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The reason 235 U size 12{ {} rSup { size 8{"235"} } U} {} and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} are easier to fission than 238 U size 12{ {} rSup { size 8{"238"} } U} {} is that the nuclear force is more attractive for an even number of neutrons in a nucleus than for an odd number. Consider that 92 235 U 143 size 12{"" lSub { size 8{"92"} } lSup { size 8{"235"} } U rSub { size 8{"143"} } } {} has 143 neutrons, and 94 239 P 145 size 12{"" lSub { size 8{"94"} } lSup { size 8{"239"} } P rSub { size 8{"145"} } } {} has 145 neutrons, whereas 92 238 U 146 size 12{"" lSub { size 8{"92"} } lSup { size 8{"238"} } U rSub { size 8{"146"} } } {} has 146. When a neutron encounters a nucleus with an odd number of neutrons, the nuclear force is more attractive, because the additional neutron will make the number even. About 2-MeV more energy is deposited in the resulting nucleus than would be the case if the number of neutrons was already even. This extra energy produces greater deformation, making fission more likely. Thus, 235 U size 12{ {} rSup { size 8{"235"} } U} {} and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} are superior fission fuels. The isotope 235 U size 12{ {} rSup { size 8{"235"} } U} {} is only 0.72 % of natural uranium, while 238 U size 12{ {} rSup { size 8{"238"} } U} {} is 99.27%, and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} does not exist in nature. Australia has the largest deposits of uranium in the world, standing at 28% of the total. This is followed by Kazakhstan and Canada. The US has only 3% of global reserves.

Most fission reactors utilize 235 U size 12{ {} rSup { size 8{"235"} } U} {} , which is separated from 238 U size 12{ {} rSup { size 8{"238"} } U} {} at some expense. This is called enrichment. The most common separation method is gaseous diffusion of uranium hexafluoride ( UF 6 size 12{"UF" rSub { size 8{6} } } {} ) through membranes. Since 235 U size 12{ {} rSup { size 8{"235"} } U} {} has less mass than 238 U size 12{ {} rSup { size 8{"238"} } U} {} , its UF 6 size 12{"UF" rSub { size 8{6} } } {} molecules have higher average velocity at the same temperature and diffuse faster. Another interesting characteristic of 235 U size 12{ {} rSup { size 8{"235"} } U} {} is that it preferentially absorbs very slow moving neutrons (with energies a fraction of an eV), whereas fission reactions produce fast neutrons with energies in the order of an MeV. To make a self-sustained fission reactor with 235 U size 12{ {} rSup { size 8{"235"} } U} {} , it is thus necessary to slow down (“thermalize”) the neutrons. Water is very effective, since neutrons collide with protons in water molecules and lose energy. [link] shows a schematic of a reactor design, called the pressurized water reactor.

The figure shows a close-shielded vessel containing fuel rod and control rods along with a moderator in one chamber from which heat is taken out to the other chamber to change water to steam. Next, the steam is taken out from the vessel to run a turbine, and then it is condensed and sent back to the closed vessel.
A pressurized water reactor is cleverly designed to control the fission of large amounts of 235 U size 12{ {} rSup { size 8{"235"} } U} {} , while using the heat produced in the fission reaction to create steam for generating electrical energy. Control rods adjust neutron flux so that criticality is obtained, but not exceeded. In case the reactor overheats and boils the water away, the chain reaction terminates, because water is needed to thermalize the neutrons. This inherent safety feature can be overwhelmed in extreme circumstances.

Control rods containing nuclides that very strongly absorb neutrons are used to adjust neutron flux. To produce large power, reactors contain hundreds to thousands of critical masses, and the chain reaction easily becomes self-sustaining, a condition called criticality    . Neutron flux should be carefully regulated to avoid an exponential increase in fissions, a condition called supercriticality    . Control rods help prevent overheating, perhaps even a meltdown or explosive disassembly. The water that is used to thermalize neutrons, necessary to get them to induce fission in 235 U size 12{ {} rSup { size 8{"235"} } U} {} , and achieve criticality, provides a negative feedback for temperature increases. In case the reactor overheats and boils the water to steam or is breached, the absence of water kills the chain reaction. Considerable heat, however, can still be generated by the reactor’s radioactive fission products. Other safety features, thus, need to be incorporated in the event of a loss of coolant accident, including auxiliary cooling water and pumps.

Practice Key Terms 9

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Source:  OpenStax, Physics for the modern world. OpenStax CNX. Sep 16, 2015 Download for free at http://legacy.cnx.org/content/col11865/1.3
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