different isotopes; different configurations of protons and neutrons in its nucleus. Not all of its isotopes are stable. For a indepth lesson on isotopes see the link's page.
The process of converting uranium to energy involves many decomposition reactions. In a decomposition reaction the first element breaks down and releases components of itself. If the reaction releases energy it is said to be endothermic. Reactions that need energy to occur are called exothermic reactions, because energy comes from outside. Decomposition reactions follow this general form:
Element A ---> Element B + Biproducts
For the nuclear reaction process, the Biproducts are neutrons and energy measured in the unit of Joules. Many other chemical equations occur within nuclear power. A special page reviews the basic forms of many chemical reactions and gives examples of them. All of the chemical equations of this page are listed at the linked page. For the decomposition of Uranium - 235 the reaction is summarized:
10n + 235
92U ---> 9236Kr + 141
56 Ba +200 MeV+ 3 1 0n
The leftover neutrons of the reaction come from the nucleus of the Uranium. These neutrons are either absorbed by system controlling fuel rods or are used to continue the uranium decomposition reaction. A process where the products of one reaction become reactants for the next reaction is called a chain reaction. The number of these rods is changed periodically to increase or decrease power production as needed.
More nuclear fuel rods means that the reaction is slowed down, and less energy is produced. The following equation summarizes this relationship:
equation of Power porport to 1/ Fuel rods#
The energy given off is 200 MeV, equivalent approximately to <value>. This energy will not be used directly, but indirectly to heat a liquid such as water to a gas. Thermal energy is the technical term applied to the energy given off from the nuclear decomposition reaction
NUCLEAR POWER GENERATION
Nuclear Power Stations use a fuel called uranium, a relatively common material. Energy is released from uranium when an atom is split by a neutron. The uranium atom is split into two and as this happens energy is released in the form of radiation and heat. This nuclear reaction is called the fission process
In a nuclear power station the uranium is first formed into pellets and then into long rods. The uranium rods are kept cool by submerging them in water. When they are removed from the water a nuclear reaction takes place causing heat. The amount of heat required is controlled by raising and lowering the rods. If more heat is required the rods are raised further out of the water and if less is needed they lower further into it.
GENERAL ADVANTAGES AND DISADVANTAGES OFNUCLEAR POWER GENERATION
1. Nuclear power is a controversial method of producing electricity. Many people and environmental organisations are very concerned about the radioactive fuel it needs. 2. There have been serious accidents with a small number of nuclear power stations. The accident at Chernobyl (Ukraine) in 1986, led to 30 people being killed and over 100,000 people being evacuated. In the preceding years another 200,00 people were resettled away from the radioactive area. Radiation was even detected over a thousand miles away in the UK as a result of the Chernobyl accident. It has been suggested that over time 2500 people died as a result of the accident. 3. There are serious questions to be answered regarding the storage of radioactive waste produced through the use of nuclear power. Some of the waste remains radioactive (dangerous) for thousands of years and is currently stored in places such as deep caves and mines.4. Storing and monitoring the radioactive waste material for thousands of years has a high cost.5. Nuclear powered ships and submarines pose a danger to marine life and the environment. Old vessels can leak radiation if they are not maintained properly or if they are dismantled carelessly at the end of their working lives.6. Many people living near to nuclear power stations or waste storage depots are concerned about nuclear accidents and radioactive leaks. Some fear that living in these areas can damage their health, especially the health of young children.7. Many Governments fear that unstable countries that develop nuclear power may also develop nuclear weapons and even use them.
1. The amount of electricity produced in a nuclear power station is equivalent to that produced by a fossil fuelled power station.
