• Nuclear Reactions

• Decisions about nuclear energy require some understanding of nuclear reactions and the nature of radioactivity. This is one of the three units of Palo Verde Nuclear Generating Station in Arizona. With all three units running, enough power is generated to meet the electrical needs of nearly 4 million people.

• Natural Radioactivity

• Introduction– Henri Becquerel discovered radioactivity in 1896

• Becquerel named the emission of invisible radiation from uranium ore radioactivity.

• Radioactive materials was the name given to materials that gave off this invisible radiation.

• Radioactivity was discovered by Henri Becquerel when he exposed a light-tight photographic plate to a radioactive mineral, then developed the plate. (A) A photographic film is exposed to an uranite ore sample. (B) The film, developed normally after a four-day exposure to uranite. Becquerel found an image like this one and deduced that the mineral gave off invisible radiation that he called radioactivity.

– Ernest Rutherford later discovered that there were three kinds of radioactivity.• Alpha particles () is a helium nucleus (2 protons

and 2 neutrons)• A beta particle () is a high energy electron• A gamma ray () is electromagnetic radiation with a

very short wavelength.

• Radiation passing through a magnetic field shows that massive, positively charged alpha particles are deflected one way, and less massive beta particles with their negative charge are greatly deflected in the opposite direction. Gamma rays, like light, are not deflected.

– Radioactivity is the spontaneous emission of particles or energy from an atomic nucleus as it disintegrates.

– Radioactive decay is the spontaneous disintegration of decomposition of a nucleus.

• Nuclear Equations– The two subatomic particles that occur in the nucleus, the

proton and the neutron, are called nucleons.• The number of protons is the atomic number which

determines the identity of the element.• The number of protons and neutrons determines the

atomic mass of the element.• Different isotopes of an element have the same atomic

number (same number of protons) but different atomic masses (different number of neutrons)

• The three isotopes of hydrogen have the same number of protons but different numbers of neutrons. Hydrogen-1 is the most common isotope. Hydrogen-2, with an additional neutron, is named deuterium, and hydrogen-3 is called tritium. Neutrons and protons are called nucleons because they are in the nucleus.

– Just like any other chemical reaction, we use symbols to show a nuclear reaction• As an example, when uranium 238 emits an

alpha particle, it loses 2 protons and 2 neutrons.

– Nuclear reactions must balance just like any other chemical reaction, but we must also be aware of balancing protons and neutrons

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• The Nature of the Nucleus.– Protons and neutrons are held together by a nuclear force

when they are very close together.– The shell model of the nucleus visualizes the protons and

neutrons moving on energy levels or shells, much like the electrons move in shells.

– We can predict the stability of a nucleus by using some simple rules• All isotopes heavier than atomic number 83 have an

unstable nucleus• Isotopes with 2, 8, 20, 28, 50, 82, or 126 protons or

neutrons in their nucleus occur in the most stable isotopes.

• Nuclei are the most stable with pairs of protons and neutrons, so those with all protons and all neutrons paired up are the most stable.

• Isotopes with an atomic number less that 83 are most stable when the ratio of protons to neutrons is 1:1.

• The dots indicate stable nuclei, which group in a band of stability according to their neutron-to-proton ratio. As the size of nuclei increases, so does the neutron-to-proton ratio that represents stability. Nuclei outside this band of stability are radioactive.

• Type of Radioactive Decay– Alpha emission

• This is the expulsion of an alpha particle– Beta Emission

• Emission of a beta particle• a beta particle is an electron that is ejected form the

nucleus– Gamma emission

• This is a high energy burst of electromagnetic radiation– Emission occurs as nuclei try to obtain a balance between

nuclear attractions, electromagnetic repulsions, and a low quantum of nuclear shell energy.

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• Unstable nuclei undergo different types of radioactive decay to obtain a more stable nucleus. The type of decay depends, in general, on the neutron-to-proton ratio, as shown.

• Radioactive Decay Series– Radioactive decay produces a simpler and more stable

nucleus.– A radioactive decay series occurs as a nucleus

disintegrates and achieves a more stable nuclei– There are 3 naturally occurring radioactive decay series.

