NATIONAL INSTITUTE OF TECHNLOGY, CALICUT

Nuclear Reactors Energy Technology

Himanshu C. Reddy B070319EC

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Table of Contents Introduction ............................................................................................................................................. 4 

Nuclear Power ..................................................................................................................................... 5 

History ................................................................................................................................................ 5 

Nuclear Fusion ........................................................................................................................................ 7 

Production Methods ............................................................................................................................ 8 

Muon-catalyzed fusion .................................................................................................................... 8 

Generally cold, locally hot fusion ................................................................................................... 8 

Hot fusion........................................................................................................................................ 8 

Nuclear Fission ..................................................................................................................................... 10 

Mechanics ......................................................................................................................................... 10 

Energytics ......................................................................................................................................... 11 

Product Nuclei and Binding Energy ................................................................................................. 12 

Chain Reaction .................................................................................................................................. 12 

Nuclear Reactors ................................................................................................................................... 14 

How It Works .................................................................................................................................... 14 

Fission ........................................................................................................................................... 14 

Heat Generation ............................................................................................................................ 15 

Cooling .......................................................................................................................................... 15 

Reactivity Control ......................................................................................................................... 15 

Electrical Power Generation ......................................................................................................... 16 

Components in a Nuclear Reactor .................................................................................................... 16 

Nuclear Fuel .................................................................................................................................. 16 

Nuclear Reactor Core .................................................................................................................... 16 

Neutron Moderator ........................................................................................................................ 17 

Neutron Poison .............................................................................................................................. 17 

Coolant .......................................................................................................................................... 17 

Control Rods ................................................................................................................................. 18 

Reactor Vessel .............................................................................................................................. 18 

Boiler Feed-Water Pump .............................................................................................................. 18 

Steam Generator ............................................................................................................................ 18 

Steam Turbine ............................................................................................................................... 19 

Electrical Generator ...................................................................................................................... 19 

Surface Condensers ....................................................................................................................... 19 

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Cooling Tower .............................................................................................................................. 20 

Spent Fuel Pool ............................................................................................................................. 20 

Containment Building ................................................................................................................... 20 

Control Room................................................................................................................................ 21 

Classification of Reactor Types ........................................................................................................ 22 

Classification by Nuclear Reaction type ....................................................................................... 22 

Classification by Moderator Material ........................................................................................... 23 

Classification by Coolant .............................................................................................................. 23 

Classification by Generation ......................................................................................................... 25 

Classification by Phase of Fuel ..................................................................................................... 25 

Classification by Use .................................................................................................................... 25 

Current Technologies ........................................................................................................................ 26 

Future and Developing Technologies ............................................................................................... 30 

Advanced Reactors ....................................................................................................................... 30 

Generation IV Reactors ................................................................................................................. 31 

Generation V Reactors .................................................................................................................. 32 

Fusion Reactors ............................................................................................................................. 32 

Nuclear Fuel Cycle ........................................................................................................................... 33 

Fuelling of Nuclear Reactors ........................................................................................................ 34 

Nuclear Power in India ......................................................................................................................... 35 

Nuclear Power Plants in India ........................................................................................................... 36 

Advantages and Disadvantages of Nuclear Energy .............................................................................. 39 

Advantages ........................................................................................................................................ 39 

Disadvantages ................................................................................................................................... 39 

Conclusion ............................................................................................................................................ 40 

References ............................................................................................................................................ 41 

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Introduction

In nuclear physics and nuclear chemistry, a nuclear reaction is the process in which two nuclei or nuclear particles collide to produce products different from the initial particles. In principle, a reaction can involve more than three particles colliding, but because the proba-bility of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare. While the transformation is spontaneous in the case of radioactive decay, it is initiated by a particle in the case of a nuclear reaction. If the particles collide and separate without changing, the process is called an elastic colli-sion rather than a reaction.

In the symbolic figure shown to the right, 63Li and deuterium (21H) react to form the

highly excited intermediate nucleus 84Be which then decays immediately into two alpha par-ticles. Protons are symbolically represented by red spheres, and neutrons by blue spheres.

63Li + 2

1H → 42He + ?

To balance the equation above, the second nucleus to the right must have atomic number 2 and mass number 4; it is therefore also helium-4. The complete equation:

63Li + 2

1H → 42He + 4

2He

Or more simply:

63Li + 21H → 2 42He

While the number of possible nuclear reactions is immense, there are several types which are more common, or otherwise notable. Some examples include:

Fusion reactions — two light nuclei join to form a heavier one, with additional parti-cles (usually protons or neutrons) thrown off to conserve momentum.

Fission reactions — a very heavy nucleus, spontaneously or after absorbing addition-al light particles (usually neutrons), splits into two or sometimes three pieces. (α de-cay is not usually called fission.)

Spallation — a nucleus is hit by a particle with sufficient energy and momentum to knock out several small fragments or, smash it into many fragments.

Induced gamma emission belongs to a class in which only photons were involved in creating and destroying states of nuclear excitation.

Out of the above mentioned types of major nuclear reaction the Fusion reactions and the Fis-sion reactions are more prominent.

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Nuclear Power

Nuclear power is produced by controlled (i.e., non-explosive) nuclear reactions. Commercial and utility plants currently use nuclear fission reactions to heat water to produce steam, which is then used to generate electricity.

Both fission and fusion appear promising for space propulsion applications, generat-ing higher mission velocities with less reaction mass. This is due to the much higher energy density of nuclear reactions: some 7 orders of magnitude (10,000,000 times) more energetic than the chemical reactions which power the current generation of rockets.

Radioactive decay has been used on a relatively small (few kW) scale, mostly to pow-er space missions and experiments.

History

The pursuit of nuclear energy for electricity generation began soon after the discovery in the early 20th century that radioactive elements, such as radium, released immense amounts of energy, according to the principle of mass–energy equivalence. However, means of harnessing such energy was impractical, because intensely radioactive elements were, by their very nature, short-lived (high energy release is correlated with short half-lives). Howev-er, the dream of harnessing "atomic energy" was quite strong; even it was dismissed by such fathers of nuclear physics like Ernest Rutherford as "moonshine." This situation, however, changed in the late 1930s, with the discovery of nuclear fission.

In 1932, James Chadwick discovered the neutron, which was immediately recognized as a potential tool for nuclear experimentation because of its lack of an electric charge. Ex-perimentation with bombardment of materials with neutrons led Frédéric and Irène Joliot-Curie to discover induced radioactivity in 1934, which allowed the creation of radium-like elements at much less the price of natural radium. Further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Exper-iments bombarding uranium with neutrons led Fermi to believe he had created a new, trans-uranic element, which he dubbed Hesperium.