2. Nuclear power stations do not burn fossil fuels to produce electricity and consequently they do not produce damaging, polluting gases. 3. Many supporters of nuclear power production say that this type of power is environmentally friendly and clean. In a world that faces global warming they suggest that increasing the use of nuclear power is the only way of protecting the environment and preventing catastrophic climate change. 4. Many developed countries such as the USA and the UK no longer want to rely on oil and gas imported from the Middle East, a politically unstable part of the world.5. Countries such as France produce approximately 90 percent of their electricity from nuclear power and lead the world in nuclear power generating technology - proving that nuclear power is an economic alternative to fossil fuel power stations. 6. Nuclear reactors can be manufactured small enough to power ships and submarines. If this was extended beyond military vessels, the number of oil burning vessels would be reduced and consequently pollution.
What is a nuclear reactor?
A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research.
Fuel, made up of heavy atoms that split when they absorb neutrons, is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause other atoms to split. Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is carried out of the reactor by coolant, which is most commonly just plain water. The coolant heats up and goes off to a turbine to spin a generator or drive shaft. So basically, nuclear reactors are exotic heat sources.
On this page:
Components of nuclear reactors Animated reactor system The nuclear core Types of nuclear reactors
Components of nuclear reactors
The control room
The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else. In the US fleet of power reactors, water is the standard.
The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete. Chernobyl did not have a containment to speak of.
Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor.
Animated reactor system
This image (reproduced from the NRC) shows a nuclear reactor heating up water and spinning a generator to produce electricity. It captures the essence of the sytem well. The water coming into the condenser and then going right back out would be water from a river, lake, or ocean. It goes out the cooling towers. As you can see, this water does not go near the radioactivity, which is in the reactor vessel.
The nuclear core
The smallest unit of the reactor is the fuel pin. These are typically uranium-oxide (UO2). They are surrounded by a zirconium clad to keep fission products from escaping into the coolant.
Fuel assemblies are bundles of fuel pins. Fuel is put in and taken out of the reactor in assemblies. The assemblies have some structural material to keep the pins close but not touching, so that there’s room for coolant. Click here to see a 3-D blowup diagram of an assembly.
This is a full core, made up of several hundred assemblies. Some assemblies are control assemblies. Various fuel assemblies around the core have different fuel in them. They vary in
enrichment and age, among other parameters. The assemblies may also vary with height, with different enrichments at the top of the core from those at the bottom.
Types of nuclear reactors
There are very many different types of nuclear reactors with different fuels, coolants, fuel cycles, purposes. Here’s an incomplete list of them. Please, add to the list by posting in the forum!
Pressurized Water Reactor
The most common type of reactor -- the PWR uses regular old water as a coolant. The primary cooling water is kept at very high pressure so it does not boil. It goes through a heat exchanger, transferring heat to a secondary coolant loop, which then spins the turbine. These use oxide fuel pellets stacked in zirconium tubes. They could possibly burn thorium or plutonium fuel as well.
Strong negative void coefficient -- reactor cools down if water starts bubbling Secondary loop keeps radioactive stuff away from turbines, making maintenance easy.
Pressurized coolant escapes rapidly if a pipe breaks, necessitating lots of back-up cooling systems.
Can’t breed new fuel -- susceptible to "uranium shortage"
Sodium Cooled Fast Reactor
The first electricity-producing nuclear reactor in the world was SFR (the EBR-1 in Arco, Idaho). As the name implies, these reactors are cooled by liquid sodium metal. Sodium is heavier than hydrogen, a fact that leads to the neutrons moving around at higher speeds (hence fast). These can use metal or oxide fuel, and burn anything you throw at them (thorium, uranium, plutonium, higher actinides).
Can breed its own fuel, effectively eliminating any concerns about uranium shortages (see what is a fast reactor?)
Can burn its own waste Metallic fuel and excellent thermal properties of sodium allow for passively safe operation -- the
reactor will shut itself down without any backup-systems working (or people around), only relying on physics (gravity, natural circulation, etc.).
Sodium coolant is explosively reactive with air, water. Thus, leaks in the pipes results in sodium fires. These can be engineered around (by making a pool and eliminating pipes, etc.) but are a major setback for these nice reactors.
