In case the reactor overheats and boils the water away, the chain reaction terminates, because water is needed to thermalize the neutrons. Control rods adjust neutron flux so that criticality is obtained, but not exceeded. A pressurized water reactor is cleverly designed to control the fission of large amounts of 235U, while using the heat produced in the fission reaction to create steam for generating electrical energy. Figure 4 shows a schematic of a reactor design, called the pressurized water reactor.įigure 4. Water is very effective, since neutrons collide with protons in water molecules and lose energy. To make a self-sustained fission reactor with 235U, it is thus necessary to slow down (“thermalize”) the neutrons. Another interesting characteristic of 235U 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. Since 235U has less mass than 238U, its UF 6 molecules have higher average velocity at the same temperature and diffuse faster. The most common separation method is gaseous diffusion of uranium hexafluoride (UF 6) through membranes. Most fission reactors utilize 235U, which is separated from 238U at some expense. This is followed by Kazakhstan and Canada. Australia has the largest deposits of uranium in the world, standing at 28% of the total. The isotope 235U is only 0.72 % of natural uranium, while 238U is 99.27%, and 239 Pu does not exist in nature. Thus, 235U and 239 Pu are superior fission fuels. This extra energy produces greater deformation, making fission more likely. About 2-MeV more energy is deposited in the resulting nucleus than would be the case if the number of neutrons was already even. 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. This is about 10 times the energy per fusion reaction, and about 100 times the energy of the average α, β, or γ decay. Thus, if a heavy nucleus splits in half, then about 1 MeV per nucleon, or approximately 240 MeV per fission, is released. The graph in Figure 2 shows BE/ A to be about 7.6 MeV/nucleon for the heaviest nuclei ( A about 240), while BE/ A is about 8.6 MeV/nucleon for nuclei having A about 120. The amount of energy per fission reaction can be large, even by nuclear standards. Figure 2 shows that BE/ A is greater for medium-mass nuclei than heavy nuclei, implying that when a heavy nucleus is split, the products have less mass per nucleon, so that mass is destroyed and energy is released in the reaction. As noted in Fusion, energy is released if the products of a nuclear reaction have a greater binding energy per nucleon (BE/ A) than the parent nuclei. (credit: Kalmthouts)įission is the opposite of fusion and releases energy only when heavy nuclei are split. The reactor is in the small domed building to the left of the towers. The cooling towers are the most prominent features but are not unique to nuclear power. About 16% of the world’s electrical power is generated by controlled nuclear fission in such plants. The people living near this nuclear power plant have no measurable exposure to radiation that is traceable to the plant. China is building nuclear power plants at the rate of one start every month.įigure 1. France provides over 75% of its electricity with nuclear power, while the US has 104 operating reactors providing 20% of its electricity. By the end of 2009, there were 442 reactors operating in 30 countries, providing 15% of the world’s electricity. Whereas nuclear power was of little interest for decades following TMI and Chernobyl (and now Fukushima Daiichi), growing concerns over global warming has brought nuclear power back on the table as a viable energy alternative. Hundreds of nuclear fission power plants around the world attest to the fact that controlled fission is practical and, at least in the short term, economical, as seen in Figure 1. Controlled fission is a reality, whereas controlled fusion is a hope for the future. Nuclear fission is a reaction in which a nucleus is split (or fissured). Describe controlled and uncontrolled chain reactions.Discuss how fission fuel reacts and describe what it produces.By the end of this section, you will be able to:
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