But in 1938, German chemists Otto Hahn and Fritz Strassmann, along with Austrian physicist Lise Meitner and Meitner's nephew, Otto Robert Frisch, conducted experiments with the products of neutron-bombarded uranium, as a means of further investigating Fermi's claims. They determined that the relatively tiny neutron split the nucleus of the massive ura-nium atoms into two roughly equal pieces, contradicting Fermi. This was an extremely sur-prising result: all other forms of nuclear decay involved only small changes to the mass of the nucleus, whereas this process—dubbed "fission" as a reference to biology—involved a com-plete rupture of the nucleus. Numerous scientists, including Leo Szilard, who was one of the

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first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. Once this was experimentally confirmed and announced by Frédéric Joliot-Curie in 1939, scientists in many countries (including the United States, the United Kingdom, France, Germany, and the Soviet Union) petitioned their governments for support of nuclear fission research, just on the cusp of World War II.

In the United States, where Fermi and Szilard had both emigrated, this led to the crea-tion of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2, 1942. This work became part of the Manhattan Project, which built large reac-tors at the Hanford Site (formerly the town of Hanford, Washington) to breed plutonium for use in the first nuclear weapons, which were used on the cities of Hiroshima and Nagasaki. A parallel uranium enrichment effort also was pursued.

After World War II, the prospects of using "atomic energy" for good, rather than simply for war, were greatly advocated as a reason not to keep all nuclear research controlled by military organizations. However, most scientists agreed that civilian nuclear power would take at least a decade to master, and the fact that nuclear reactors also produced weapons-usable plutonium created a situation in which most national governments (such as those in the United States, the United Kingdom, Canada, and the USSR) attempted to keep reactor re-search under strict government control and classification. In the United States, reactor re-search was conducted by the U.S. Atomic Energy Commission, primarily at Oak Ridge, Ten-nessee, Hanford Site, and Argonne National Laboratory.

Work in the United States, United Kingdom, Canada, and USSR proceeded over the course of the late 1940s and early 1950s. Electricity was generated for the first time by a nu-clear reactor on December 20, 1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW. Work was also strongly researched in the US on nu-clear marine propulsion, with a test reactor being developed by 1953. (Eventually, the USS Nautilus, the first nuclear-powered submarine, would launch in 1955.) In 1953, US President Dwight Eisenhower gave his "Atoms for Peace" speech at the United Nations, emphasizing the need to develop "peaceful" uses of nuclear power quickly. This was followed by the 1954 Amendments to the Atomic Energy Act which allowed rapid declassification of U.S. reactor technology and encouraged development by the private sector.

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Nuclear Fusion

In nuclear physics and nuclear chemistry, nuclear fusion is the process by which mul-tiple atomic nuclei join together to form a single heavier nucleus. It is accompanied by the release or absorption of large quantities of energy. Large scale fusion processes, involving many atoms fusing at once, must occur in matter at very high densities.

The fusion of two nuclei with lower mass than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy while the fusion of nuclei heav-ier than iron absorbs energy; vice-versa for the reverse process, nuclear fission. In the sim-plest case of hydrogen fusion, two protons have to be brought close enough for the weak force to convert either of the identical protons into a neutron forming deuterium. In more complex cases of heavy ion fusion involving many nucleons, the reaction mechanism is dif-ferent, but we achieve the same result of assembling larger nuclei from smaller nuclei.

Fusion reactions power the stars and produce virtually all elements in a process called nucleo-synthesis. Although the fusion of lighter elements in stars releases energy, production of elements heavier than iron absorbs energy. When the fusion reaction is a sustained uncon-trolled chain, it can result in a thermonuclear explosion, such as that generated by a hydrogen bomb. Non-self sustaining reactions can still release considerable energy, as well as large numbers of neutrons.

It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. This is because all nuclei have a positive charge (due to their protons), and as like charges repel, nuclei strongly resist being put too close together. Accelerated to high speeds (that is, heated to thermonuclear temperatures), they can overcome this electromagnetic re-pulsion and get close enough for the attractive nuclear force to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and a free neu-tron, generally releases more energy than it takes to force the nuclei together; this is an exo-thermic process that can produce self-sustaining reactions. The National Ignition Facility, which uses laser-driven inertial confinement fusion, is thought to be capable of break-even fusion. The first large-scale laser target experiments were performed in June 2009 and igni-tion experiments will begin in 2010.

The energy released in most nuclear reactions is much larger than that in chemical reac-tions, because the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17 MeV released in the deuterium–tritium (D–T) reaction shown in the diagram to the right. Fusion reactions have an energy density many times greater than nuclear fission; the reactions produce far greater energies per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones, which are themselves millions of times more en-ergetic than chemical reactions. Only direct conversion of mass into energy, such as that

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caused by the collision of matter and antimatter, is more energetic per unit of mass than nu-clear fusion.

Production Methods

A variety of methods are known to effect nuclear fusion. Some are "cold" in the strict sense that no part of the material is hot (except for the reaction products), some are "cold" in the limited sense that the bulk of the material is at a relatively low temperature and pressure but the reactants are not, and some are "hot" fusion methods that create macroscopic regions of very high temperature and pressure.

Muon-catalyzed fusion Muon-catalyzed fusion is a well-established and reproducible fusion process that oc-

curs at ordinary temperatures. It was studied in detail by Steven Jones in the early 1980s. It has not been reported to produce net energy. Net energy production from this reaction cannot occur because of the energy required to create muons, their 2.2 μs half-life, and the chance that a muon will bind to the new alpha particle and thus stop catalyzing fusion.

Generally cold, locally hot fusion Accelerator-based light-ion fusion is a technique using particle accelerators to achieve

particle kinetic energies sufficient to induce light-ion fusion reactions. Accelerating light ions is relatively easy, and can be done in an efficient manner—all it takes is a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kV between electrodes. The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb in-teraction cross sections. Therefore the vast majority of ions end up expending their energy on bremsstrahlung and ionization of atoms of the target. Devices referred to as sealed-tube neu-tron generators are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement that allows ions of these nuclei to be accelerated against hydride targets, also containing deuterium and triti-um, where fusion takes place. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves. Despite periodic reports in the popular press by scientists claiming to have invented "tabletop" fusion machines, neutron generators have been around for half a century. The sizes of these devices vary but the smallest instruments are often packaged in sizes smaller than a loaf of bread. These devices do not produce a net power output.

Hot fusion In hot fusion, the fuel reaches tremendous temperature and pressure inside a fusion

reactor or nuclear weapon (or star).

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The methods in the second group are examples of non-equilibrium systems, in which very high temperatures and pressures are produced in a relatively small region adjacent to material of much lower temperature. In his doctoral thesis for MIT, Todd Rider did a theoret-ical study of all quasi-neutral, isotropic, non-equilibrium fusion systems. He demonstrated that all such systems will leak energy at a rapid rate due to bremsstrahlung produced when electrons in the plasma hit other electrons or ions at a cooler temperature and suddenly decel-erate. The problem is not as pronounced in hot plasma because the range of temperatures, and thus the magnitude of the deceleration, is much lower. Note that Rider's work does not apply to non-neutral and/or anisotropic non-equilibrium plasmas.