To fully burn waste, these require reprocessing facilities which can also be used for nuclear proliferation.
Positive void coefficients are inherent to all fast reactors. This is a safety concern.
Liquid Fluoride Thorium Reactor
LFTRs have gotten a lot of attention lately in the media. They are unique so far in that they use molten fuel. So there's no worry of meltdown because they’re already melted. The folks over at Energy from thorium are totally stoked about this technology.
Can constantly breed new fuel, eliminating concerns over energy resources Can be maintained online with chemical fission product removal, eliminating the need to shut
down during refueling. No cladding means less neutron-absorbing material in the core, which leads to better neutron
efficiency and thus higher fuel utilization Liquid fuel also means that structural dose does not limit the life of the fuel, allowing the reactor
to extract very much energy out of the loaded fuel.
Radioactive gaseous fission products are not contained in small pins, as they are in typical reactors. So if there is a containment breach, all the fission gases can release instead of just the gases from one tiny pin. This necessitates things like triple-redundant containments, etc. and can be handled, but is certainly a challenge and disadvantage. All liquid fuel reactors have this problem.
The presence of an online reprocessing facility with incoming pre-melted fuel is a proliferation concern. The operator could easily divert Pa-233 to provide a small stream of nearly pure weapons-grade U-233. Also, the entire uranium inventory can be separated without much effort. In his autobiography, Alvin Weinberg explains how this was done at Oak Ridge National Lab: "It was a remarkable feat! In only 4 days all of the 218 kg of uranium in the reactor were separated from the intensely radioactive fission products and its radioactivity reduced five
billion-fold." Thus, anyone who operates this kind of reactor will have easy access to bomb material.
Nuclear Power Plant Emergency Information & Safety Tips
FACT SHEET: NUCLEAR POWER PLANT EMERGENCY Since 1980, each utility that owns a commercial nuclear power plant in the United States has been required to have both an onsite and offsite emergency response plan as a condition of obtaining and maintaining a license to operate that plant. Onsite emergency response plans are approved by the Nuclear Regulatory Commission (NRC). Offsite plans (which are closely coordinated with the utility's onsite emergency response plan) are evaluated by the Federal Emergency Management Agency (FEMA) and provided to the NRC, who must consider the FEMA findings when issuing or maintaining a license. Federal law establishes the criterion for determining the adequacy of offsite planning and
preparedness, i.e: "Plans and preparedness must be determined to adequately protect the public health and safety by providing reasonable assurance that appropriate measures can be taken offsite in the event of a radiological emergency."
Although construction and operation of nuclear power plants are closely monitored and regulated by the NRC, an accident, though unlikely, is possible. The potential danger from an accident at a nuclear power plant is exposure to radiation. This exposure could come from the release of radioactive material from the plant into the environment, usually characterized by a plume (cloud-like) formation. The area the radioactive release may affect is determined by the amount released from the plant, wind direction and speed and weather conditions (i.e., rain, snow, etc.) which would quickly drive the radioactive material to the ground, hence causing increased deposition of radionuclides.
If a release of radiation occurs, the levels of radioactivity will be monitored by authorities from Federal and State governments, and the utility, to determine the potential danger in order to protect the public.
What Is Radiation?
Radiation is any form of energy propagated as rays, waves or energetic particles that travel through the air or a material medium.
Radioactive materials are composed of atoms that are unstable. An unstable atom gives off its excess energy until it becomes stable. The energy emitted is radiation. The process by which an atom changes from an unstable state to a more stable state by emitting radiation is called radioactive decay or radioactivity.
People receive some natural or background radiation exposure each day from the sun, radioactive elements in the soil and rocks, household appliances (like television sets and microwave ovens), and medical and dental x-rays. Even the human body itself emits radiation. These levels of natural and background radiation is normal. The average American receives 360 millirems of radiation each year, 300 from natural sources and 60 from man-made activities. (A rem is a unit of radiation exposure.)