• Thorium 232 ending in lead 208• Uranium 235 ending in lead 207• Uranium 238 ending in lead 206

• The radioactive decay series for uranium-238. This is one of three naturally occurring series.

– When there are a large number of nuclei the ration of the rate of nuclear decay per unit time to the total number of radioactive nuclei will be a constant• k=rate• n• The radioactive decay constant is specific for each

isotope

– The rate of radioactive decay is expressed in terms of half-life• The half-life of an element is the time required from

one-half of its unstable nuclei to decay• The half-life of an element is related to the ration of

0.693 to its radioactive decay constant– t ½ = 0.693/k

• The decay constant for U238 is 4.87 X 10-18/s• The half life is therefore

– t ½ = 0.693/4.87 X 10-18/s = 1.42 X 1017s = 4.5 X 109 years

• The half-life of U238 is 4.5 billion years.

• Radioactive decay of a hypothetical isotope with a half-life of one day. The sample decays each day by one-half to some other element. Actual half-lives may be in seconds, minutes, or any time unit up to billions of years.

• The half-life of each step in the uranium-238

radioactive decay series.

• Measurement of Radiation

• Measurement Methods– Film badges

• Workers who are exposed to radioactivity carry film badges

• The film is exposed and the optical density of the film shows the workers exposure levels during the time the film badge was worn.

– Ionization counter.• Measure ions that are produced by radiation

– Scintillation counter.• Measures the flashes of light that occur when radiation

strikes a phosphor.– Geiger counter

• Measures pulses of electrons released from the ionization of gas molecules in a metal cylinder

• Each pulse of electrons is heard as a pop or click

• This is a beta-gamma probe, which can measure beta and gamma radiation in millirems per unit of time.

• The working parts of a Geiger counter.

• Radiation Units– Curie (Ci)

• Measurement of the activity of a radioactive source.• Measured as the number of nuclear disintegrations per

unit of time• A curie is 3.70 X 1010 nuclear disintegrations per

second.

– Rad• Measures the amount of energy released by radiation

striking living tissue• Short for radiation absorbed dose• One rad releases 1 X 10-2 J/kg

– Rem• Short for roetgen equivalent man• This takes into account the possible biological damage

to humans of certain types of radiation.

• Radiation Exposure– Background radiation is constantly present in our

environment.• Most people are exposed to between 100 to 500

millirems per year.• This background radiation comes from many natural

source.– The harm that radiation does to living organisms is due to

the fact that it produces ionization which can:• Disrupt chemical bonds in biological macromolecules

such as DNA• Produce molecular fragments which can interfere with

enzyme action and essential cell functions.

– The linear model of exposure proposes that any exposure above zero is damaging and can produce cancer and genetic damage, mostly through its effect on DNA

– The threshold model proposes that there is a threshold limit of exposure up to which the human body can repair damage caused by the exposure• It is not until we reach and exceed this threshold that

we begin to see irreversible damage.

• Graphic representation of the (A) threshold model and (B) linear model of low-level radiation exposure. The threshold model proposes that the human body can repair damage up to a threshold. The linear model proposes that any radiation exposure is damaging.

• Nuclear Energy

• Introduction.– Albert Einstein showed us that energy and matter are the

same thing, both are inter-convertible.• E=mc2

– Using mass losses during nuclear reactions, one can calculate the energy change of a system.E=mc2

– There is a difference between the mass of the individual nucleons that make up a nucleus and the actual mass of the nucleus.

• This is called the mass defect of the nucleus.• The mass defect occurs as energy is released when nucleons

join to form a nucleus.

– The energy that is released is called the binding energy.• This is also the energy that is required to break the

nucleus into its individual protons and neutrons.• The ratio of the binding energy to the nucleon number

is a measure of a nucleus’ stability• Massive nuclei can gain stability by breaking into

smaller nuclei with a release of energy.• Smaller nuclei can gain stability by joining together

with the release of energy.

• The maximum binding energy per nucleon occurs around mass number 56, then decreases in both directions. As one result, fission of massive nuclei and fusion of less massive nuclei both release energy.

– Splitting massive nuclei apart with the release of energy is called nuclear fission.

– The joining together of less massive nuclei with the release of energy is called nuclear fusion

• Nuclear Fission– As a nuclear reaction occurs, it has the ability to produce

a chain reaction• A chain reaction is a reaction where the products are

able to produce more products in a self-sustaining reaction series.