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Nuclear Fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), as well. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radia-tion and as kinetic energy of the fragments (heating the bulk material where fission takes place). For fission to produce energy, the total binding energy of the resulting elements has to be lower than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom.

Nuclear fission produces energy for nuclear power and to drive the explosion of nu-clear weapons. Both uses are made possible because certain substances called nuclear fuels undergo fission when struck by free neutrons and in turn generate neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.

The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fis-sion a very tempting source of energy. The products of nuclear fission, however, are on aver-age far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power.

Mechanics

Nuclear fission can occur without neutron bombardment as a type of radioactive de-cay. This type of fission (called spontaneous fission) is rare except in a few heavy isotopes. In engineered nuclear devices, essentially all nuclear fission occurs as a "nuclear reaction" — a bombardment-driven process that results from the collision of two subatomic particles. In nu-clear reactions, a subatomic particle collides with an atomic nucleus and causes changes to it. Nuclear reactions are thus driven by the mechanics of bombardment, not by the relatively constant exponential decay and half-life characteristic of spontaneous radioactive processes.

A great amount of nuclear reactions are known. Nuclear fission differs importantly from other types of nuclear reactions in that it can be amplified and sometimes controlled via a nuclear chain reaction. In such a reaction, free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions.

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The chemical element isotopes that can sustain a fission chain reaction are called nu-clear fuels, and are said to be fissile. The most common nuclear fuels are 235U (the isotope of uranium with an atomic mass of 235 and of use in nuclear reactors) and 239Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centring near 95 and 135 u (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha/beta decay chain over periods of millennia to eons. In a nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with an-other particle, a neutron, which is itself produced by prior fission events.

Energytics

Typical fission events release about two hundred million eV (200 MeV) of energy for each fission event. By contrast, most chemical oxidation reactions (such as burning coal or TNT) release at most a few eV per event, so nuclear fuel contains at least ten million times more usable energy per unit mass than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic ra-diation in the form of gamma rays; in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working flu-id, usually water or occasionally heavy water.

When a uranium nucleus fissions into two daughter nuclei fragments, an energy of ~200 MeV is released. For uranium-235 (total mean fission energy 202.5 MeV), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 neutrons are emitted with a kinetic energy of ~2 MeV each (total of 4.8 MeV). The fission reaction also releases ~7 MeV in prompt gamma ray photons. The latter figure means that a nuclear explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its ener-gy as fast neutrons, and the rest as kinetic energy of fission fragments ("heat"). In an atomic bomb, this heat may serve to raise the temperature of the bomb core to 100 million kelvin and cause secondary emission of soft X-rays, which convert some of this energy to ionizing radia-tion. However, in nuclear reactors, the fission fragment kinetic energy remains as low-temperature heat which causes little or no ionization.

The total prompt fission energy amounts to about 181 MeV or ~ 89% of the total en-ergy. The remaining ~11% is released in beta decays which have various half-lives, but begin as a process in the fission products immediately; and in delayed gamma emissions associated with these beta decays. For example, in uranium-235 this delayed energy is divided into about 6.5 MeV in betas, 8.8 MeV in antineutrinos (released at the same time as the betas), and finally, an additional 6.3 MeV in delayed gamma emission from the excited beta-decay products (for a mean total of ~10 gamma ray emissions per fission, in all).

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The 8.8 MeV/202.5 MeV = 4.3% of the energy which is released as antineutrinos is not captured by the reactor material as heat, and escapes directly through all materials (in-cluding the Earth) at nearly the speed of light, and into interplanetary space. Almost all of the remaining radiation is converted to heat, either in the reactor core or it’s shielding.

Product Nuclei and Binding Energy

In fission there is a preference to yield fragments with even proton numbers, which is called the odd-even effect on the fragments charge distribution. However, no odd-even effect is observed on fragment mass number distribution. This result is attributed to nucleon pair breaking.

In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120; the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u. Unequal fissions are energetically more favourable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei (another statement of this, is that the atomic binding energy curve is slightly steeper to the left of mass 120 u than to the right of it).

Chain Reaction

Several heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission, a form of radioactive decay and induced fission, a form of nuclear reac-tion. Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable; isotopes that undergo fission when struck by a thermal, slow moving neutron are also called fissile. A few particularly fissile and readily obtainable isotopes (notably 235U and 239Pu) are called nuclear fuels because they can sustain a chain reaction and can be ob-tained in large enough quantities to be useful.

All fissionable and fissile isotopes undergo a small amount of spontaneous fission which releases a few free neutrons into any sample of nuclear fuel. Such neutrons would es-cape rapidly from the fuel and become a free neutron, with a mean lifetime of about 15 minutes before decaying to protons and beta particles. However, neutrons almost invariably impact and are absorbed by other nuclei in the vicinity long before this happens (newly-created fission neutrons move at about 7% of the speed of light, and even moderated neutrons move at about 8 times the speed of sound). Some neutrons will impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough nuclear fuel is assembled in one

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place, or if the escaping neutrons are sufficiently contained, then these freshly generated neu-trons outnumber the neutrons that escape from the assembly, and a sustained nuclear chain reaction will take place.

An assembly that supports a sustained nuclear chain reaction is called a critical as-sembly or, if the assembly is almost entirely made of a nuclear fuel, a critical mass. The word "critical" refers to a cusp in the behaviour of the differential equation that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by radioactive decay, but if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual mass of a critical mass of nuclear fuel depends strongly on the geometry and surrounding materi-als.

Not all fissionable isotopes can sustain a chain reaction. For example, 238U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But too few of the neu-trons produced by 238U fission are energetic enough to induce further fissions in 238U, so no chain reaction is possible with this isotope. Instead, bombarding 238U with slow neutrons causes it to absorb them (becoming 239U) and decay by beta emission to 239Np which then decays again by the same process to 239Pu; that process is used to manufacture 239Pu in breeder reactors. In-situ plutonium production also contributes to the neutron chain reaction in other types of reactors after sufficient plutonium-239 has been produced, since plutonium-239 is also a fissile element which serves as fuel. It is estimated that up to half of the power produced by a standard "non-breeder" reactor is produced by the fission of plutonium-239 produced in place, over the total life-cycle of a fuel load.

Fissionable, non-fissile isotopes can be used as fission energy source even without a chain reaction. Bombarding 238U with fast neutrons induces fissions, releasing energy as long as the external neutron source is present. This is an important effect in all reactors where fast neutrons from the fissile isotope can cause the fission of nearby 238U nuclei, which means that some small part of the 238U is "burned-up" in all nuclear fuels, especially in fast breeder reactors that operate with higher-energy neutrons. That same fast-fission effect is used to augment the energy released by modern thermonuclear weapons, by jacketing the weapon with 238U to react with neutrons released by nuclear fusion at the centre of the de-vice.

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Nuclear Reactors

A nuclear reactor is a device to initiate, and control, a sustained nuclear chain reac-tion. The most common use of nuclear reactors is for the generation of electrical power and for the power in some ships. This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines. There are also other less common uses as discussed below.