Radioactive materials--if handled improperly--or radiation accidentally released into the environment, can be dangerous because of the harmful effects of certain types of radiation on the body. The longer a person is exposed to radiation and the closer the person is to the radiation, the greater the risk.
Although radiation cannot be detected by the senses (sight, smell, etc.), it is easily detected by scientists with sophisticated instruments that can detect even the smallest levels of radiation.
Preparing For An Emergency
Federal, State and local officials work together to develop site-specific emergency response plans for nuclear power plant accidents. These plans are tested through exercises that include protective actions for schools and nursing homes.
The plans also delineate evacuation routes, reception centers for those seeking radiological monitoring and location of congregate care centers for temporary lodging.
State and local governments, with support from the Federal government and utilities, develop plans that include a plume emergency planning zone with a radius of 10 miles from the plant, and an ingestion planning zone within a radius of 50 miles from the plant.
Residents within the 10-mile emergency planning zone are regularly disseminated emergency information materials (via brochures, the phone book, calendars, utility bills, etc.). These materials contain educational information on radiation, instructions for evacuation and sheltering, special arrangements for the handicapped, contacts for additional information, etc. Residents should be familiar with these
emergency information materials.
Radiological emergency plans call for a prompt Alert and Notification system. If needed, this prompt Alert and Notification System will be activated quickly to inform the public of any potential threat from natural or man-made events. This system uses either sirens, tone alert radios, route alerting (the "Paul Revere" method), or a combination to notify the public to tune their radios or television to an Emergency Alert System (EAS) station.
The EAS stations will provide information and emergency instructions for the public to follow. If you are alerted, tune to your local EAS station which includes radio stations, television stations, NOAA weather radio, and the cable TV system.
Special plans must be made to assist and care for persons who are medically disabled or handicapped. If you or someone you know lives within ten miles of a nuclear facility, please notify and register with your local emergency management agency. Adequate assistance will be provided during an emergency.
In the most serious case, evacuations will be recommended based on particular plant conditions rather than waiting for the situation to deteriorate and an actual release of radionuclides to occur.
Emergency Classification Levels
Preparedness for commercial nuclear power plants includes a system for notifying the public if a problem occurs at a plant. The emergency classification level of the problem is defined by these four categories:
Notification of Unusual Event is the least serious of the four levels. The event poses no threat to you or to plant employees, but emergency officials are notified. No action by the public is necessary.
Alert is declared when an event has occurred that could reduce the plant's level of safety, but backup plant systems still work. Emergency agencies are notified and kept informed, but no action by the public is necessary.
Site Area Emergency is declared when an event involving major problems with the plant's safety systems has progressed to the point that a release of some radioactivity into the air or water is possible, but is not expected to exceed Environmental Protection Agency Protective Action Guidelines (PAGs) beyond the site boundary. Thus, no action by the public is necessary.
General Emergency is the most serious of the four classifications and is declared when an event at the plant has caused a loss of safety systems. If such an event occurs, radiation could be released that would travel beyond the site boundary. State and local authorities will take action to protect the residents living near the plant. The alert and notification system will be sounded. People in the affected areas could be advised to evacuate promptly or, in some situations, to shelter in place. When the sirens are sounded, you should listen to your radio, television and tone alert radios for site-specific information and instructions.
If You Are Alerted
Remember that hearing a siren or tone alert radio does not mean you should evacuate. It means you should promptly turn to an EAS station to determine whether it is only a test or an actual emergency.
Tune to your local radio or television station for information. The warning siren could mean a nuclear power plant emergency or the sirens could be used as a warning for tornado, fire, flood, chemical spill, etc.
Check on your neighbors.
Do not call 911. Special rumor control numbers and information will be provided to the public for a nuclear power plant emergency, either during the EAS message, in the utilities' public information brochure, or both.