– In order to achieve a chain reaction there must be:• A sufficient mass.• A large concentration of fissionable nuclei

– The critical mass is when the mass and concentration are high enough to sustain a chain reaction.

– A sub-critical mass is one that is too small to achieve a chain reaction.

• The fission reaction occurring when a neutron is absorbed by a uranium-235 nucleus. The deformed nucleus splits any number of ways into lighter nuclei, releasing neutrons in the process.

• A schematic representation of a chain reaction. Each fissioned nucleus releases neutrons, which move out to fission other nuclei. The number of neutrons can increase quickly with each series.

• Nuclear Power Plants– The nuclear reactor contains the material and is the vessel

for the controlled chain reaction of fissionable materials that will release the energy.

– Usually there is a fissionable enriched material made of 3% U235 and 97% U238 the is fabricated into small beads.

– The beads are enclosed in a fuel rod.– The fuel rods are locked in a fuel rod assembly by

locking collars and arranged so that pressurized water can flow around the rods.

– Control rods are made of material that can absorb neutrons and are inserted between the fuel rods.

– The rate of the chain reaction is controlled by raising or lowering the control rod.

– In a pressurized water reactor the energy is carried way from the reactor by water in a closed pipe called a primary loop.• This water acts as a coolant and also as a moderator to

slow neutrons so they can more readily be absorbed by the U235 nuclei.

– The water from the primary loop is circulated to a heat exchanger called a steam generator.• The water in the heat exchanger moves through hundreds

of small loops and heats the feedwater in the steam generator.

• The water then returns to the core to be heated again.

• A schematic representation of the basic parts of a nuclear reactor. The largest commercial nuclear power plant reactors are nine- to eleven-inch-thick steel vessels with a stainless steel liner, standing about 40 feet high with a diameter of 16 feet. Such a reactor has four pumps, which move 440,000 gallons of water per minute through the primary loop.

• (A)These are uranium oxide fuel pellets that are stacked inside fuel rods, which are then locked together in a fuel rod assembly. (B) A fuel rod assembly.

• A schematic general system diagram of a pressurized water nuclear power plant, not to scale. The containment building is designed to withstand an internal temperature of 300OF at a pressure of 60 lbs/in2 and still maintain its leak-tight integrity.

• Spent fuel rod assemblies are removed and new ones are added to a reactor head during refueling. This shows an initial fuel load to a reactor, which has the upper part removed and set aside for the loading.

• The turbine deck of a nuclear generating station. There is one large generator in line with four steam turbines in this non-nuclear part of the plant. The large silver tanks are separators that remove water from the steam after it has left the high-pressure turbine and before it is recycled back into the low-pressure turbines.

• The composition of the nuclear fuel in a fuel rod (A) before and (B) after use over a three-year period in a nuclear reactor.

• Nuclear Fusion– Nuclear fusion is the source of the energy from the Sun

and other stars.– Fusion is a very desirable energy source as:

• Two isotopes of hydrogen (deuterium and tritium) undergo fusion at a relatively low temperature.

• The supply of deuterium is unlimited with seawater being a very large source

• Enormous amounts of energy are released with no radioactive byproducts.

– The problems with utilizing fusion as an energy source are:• Temperature.

– The amount of energy required to bring two nuclei together is enormous.

• Density– The density of the reacting hydrogen nuclei must

be significantly high so that there are enough reactions occurring in a short period of time.

• time– These nuclei need to be confined to up to a second

or more at 10 atmospheres of pressure in order for enough reactions to take place.

• A fusion reaction between a tritium nucleus and a deuterium nucleus requires a certain temperature, density, and time of containment to take place.

– Plasma.• A very hot gas consisting of atoms that have been

stripped of their electrons and utilized as a confining mechanism

– Inertial confinement• An attempt to heat and compress small frozen pellets

of deuterium and tritium with energetic laser beams or particle beams, producing fusion.

• The Source of Nuclear Energy– When elements undergo the natural radioactive decay

process, energy is released and the decay products have less energy than the reacting nucleus.• When massive nuclei undergo fission, a great deal of

energy is Released