How It Works

Just as conventional power stations generate electricity by harnessing the thermal en-ergy released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission.

Fission When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs

a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission products. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.

The reaction can be controlled by using neutron poisons, which absorb excess neu-trons; and neutron moderators which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons, which are more likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reac-tor.

Commonly used moderators include regular (light) water (75% of the world's reac-tors) solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

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Heat Generation The reactor core generates heat in a number of ways:

The kinetic energy of fission products is converted to thermal energy when these nu-clei collide with nearby atoms.

Some of the gamma rays produced during fission are absorbed by the reactor, their energy being converted to heat.

Heat produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shutdown.

A kilogram of uranium-235 (U-235) converted via nuclear processes contains approx-imately three million times the energy of a kilogram of coal burned conventionally (7.2 × 1013 Joules per kilogram of uranium-235 versus 2.4 × 107 Joules per kilogram of coal).

Cooling A nuclear reactor coolant — usually water but sometimes a gas or a liquid metal

or molten salt — is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for example the boiling water reactor.

Reactivity Control The power output of the reactor is controlled by controlling how many neutrons are

able to create more fissions.

Control rods that are made of a nuclear poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.

In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose en-ergy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the cool-ant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.

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In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors power output can be increased by heating the cool-ant, which makes it a less dense poison. Nuclear reactors generally have automatic and man-ual systems to insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.

Electrical Power Generation The energy released in the fission process generates heat, some of which can be con-

verted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that generates electricity.

Components in a Nuclear Reactor

Nuclear Fuel Nuclear fuel is a material that can be consumed to derive nuclear energy, by analogy

to chemical fuel that is burned for energy. Nuclear fuels are the densest sources of energy available. Nuclear fuel in a nuclear fuel cycle can refer to the fuel itself, or to physical objects (for example bundles composed of fuel rods) composed of the fuel material, mixed with structural, neutron moderating, or neutron reflecting materials.

Most nuclear fuels contain heavy fissile elements that can be made to undergo a nuclear fission reaction in a nuclear reactor. The most common fissile nuclear fuels are 235U and 239Pu. The actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle.

Not all nuclear fuels are used in fission reactors. Plutonium-238 and some other ele-ments are used to produce small amounts of nuclear power by radioactive de-cay in radioisotope thermoelectric generators and other atomic. Light nuclides such as 3H (tritium) are used as fuel for nuclear fusion.

Nuclear Reactor Core The nuclear reactor core (also referred to as the "reactor core” or the "core") is the re-

gion within a nuclear reactor where the nuclear fuel assemblies are located and the nuclear reaction consequently takes place.

Water-Moderated Reactors

Graphite Moderated Reactors

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Neutron Moderator In nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involvinguranium-235.

Commonly used moderators include regular (light) water (roughly 75% of the world's reac-tors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Neutron Poison A neutron poison (also called a 'neutron absorber' or a 'nuclear poison') is a substance

with a large neutron absorption cross-section in applications, such as nuclear reactors, when absorbing neutrons is an undesirable effect. However neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reac-tivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation, while others remain relatively constant.

The capture of neutrons by short-halftime fission products is known as reactor poi-soning; neutron capture by long-lived or stable fission products is called reactor slagging.

Coolant A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor promoting corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

While the term coolant is commonly used in automotive, residential and commercial temperature-control applications, in industrial processing, heat transfer fluid is one technical term more often used, in high temperature as well as low temperature manufacturing applica-tions.

The coolant can either keep its phase and stay liquid or gaseous, or can undergo a phase change, with the latent heat adding to the cooling efficiency. The latter, when used to achieve low temperatures, is more commonly known as refrigerant.

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Control Rods A control rod is a rod made of chemical elements capable of absorbing many neu-

trons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Because these elements have different capture cross sec-tions for neutrons of varying energies, the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control. Light water reactors (BWR, PWR) and heavy water reactors (HWR) operate with "thermal" neutrons, whereas breeder reactors operate with "fast" neutrons.

Reactor Vessel In a nuclear power plant, the reactor vessel is a pressure vessel containing

the coolant and reactor core. It is a device for containing and controlling a chemical reaction. The chemical process enables the conversion of raw material into a final product under given pressure and temperature. During the reaction it becomes necessary to remove excess heat in the process to order to keep the process under control. Vessels are built to withstand high pressure in the system.

Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to contain the coolant.

Boiler Feed-Water Pump A boiler feed-water pump is a specific type of pump used to pump feed water into

a steam boiler. The water may be freshly supplied or returning condensate produced as a re-sult of the condensation of the steam produced by the boiler. These pumps are normally high pressure units that take suction from a condensate return system and can be of the centrifugal pump type or positive displacement type.

Steam Generator Steam generators are heat exchangers used to convert water into steam from heat

produced in a nuclear reactor core. They are used in pressurized between the primary and sec-ondary coolant loops.

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Steam Turbine A steam turbine is a mechanical device that extracts thermal energy from pressur-

ized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884.

It has almost completely replaced the reciprocating piston steam engine primarily be-cause of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic effi-ciency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.

Electrical Generator In electricity generation, an electric generator is a device that converts mechanical

energy to electrical energy. The reverse conversion of electrical energy into mechanical ener-gy is done by a motor; motors and generators have many similarities. A generator forc-es electrons in the windings to flow through the external electrical. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water fall-ing through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.

Surface Condensers Surface condenser is the commonly used term for a water-cooled shell and tube heat

exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however significantly more ex-pensive and cannot achieve as low a steam turbine exhaust pressure as a water cooled surface condenser.

Surface condensers are also used in applications and industries other than the con-densing of steam turbine exhaust in power plants.

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Cooling Tower Cooling towers are heat removal devices used to transfer process waste heat to

the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power stations and build-ing cooling. The towers vary in size from small roof-top units to very large hyperboloid struc-tures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectan-gular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site. They are often associated with nuclear power plants in popular culture.

Spent Fuel Pool Spent fuel pools (SFP) are storage pools for spent fuel from nuclear reactors. Typi-

cally 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from the reactor. These fuel pools are specially designed at the reactor in which the fuel was used and situated at the reactor site. In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years before being sent for reprocessing or dry cask storage. The water cools the fuel and provides shielding from radiation.

While only about 8 feet of water is needed to keep radiation levels below acceptable levels, the extra depth provides a safety margin and allows fuel assemblies to be manipulated without special shielding to protect the operators.

Containment Building A containment building, in its most common usage, is a steel or reinforced con-

crete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi (410 to 1400 kPa). The containment is the final barrier to radioactive release (part of a nuclear reactor's defence in depth strategy), the first being the fuel ceramic itself, the second being the metal fuel clad-ding tubes, the third being the reactor vessel and coolant system.