In a nuclear power plant emergency, you may be advised to go indoors and, if so, to close all windows, doors, chimney dampers, other sources of outside air, and turn off forced air heating and cooling equipment, etc. If You Are Advised to Evacuate the Area Stay calm and do not rush Listen to emergency information Close and lock windows and doors Turn off air conditioning, vents, fans, and furnace Close fire place dampers Take a few items with you. Gather personal items you or your family might need: Flash light and extra batteries Portable, battery operated radio and extra batteries First aid kit and manual Emergency food and water Essential medicines Cash and credit cards Use your own transportation or make arrangements to ride with a neighbor. Public transportation should be available for those who have not made arrangements. Keep car windows and air vents closed and listen to an EAS radio station. Follow the evacuation routes provided. If you need a place to stay, congregate care information will be provided.
If Advised to remain at Home
Bring pets inside. Close and lock windows and doors Turn off air conditioning, vents, fans and furnace Close fireplace dampers Go to the basement or other underground area Stay inside until authorities say it is safe When Coming In From Outdoors Shower and change clothing
and shoes Put items worn outdoors in a plastic bag and seal it. The thyroid gland is vulnerable to the uptake of radioactive iodine. If a radiological release occurs at a nuclear power plant, States may decide to provide the public with a stable iodine, potassium iodide, which saturates the thyroid and protects it from the uptake of radioactive iodine. Such a protective action is at the option of State, and in some cases, local government. Remember your neighbors may require special assistance--infants, elderly people, and people with disabilities.
If an incident involving an actual or potential radiological release occurs, consideration is given to the safety of the children. If an emergency is declared, students in the 10-mile emergency planning zone will be relocated to designated facilities in a safe area. Usually, as a precautionary measure, school children are relocated prior to the evacuation of the general public.
For Farmers and Home Gardeners
If a radiological incident occurs at the nuclear facility, periodic information concerning the safety of farm and home grown products will be provided. Information on actions you can take to protect crops and livestock is available from your agricultural extension agent.
Normal harvesting and processing may still be possible if time permits. Unharvested crops are hard to protect.
Crops already harvested should be stored inside if possible.
Wash and peel vegetables and fruits before use if they were not already harvested.
Provide as much shelter as possible. Take care of milk-producing animals.
Provide plenty of food and water and make sure shelters are well-ventilated. Use stored feed and water, when possible.
Three Ways to Minimize Radiation Exposure
There are three factors that minimize radiation exposure to your body: Time, Distance, and Shielding.
Time--Most radioactivity loses its strength fairly quickly. Limiting the time spent near the source of radiation reduces the amount of radiation exposure you will receive. Following an accident, local authorities will monitor any release of radiation and determine the level of protective actions and when the threat has passed.
Distance--The more distance between you and the source of the radiation, the less radiation you will receive. In the most serious nuclear power plant accident, local officials will likely call for an evacuation, thereby increasing the distance between you and the radiation.
Shielding--Like distance, the more heavy, dense materials between you and the source of the radiation, the better. This is why local officials could advise you to remain indoors if an accident occurs. In some cases, the walls in your home or workplace would be sufficient shielding to protect you for a short period of time.
What you can do to stay informed:
Attend public information meetings. You may also want to attend post-exercise meetings that include the media and the public.
Contact local emergency management officials, who can provide information about radioactivity, safety precautions, and state, local, industry and federal plans.
Ask about the hazards radiation may pose to your family, especially with respect to young children, pregnant women and the elderly.
Ask where nuclear power plants are located.
Learn your community's warning systems.
Learn emergency plans for schools, day care centers, nursing homes--anywhere family members might be.
Be familiar with emergency information materials that are regularly disseminated to your home (via brochures, the phone book, calendars, utility bills, etc.) These materials contain educational information on radiation, instructions for evacuation and sheltering, special arrangements for the handicapped, contacts for additional information, etc
Safety of Nuclear Power Plants
Safety is taken very seriously by those working in nuclear power plants. The main safety concern is the emission of uncontrolled radiation into the environment which could cause harm to humans both at the reactor site and off-site. A summary by the nuclear world association on environmental, health and safety issues can be found at the Nuclear World Association website.