The containment building itself is typically an airtight steel structure enclosing the re-actor normally sealed off from the outside atmosphere. The steel is either free-standing or attached to the concrete missile shield. In the United States, the design and thickness of the containment and the missile shield are governed by federal regulations (10 CFR 50.55a), and must be strong enough to withstand the impact of a fully loaded passenger airliner without rupture.

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Control Room A control room is a room serving as an operations centre where a facility or service

can be monitored and controlled. Examples include:

in television production, the master control room is an operations centre for a network or station, or the production control room of a television studio or colour

each recording studio typically has its own control room where the recording is actu-ally made;

a NASA flight controller works in a "Flight Control Room" in a Mission Control Cen-tre; affiliated facilities such as the Jet Propulsion Laboratory have their own control rooms;

Nuclear power plants and other power-generating stations, many oil refiner-ies and chemical plants have control rooms, sometimes also serving as an area of ref-uge;

Various military facilities, ranging in scale from a missile silo to NORAD, have con-trol rooms.

Call centres use a control room to monitor incoming and outgoing communications of customer service representatives and provide general oversight of the call centre.

Fire service control rooms (UK) see: FiRe Control for article about nine new regional control rooms to handle emergency calls in England.

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Classification of Reactor Types

Nuclear Reactors are classified by several methods; a brief outline of these classifica-tion schemes is provided.

Classification by Nuclear Reaction type Nuclear fission. All commercial power reactors are based on nuclear fission. They

generally use uranium and its product plutonium as nuclear fuel, though a thorium fuel cycle is also possible. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission chain reaction:

Thermal reactors use slowed or thermal neutrons. Almost all current reactors are of this type. These contain moderator materials that slow neutrons until their neutron temperature is thermalized, that is, until their kinetic ener-gy approaches the average kinetic energy of the surrounding particles. Ther-mal neutrons have a far higher cross section (probability) of fissioning the fissile nucleiuranium-235, plutonium-239, and plutonium-241, and a rela-tively lower probability of neutron capture by uranium-238 (U-238) compared to the faster neutrons that originally result from fission, allowing use of low-enriched uranium or even natural uranium fuel. The moderator is often also the coolant, usually water under high pressure to increase the boiling point.

Fast neutron reactors use fast neutrons to cause fission in their fuel. They do not have a neutron moderator, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched in fissile material (about 20% or more) due to the relatively lower probability of fission versus capture by U-238. Fast reactors have the potential to produce less trans-uranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reac-tors are less common than thermal reactors in most applications.

Nuclear fusion. Fusion power is an experimental technology, generally with hydrogen as fuel. While not suitable for power production, Farnsworth-Hirsch fusers are used to produce neutron radiation.

Radioactive decay. Examples include radioisotope thermoelectric generators as well as other types of atomic batteries, which generate heat and power by exploiting pas-sive radioactive decay.

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Classification by Moderator Material Graphite moderated reactors

Water moderated reactors

Heavy water reactors

Light water moderated reactors (LWRs). Light water reactors use ordinary wa-ter to moderate and cool the reactors. When at operating temperature, if the temperature of the water increases, its density drops, and fewer neutrons pass-ing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised than light water reactors. Due to the extra thermalization, these types can use natural uranium/un-enriched fuel.

Light element moderated reactors. These reactors are moderated by lithium or berylli-um.

Molten salt reactors (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.

Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator.

Organically moderated reactors (OMR) use biphenyl and terphenyl as moderator and coolant.

Classification by Coolant Water cooled reactor. There are 104 operating reactors in the United States. Of these,

69 are pressurized water reactors (PWR), and 35 are boiling water reactors (BWR).

Pressurized water reactor (PWR)

A primary characteristic of PWRs is a pressurizer, a special-ized pressure vessel. Most commercial PWRs and naval reactors use pressurizers. During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. During normal operation, the pressuriz-er is connected to the primary reactor pressure vessel (RPV) and the pressurizer "bubble" provides an expansion space for changes in water volume in the reactor. This arrangement also provides a means of pres-sure control for the reactor by increasing or decreasing the steam pres-sure in the pressurizer using the pressurizer heaters.

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Pressurised heavy water reactors are a subset of pressurized water reac-tors, sharing the use of a pressurized, isolated heat transport loop, but using heavy water as coolant and moderator for the greater neutron economies it offers.

Boiling water reactor (BWR)

BWRs are characterized by boiling water around the fuel rods in the lower portion of a primary reactor pressure vessel. A boiling water re-actor uses 235U, enriched as uranium dioxide, as its fuel. The fuel is assembled into rods that are submerged in water and housed in a steel vessel. The nuclear fission causes the water to boil, generating steam. This steam is pumped through pipes into turbines. The turbines are driven by the steam, and this process generates electricity. During normal operation, pressure control is accomplished by controlling the amount of steam flowing from the reactor pressure vessel to the tur-bine.

Pool-type reactor

Liquid metal cooled reactor. Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid metal coolants have included sodium, NaK, lead, lead-bismuth eutectic, and in early reactors, mercury.

Sodium-cooled fast reactor

Lead-cooled fast reactor

Gas cooled reactors are cooled by a circulating inert gas, often helium in high-temperature designs, while carbon dioxide has been used in past British and French nuclear power plants. Nitrogen has also been used. Utilization of the heat varies, de-pending on the reactor. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine.

Molten Salt Reactors (MSRs) are cooled by circulating a molten salt, typically a eu-tectic mixture of fluoride salts, such as FLiBe. In a typical MSR, the coolant is also used a matrix in which the fissile material is dissolved.

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Classification by Generation Generation I reactor

Generation II reactor (most current nuclear power plants)

Generation III reactor (evolutionary improvements of existing designs)

Generation IV reactor (technologies still under development)

Classification by Phase of Fuel Solid fuelled

Fluid fuelled

Aqueous homogeneous reactor

Molten salt reactor

Gas fuelled

Classification by Use Electricity

Nuclear power plants

Propulsion, see nuclear propulsion

Nuclear marine propulsion

Various proposed forms of rocket propulsion

Other uses of heat

Desalination

Heat for domestic and industrial heating

Hydrogen production for use in a hydrogen economy

Production reactors for transmutation of elements

Breeder reactors. Fast breeder reactors are capable of producing more fissile materials than they consume during the fission chain reaction (by convert-ing fertile U-238 to Pu-239) which allows an operational fast reactor to gener-ate more fissile material than it consumes. Thus, a breeder reactor, once run-ning, can be re-fuelled with natural or even depleted uranium.

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Creating various radioactive isotopes, such as americium for use in smoke de-tectors, and cobalt-60, molybdenum-99 and others, used for imaging and med-ical treatment.

Production of materials for nuclear weapons such as weapons-grade plutonium

Providing a source of neutron radiation (for example with the pulsed Godiva device) and positron radiation (e.g. neutron activation analysis and potassium-argon dating)

Research reactor: Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.

Current Technologies

There are two types of nuclear power in current use:

The Radioisotope thermoelectric generator produces heat through passive radioactive decay. Some radioisotope thermoelectric generators have been created to power space probes (for example, the Cassini probe), some lighthouses in the former Soviet Union, and some pacemakers. The heat output of these generators diminishes with time; the heat is converted to electricity utilising the thermoelectric effect.

Nuclear fission reactors produce heat through a controlled nuclear chain reaction in a critical mass of fissile material. All current nuclear power plants are critical fission reactors, which are the focus of this article. The output of fission reactors is controlla-ble. There are several subtypes of critical fission reactors, which can be classified as Generation I, Generation II and Generation III. All reactors will be compared to the Pressurized Water Reactor (PWR), as that is the standard modern reactor design.

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Pressurized Water Reactors (PWR)

These reactors use a pressure vessel to contain the nuclear fuel, control rods, modera-tor, and coolant. They are cooled and moderated by high pressure liquid water. The hot radio-active water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (non-radioactive) loop of water to steam that can run turbines. They are the majority of current reactors, and are generally considered the safest and most reliable tech-nology currently in large scale deployment. This is a thermal neutron reactor design, the new-est of which are the VVER-1200, Advanced Pressurized Water Reactor and the European Pressurized Reactor. United States Naval reactors are of this type.

Boiling Water Reactors (BWR)

A BWR is like a PWR without the steam generator. A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure, which allows the water to boil inside the pressure vessel producing the steam that runs the turbines. Unlike a PWR, there is no primary and secondary loop. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. This is a thermal neutron reac-tor design, the newest of which are the Advanced Boiling Water Reactor and the Economic Simplified Boiling Water Reactor.

Pressurized Heavy Water Reactor (PHWR)

A Canadian design (known as CANDU), these reactors are heavy-water-cooled and -moderated Pressurized-Water reactors. Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fuelled with nat-ural uranium and are thermal neutron reactor designs. PHWRs can be re-fuelled while at full power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWRs have been built in Cana-da, Argentina, China,India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea. India also operates a number of PHWRs, often termed 'CANDU-derivatives', built after the Gov-ernment of Canada halted nuclear dealings with India following the 1974 Smiling Bud-dha nuclear weapon test.

Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (RBMK)

A Soviet design, built to produce plutonium as well as power. RBMKs are water cooled with a graphite moderator. RBMKs are in some respects similar to CANDU in that they are re-fuel able during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are very unstable and large, mak-ing containment buildings for them expensive. A series of critical safety flaws have also been identified with the RBMK design, though some of these were corrected following the Cherno-

byl. Their main attraction is their use of light water and un-enriched uranium. As of 2010, 11 remain open, mostly due to safety improvements and help from international safety agencies

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such as the DOE. Despite these safety improvements, RBMK reactors are still considered one of the most dangerous reactor designs in use. RBMK reactors were deployed only in the for-mer Soviet.

Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)

These are generally graphite moderated and CO2 cooled. They can have a high ther-mal efficiency compared with PWRs due to higher operating temperatures. There are a num-ber of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. Older designs (i.e. Magnox stations) are either shut down or will be in the near future. However, the AGCRs have an anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design. Decommissioning costs can be high due to large volume of reactor core.

Liquid Metal Fast Breeder Reactor (LMFBR)

This is a reactor design that is cooled by liquid metal, totally un-moderated, and pro-duces more fuel than it consumes. They are said to "breed" fuel, because they produce fis-sionable fuel during operation because of neutron capture. These reactors can function much like a PWR in terms of efficiency, and do not require much high pressure containment, as the liquid metal does not need to be kept at high pressure, even at very high temperatures. BN-350 and BN-600 in USSR and Superphénix in France were a reactor of this type, as was Fermi-I in the United States. The Monju reactor in Japan suffered a sodium leak in 1995 and is pending restart earliest in February 2010. All of them use/used liquid sodium. These reactors are fast neutron, not thermal neutron designs. These reactors come in two types:

Lead cooled

Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a lead-bismuth eutectic mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead. The Russian Alfa class submarine uses a lead-bismuth-cooled fast reactor as its main power plant.

Sodium cooled

Most LMFBRs are of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually prevent corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions wouldn't be vastly more violent than (for example) a leak of superheated fluid from a SCWR or PWR. EBR-I, the first reactor to have a core meltdown, was of this type.

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Pebble Bed Reactors (PBR)

These use fuel moulded into ceramic balls, and then circulate gas through the balls. The result is an efficient, low-maintenance, very safe reactor with inexpensive, standardized fuel. The prototype was the AVR.

Molten Salt Reactors

These dissolve the fuels in fluoride salts, or use fluoride salts for coolant. These have many safety features, high efficiency and a high power density suitable for vehicles. Notably, they have no high pressures or flammable components in the core. The prototype was the MSRE, which also used Thorium's fuel cycle to produce 0.1% of the radioactive waste of standard reactors.

Aqueous Homogeneous Reactor (AHR)

These reactors use soluble nuclear salts dissolved in water and mixed with a coolant and a neutron moderator.

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Future and Developing Technologies

Advanced Reactors More than a dozen advanced reactor designs are in various stages of development.

Some are evolutionary from the PWR, BWR and PHWR designs above, some are more radi-cal departures. The former include the Advanced Boiling Water Reactor (ABWR), two of which are now operating with others under construction, and the planned passively safe ESBWR and AP1000 units (see Nuclear Power 2010 Program).

The Integral Fast Reactor (IFR) was built, tested and evaluated during the 1980s and then retired under the Clinton administration in the 1990s due to nuclear non-proliferation policies of the administration. Recycling spent fuel is the core of its de-sign and it therefore produces only a fraction of the waste of current reactors.

The Pebble Bed Reactor, a High Temperature Gas Cooled Reactor (HTGCR), is de-signed so high temperatures reduce power output by Doppler of the fuel's neutron cross-section. It uses ceramic fuels so its safe operating temperatures exceed the pow-er-reduction temperature range. Most designs are cooled by inert helium. Helium is not subject to steam explosions, resists neutron absorption leading to radioactivity, and does not dissolve contaminants that can become radioactive. Typical designs have more layers (up to 7) of passive containment than light water reactors (usually 3). A unique feature that may aid safety is that the fuel-balls actually form the core's mech-anism, and are replaced one-by-one as they age. The design of the fuel makes fuel re-processing expensive.

The Small Sealed Transportable Autonomous Reactor (SSTAR) is being primarily re-searched and developed in the US, intended as a fast breeder reactor that is passively safe and could be remotely shut down in case the suspicion arises that it is being tam-pered with.

The Clean And Environmentally Safe Advanced Reactor (CAESAR) is a nuclear re-actor concept that uses steam as a moderator — this design is still in development.

The Hydrogen Moderated Self-regulating Nuclear Power Module (HPM) is a reactor design emanating from the Los Alamos National Laboratory that uses uranium hy-dride as fuel.

Subcritical reactors are designed to be safer and more stable, but pose a number of engineering and economic difficulties. One example is the Energy amplifier.