The Union of Concerned Scientists has an extensive website devoted to the detailed safety issues faced by American Nuclear Power Industry. These provide an interesting perspective on the importance both of a vigilent safety culture and a pro-active regulatory oversight.
Safety Mechanisms of a Nuclear Power Reactor
By regulation, the design of the nuclear reactor must include provisions for human (operator) error and equipment failure. Nuclear Plants in the western world use a "Defense in Depth" concept which is a system with multiple safety components, each with back-up and design to accommodate human error. The components include:
1. Control of Radioactivity
This requires being able to control the neutron flux. Recall that in a nuclear reactor when a neutron is captured by a fuel nucleus (generally uranium) the nucleus splits releasing radioactive particles (or undergoes fission). Hence if we decrease the neutron flux we decrease the radioactivity. The most common way to reduce the neutron flux is include neutron-absorbing control rods. These control rods can be partially inserted into the reactor core to reduce the reactions. The control rods are very important because the reaction could run out of control if fission events are extremely frequent. In modern nuclear power plants, the insertion of all the control rods into the reactor core occurs in a few seconds, thus halting the nuclear reaction as rapidly as possible. In addition, most reactors are designed so that beyond optimal level, as the temperature increases the efficiency of reactions decreases, hence fewer neutrons are able to cause fission and the reactor slows down automatically.
2. Maintenance of Core Cooling
In any nuclear reactor some sort of cooling is necessary. Generally nuclear reactors use water as a coolant. However some reactors which cannot use water use sodium or sodium salts.
3. Maintenance of barriers that prevent the release of radiation
There is a series of physical barriers between the radioactive core and the environment. For instance at the Darling Nuclear Generation Station in Canada the reactors are enclosed in heavily reinforced concrete which is 1.8m thick. Workers are shielded from radiation via interior concrete walls. A vacuum building is connected to the reactor buildings by a pressure relief duct. The vacuum building is a 71m high concrete structure and is kept at negative atmospheric pressure. This means that if any radiation were to leak from the reactor it would be sucked into the vacuum building and therefore prevented from being released into the
The design of the reactor also includes multiple back-up components, independent systems (two or more systems performing the same function in parallel), monitoring of instrumentation and the prevention of a failure of one type of equipment affecting any other.
Further, regulation requires that a core-meltdown incident must be confined only to the plant itself without the need to evacuate nearby residence.
Safety is also important for the workers of nuclear power plants. Radiation doses are controlled via the following procedures,
The handling of equipment via remote in the core of the reactor Physical shielding Limit on the time a worker spends in areas with significant radiation levels Monitoring of individual doses and of the work environment
Current nuclear power plants are powered by nuclear fission, with the science of nuclear fusion emerging. The key parts to a nuclear power plant (fission) reactor are:
Fuel rods - Fuel rods are usually composed of fissionable isotopes such as 235U, 233U and 239Pu. Any isotope present in critical mass will do. In the U.S. enriched tri-uranium oct-oxide, U3O 8, pellets are placed in a Zirconium alloyed tube where they are lowered into its core.
Control rods - Control rods, normally composed of 10B or Cd, absorb neutrons. Neutrons are absorbed so as to prevent the reaction from going at an unsafe rate (i.e. meltdown).
Moderators - It is job the of the moderator to slow down neutrons without absorbing them. Moderators must slow down neutrons without absorbing or reacting with them. D2O (deuterium oxide--heavy water), H2O, and graphi te are common moderators. If neutrons were permitted to continue uninhibited a chain-reaction would occur causing a meltdown of the facility.
Shielding/Containment - Several products are produced by nuclear reactions, most of which are lethal to humans. The storage and disposal of these materials is an enormous task. Should the facilities where the materials are being held be com promised, that area must be sealed for hundreds to millions of years, until the level of radioactivity has dropped to suitable levels. Inside the reactor several layers of concrete and steel must be laid to prevent radiation from escaping. The steel mus t be replaced periodically be cause exposure to radiation causes it to warp.