Thorium based reactors. It is possible to convert Thorium-232 into U-233 in reactors specially designed for the purpose. In this way, thorium, which is more plentiful than uranium, can be used to breed U-233 nuclear fuel. U-233 is also believed to have fa-vourable nuclear properties as compared to traditionally used U-235, including better neutron economy and lower production of long lived trans-uranic waste.

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Advanced Heavy Water Reactor (AHWR) — A proposed heavy water moderated nu-clear power reactor that will be the next generation design of the PHWR type. Under development in the Bhabha Atomic Research Centre (BARC), India.

KAMINI — A unique reactor using Uranium-233 isotope for fuel. Built in India by BARC and Indira Gandhi Centre for Atomic Research (IGCAR).

India is also planning to build fast breeder reactors using the thorium - Uranium-233 fuel cycle. The FBTR (Fast Breeder Test Reactor) in operation at Kalpakkam (India) uses Plutonium as a fuel and liquid sodium as a coolant.

Generation IV Reactors Generation IV reactors are a set of theoretical nuclear reactor designs currently being

researched. These designs are generally not expected to be available for commercial con-struction before 2030. Current reactors in operation around the world are generally consid-ered second- or third-generation systems, with the first-generation systems having been re-tired some time ago. Research into these reactor types was officially started by the Genera-tion IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural re-source utilization, and to decrease the cost to build and run such plants.

Gas cooled fast reactor

Lead cooled fast reactor

Molten salt reactor

Sodium-cooled fast reactor

Supercritical water reactor

Very high temperature reactor

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Generation V Reactors Designs which are theoretically possible, but which are not being actively considered

or researched at present. Though such reactors could be built with current or near term tech-nology, they trigger little interest for reasons of economics, practicality, or safety.

Liquid Core reactor. A closed loop liquid core nuclear reactor, where the fissile mate-rial is molten uranium cooled by a working gas pumped in through holes in the base of the containment vessel.

Gas core reactor. A closed loop version of the nuclear light-bulb rocket, where the fis-sile material is gaseous uranium-hexafluoride contained in a fused silica vessel. A working gas (such as hydrogen) would flow around this vessel and absorb the UV light produced by the reaction. In theory, using UF6 as a working fuel directly (rather than as a stage to one, as is done now) would mean lower processing costs, and very small reactors. In practice, running a reactor at such high power densities would prob-ably produce unmanageable neutron flux.

Gas core EM reactor. As in the Gas Core reactor, but with photovoltaic arrays con-verting the UV light directly to electricity.

Fission fragment reactor

Fusion Reactors Controlled nuclear fusion could in principle be used in fusion power plants to produce

power without the complexities of handling actinides, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as yet none has 'produced' more thermal energy than electrical energy consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER project is currently leading the effort to commercialize fusion power.

The only fusion reactors that have been successful are the stars and the Hydrogen bomb.

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Nuclear Fuel Cycle

Thermal reactors generally depend on refined and enriched uranium. Some nuclear re-actors can operate with a mixture of plutonium and uranium (see MOX). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle.

Less than 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired into pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor.

Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, and some commercial reactors with a high neutron economy do not require the fuel to be en-riched at all (that is, they can use natural uranium). According to the International Atomic Energy Agency there are at least 100 research reactors in the world fuelled by highly en-riched (weapons-grade/90% enrichment uranium). Theft risk of this fuel (potentially used in the production of a nuclear weapon) has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium (which poses less threat of proliferation).

Fissile U-235 and non-fissile but fissionable and fertile U-238 are both used in the fis-sion process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A thermal neutron is one which is moving about the same speed as the atoms around it. Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron has the best opportunity to fis-sion U-235 when it is moving at this same vibration speed. On the other hand, U-238 is more likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239, which is another fuel. Pu-239 is a viable fuel and must be ac-counted for even when a highly enriched uranium fuel is used. Plutonium fissions will domi-nate the U-235 fissions in some reactors, especially after the initial loading of U-235 is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs.

Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a fast breeder reactor, some other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap un-enriched uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel.

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Fuelling of Nuclear Reactors The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms

of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refuelling outage times) is related to the amount of fissile uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent" and is discharged and replaced with new (fresh) fuel assemblies, although in practice it is the build-up of reaction poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the build up of long-lived neutron absorbing fission by products impedes the chain reaction. The fraction of the reactor's fuel core replaced during refuelling is typically one-fourth for a boiling-water reactor and one-third for a pres-surized-water reactor. The disposition and storage of this spent fuel is one of the most chal-lenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years.

Not all reactors need to be shut down for refuelling; for example, pebble bed reac-tors, RBMK reactors, molten salt reactors, Magnox, AGR and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows indi-vidual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its burn up, which is ex-pressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is com-monly expressed as megawatt days thermal per metric ton of initial heavy metal.

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Nuclear Power in India

Nuclear power is the fourth-largest source of electricity in India - af-ter thermal, hydro and renewable sources of electricity. As of 2010, India has 19 nuclear power plants in operation generating 4,560 MW while 4 other are under construction and are expected to generate an additional 2,720 MW. India is also involved in the development of fusion reactors through its participation in the ITER project.

Since early 1990s, Russia has been a major source of nuclear fuel to India. Due to dwindling domestic uranium reserves, electricity generation from nuclear power in India de-clined by 12.83% from 2006 to 2008. Following a waiver from the Nuclear Suppliers Group in September 2008 which allowed it to commence international nuclear trade, India has signed nuclear deals with several other countries including France, United States, United Kingdom, Canada, Namibia, Mongolia, Argentina, Kazakhstan. In February 2009, India also signed a $700 million deal with Russia for the supply of 2000 tons nuclear fuel.

India now envisages increasing the contribution of nuclear power to overall electricity generation capacity from 4.2% to 9% within 25 years. In 2010, India's installed nuclear pow-er generation capacity will increase to 6,000 MW. As of 2009, India stands9th in the world in terms of number of operational nuclear power reactors and is constructing 9 more, including two EPRs being constructed by France's Areva. Indigenous atomic reactors include TAPS-3, and -4, both of which are 540 MW reactors. India's $717 million fast breeder reactor project is expected to be operational by 2010.

India has already been using imported enriched uranium and is currently un-der International Atomic Energy Agency (IAEA) safeguards, but it has developed various aspects of the nuclear fuel cycle to support its reactors. Development of select technologies has been strongly affected by limited imports. Use of heavy water reactors has been particu-larly attractive for the nation because it allows Uranium to be burnt with little to no enrich-ment capabilities. India has also done a great amount of work in the development of a Thorium centred fuel cycle. While Uranium deposits in the nation are limited (see next par-agraph) there are much greater reserves of Thorium and it could provide hundreds of times the energy with the same mass of fuel. The fact that Thorium can theoretically be utilized in heavy water reactors has tied the development of the two. A prototype reactor that would burn Uranium-Plutonium fuel while irradiating a Thorium blanket is under construction at the Madras/Kalpakkam Atomic Power Station.