Coolant - The job of the coolant is to carry the heat from the reactor to a steam turbine system where it is converted to electricity, as well as keeping the reactor cool enough to prevent a meltdown.
How Nuclear Reactors Work ... And the Dangers When They Don't
How does a nuclear reactor work?
The core of a nuclear reactor contains both water and fuel rods made of zirconium and pellets of nuclear fuel, such as uranium, that set off a controlled nuclear reaction. The reaction, heats the water, creating 550-degree Fahrenheit steam, which powers a turbine, generating electricity.
What is a meltdown?
If the core gets too hot, the fuel rods can crack and release radioactive gases. In the worst case, the fuel pellets themselves can melt and fall to the reactor floor, where the hot, radioactive material may be able to eat through protective barriers and ultimately reach the surrounding environment.
In a partial meltdown, only some of the fuel or the reactor core melts, reducing the likelihood of breaching the containment structure.
WSJ's Yukimo Ono reports from Tokyo on a third explosion at Japan's Fukushima Daiichi nuclear power plant, along with continued rescue efforts in the wake of last week's earthquake.
Keep track of reactor incidents at the Fukushima Daiichi nuclear plant, 170 miles northeast of Tokyo.
What went wrong in Japan?
Nuclear reactors in Japan are designed to turn off automatically anytime a disaster knocks out the electric grid. That system worked properly in this case, shutting down the nuclear reaction.
Even with the plant shutdown, though, the nuclear fuel still held tremendous heat. Diesel-powered backup generators are meant to pump water into the plant to cool the fuel, but those systems failed in the tsunami that followed the quake. Emergency batteries provided some power, but not enough to run the water pumps.
The health of the badly damaged nuclear plant in Japan is deteriorating by the hour. Video courtesy of Reuters
What happens when the water pumps fail?
Without power to fuel the pumps, plant operators couldn't circulate water through the reactors to cool them down. The fuel rods began to boil off the remaining water, allowing water levels to drop and leaving the fuel at least partially exposed. That allowed temperatures to rise dangerously.
What caused the explosions at Reactors 1 and 3?
Details are unclear, but as the fuel rods begin to break down they can release gases that react with surrounding steam, generating hydrogen and allowing pressure inside the core to rise dangerously.
To prevent more serious damage, the plant's operators decided to release some of the pressure from the core by venting the built-up gas and steam. The escaping hydrogen reacted with oxygen in the atmosphere, causing an explosion that damaged nearby structures at Reactors 1 and 3. It is unclear what other systems were damaged, but officials say the main containment structures around the reactor cores—a key safety barrier—remained intact.
What is the situation at the different reactors now?
Plant operators are trying to pump seawater into all three reactors to cool the fuel. That will cause irreversible damage, but, it's hoped, will stabilize the plants. Workers are struggling to pump in water, however, for reasons that remain unclear.
At units 1 and 3, fuel rods were left exposed for long enough to allow at least some melting, experts believe. But workers have managed to restore water levels for the time being, and temperatures appear to be falling.
Unit 2 initially appeared to be in better shape than the other two reactors. But on Monday, water levels dropped, leaving fuel rods almost entirely exposed. Officials now consider Reactor 2 to be the most vulnerable of the three reactors to further damage, but no explosion had taken place as of early Tuesday morning.
What is the risk to the surrounding area?
Workers have released built-up gases to ease pressure inside the plant. That has released at least some radioactive material into the atmosphere, but officials say it isn't enough to be dangerous to anyone outside the immediate vicinity of the plant.
The worst-case scenario is a full meltdown, in which radioactive material eats through the various protective barriers and reaches the outside. Many experts consider that unlikely as long as workers can pump seawater into the plant and so long as containment structures remain intact.