Uranium used for the weapons program has been separate from the power program, using Uranium from indigenous reserves. This domestic reserve of 80,000 to 112,000 tons of uranium (approx 1% of global uranium reserves) is large enough to supply all of India's commercial and military reactors as well as supply all the needs of India's nuclear weapons arsenal. Currently, India's nuclear power reactors consume, at most, 478 metric tonnes of uranium per year. Even if India were quadruple its nuclear power output (and reactor base) to 20GW by 2020, nuclear power generation would only consume 2000 metric tonnes of urani-

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um per annum. Based on India's known commercially viable reserves of 80,000 to 112,000 tons of uranium, this represents a 40 to 50 years uranium supply for India's nuclear power reactors (note with reprocessing and breeder reactor technology, this supply could be stretched out many times over). Furthermore, the uranium requirements of India's Nuclear Arsenal are only a fifteenth (1/15) of that required for power generation (approx. 32 tonnes), meaning that India's domestic fissile material supply is more than enough to meet all needs for it strategic nuclear arsenal. Therefore, India has sufficient uranium resources to meet its strategic and power requirements for the foreseeable future.

Nuclear Power Plants in India

Currently, nineteen nuclear power reactors produce 4,560.00 MW (2.9% of total installed base).

Power station Operator State Type Units Total capacity (MW)

Kaiga NPCIL Karnataka PHWR 220 x 3 660

Kakrapar NPCIL Gujarat PHWR 220 x 2 440

Kalpakkam NPCIL Tamil Nadu PHWR 220 x 2 440

Narora NPCIL Uttar Pradesh PHWR 220 x 2 440

Rawatbhata NPCIL Rajasthan PHWR

100 x 1200 x 1220 x 4

1180

Tarapur NPCIL Maharashtra BWR (PHWR)160 x 2540 x 2

1400

Total 19 4560

The projects under construction are:

Power station Operator State Type Units Total capacity (MW)

Kaiga NPCIL Karnataka PHWR 220 x 1 220

Kudankulam NPCIL Tamil Nadu VVER-1000 1000 x 2 2000

Kalpakkam NPCIL Tamil Nadu PFBR 500 x 1 500

Total 4 2720

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The planned projects are:

Power station Operator State Type Units Total capacity (MW)

Kakrapar NPCIL Gujarat PHWR 640 x 2 1280

Rawatbhata NPCIL Rajasthan PHWR 640 x 2 1280

Kudankulam NPCIL Tamil Nadu VVER-1200 1200 x 2 2400

Jaitapur NPCIL Maharashtra EPR 1600 x 4 6400

Kaiga NPCIL Karnataka PWR 1000 x 1, 1500 x 1 2500

Bhavini

PFBR 470 x 4 1880

NPCIL

AHWR 300 300

NTPC

PWR 1000 x 2 2000

NPCIL

PHWR 640 x 4 2560

Total 10 20600

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The following projects are firmly proposed.

Power sta-tion

Operator State Type Units Total capacity (MW)

Kudankulam NPCIL Tamil Nadu

VVER-1200

1200 x 2

2400

Jaitapur NPCIL Maharastra EPR

1600 x 2

3200

Pati Sonapur

Orissa PWR 6000

Kumaharia

Haryana PWR 2800

Saurashtra

Gujarat PWR

Pulivendula NPCIL 51%, AP Genco 49%

Andhra Pra-desh

PWR

2000 x 1

2000

Kovvada

Andhra Pra-desh

PWR

Haripur

West Bengal PWR

Total 15

The following projects are proposed and to be confirmed soon.

Power station Operator State Type Units Total capacity (MW)

Kudankulam NPCIL Tamil Nadu VVER-1200 1200 x 2 2400

Total 2 2400

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Advantages and Disadvantages of Nuclear Energy

Advantages Nuclear reactions release a million times more energy, as compared to hydro or wind

energy. Hence, a large amount of electricity can be generated. Presently, 12-18% of the world's electricity is generated through nuclear energy.

The biggest advantage of nuclear energy is that there is no release of greenhouse gas-es (carbon dioxide, methane, ozone, and chlorofluorocarbon) during nuclear reaction. The greenhouse gases are a major threat in the current scenario, as they cause global warming and climate change. As there is no emission of these gases during nuclear reaction, there is very little effect on the environment.

The burning of fossil fuels result in emission of the poisonous carbon dioxide. It is a menace to the environment as well as human life. There is no release of carbon d-oxide at the time of nuclear reaction.

Nuclear reactors make use of uranium as fuel. Fission reaction of a small amount of uranium generates large amount of energy. Currently, the high reserves of uranium found on Earth, are expected to last for another 100 years.

High amount of energy can be generated from a single nuclear power plant. Also, nu-clear fuel is inexpensive and easier to transport.

Disadvantages Nuclear energy can be used for production and proliferation of nuclear weapons. Nu-

clear weapons make use of fission, fusion or combination of both reactions for de-structive purposes. They are a major threat to the world as they can cause a large-scale devastation.

Though large amount of energy can be produced from a nuclear power plant, it re-quires large capital cost. Around 15-20 years are required to develop a single plant. Hence, it is not very feasible to build a nuclear power plant. The nuclear reactors will work only as long as uranium is available. Its extinction can again result in a grave problem.

The waste produced after fission reactions contains unstable elements and is high-ly radioactive. It is very dangerous to the environment as well as human health, and remains so, for thousands of years. It needs professional handling and should be kept isolated from the living environments. The radioactivity of these elements reduces over a period of time, after decaying. Hence, they have to be carefully stored. It is very difficult to store radioactive elements for a long period.

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Conclusion

Everyone is familiar with Hiroshima and Nagasaki. America dropped atomic bombs on these Japanese cities. They demonstrated the power and destructive capability of the atom. However, nuclear technology is not limited to use in war. It is a major source of energy. Nu-clear energy is produced by harnessing the power of the atom. The Sun and other stars are sustained by nuclear reactions. Manmade nuclear power is produced in nuclear reactors. Nu-clear fission is used to generate nuclear power. Scientists are currently working on ways of using nuclear fusion to generate nuclear power.

We can finally say that nuclear reactors can be an alternative for the non-renewable sources. When controlled this is an immense energy resource, and when it is out of control it is an unimaginable disaster. But still, there are proponents and opponents of nuclear energy. However, only the future will make it clear whether nuclear energy is a boon or a bane!

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References  

[1] Wikipedia ‐ http://en.wikipedia.org/wiki/Nuclear_energy 

[2] Wikipedia ‐ http://en.wikipedia.org/wiki/Nuclear_fusion 

[3] Wikipedia ‐ http://en.wikipedia.org/wiki/Nuclear_fusion 

[4] Wikipedia ‐ http://en.wikipedia.org/wiki/Nuclear_reactor_technology 

[5] Buzzle, Intelligent Life on Web ‐ http://www.buzzle.com/articles/advantages‐and‐

disadvantages‐of‐nuclear‐power.html