Nuclear fission power

Commercial nuclear power plants and most forms of nuclear weaponry depend on nuclear fission for their operation. The first nuclear fission reaction to be discovered was that of uranium-235. This nucleus, as well as those of uranium-233 and plutonium-239, undergoes fission when struck by a slow-moving neutron (V FIGURE 21.13).  

This is just one of many fission patterns, in this reaction, 3.5 X 10 J of energy is produced per nucieus. 

A heavy nucleus can split in many ways. Two ways that the uraniuni-235 nucleus splits, for instance, are 

More than 200 isotopes of 35 elements have been found among the fission products of uranium-235. Most of them are radioactive. 

Slow-moving neutrons are required in fission because the process involves initial absorption of the neutron by the nucleus. The resulting more massive nucleus is often unstable and spontaneously undergoes fission. Fast neutrons tend to bounce off the nucleus, and little fission occurs. 

The nuclei produced in equations 21.24 and 21.25—called the fission products—are themselves radioactive and undergo further nuclear decay. More than 200 isotopes of 35 elements have been foimd among the fission products of uranium-235. Most of them are radioactive. 

For a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass. Otherwise, neutrons escape from the sample before they have the opportunity to strike other nuclei and cause additional fission. The amoimt of fissionable material large enough to maintain a chain reaction with a constant rate of fission is called the critical mass. When a critical mass of material is present, one neutron on average from each fission is subsequently effective in producing another fission and 

What is the relationship between the sum of the mass numbers on the two sides of this reaction  

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The fact that neutrons can be absorbed by nuclei without overcoming a threshold (1 = 0 or s-wave reactions) makes neutrons extremely effective nuclear reactants. Neutron-induced reactions are the energy source for present-day commercial nuclear power (fission reactors) while charged-particle-induced reactions remain under study as power sources (fusion reactors). In this chapter we will consider the general features of nuclear fission reactors, following by the general features... 

NUCLEAR POWER FISSION We explore nuclear fission, in which a heavy nucleus splits to form two or more product nuclei. Nuclear fission is the energy source for nuclear power plants, and we look at the operating principles of these plants. 

Large Scale Production of Hydrogen from Nuclear Power—Fission and Fusion... 

Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world s nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. The various nuclear applications of plutonium are well known. 238Pu has been used in the Apollo lunar missions to power seismic and other equipment on the lunar surface. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals. 

Del y for Dec y. Nuclear power plants generate radioactive xenon and krypton as products of the fission reactions. Although these products ate trapped inside the fuel elements, portions can leak out into the coolant (through fuel cladding defects) and can be released to the atmosphere with other gases through an air ejector at the main condenser. 

Krypton and Xenon from Huclear Power Plants. Both xenon and krypton are products of the fission of uranium and plutonium. These gases are present in the spent fuel rods from nuclear power plants in the ratio 1 Kr 4 Xe. Recovered krypton contains ca 6% of the radioactive isotope Kr-85, with a 10.7 year half-life, but all radioactive xenon isotopes have short half-Hves. 

Spent fuel can be stored or disposed of intact, in a once-through mode of operation, practiced by the U.S. commercial nuclear power industry. Alternatively, spent fuel can be reprocessed, ie, treated to separate the uranium, plutonium, and fission products, for re-use of the fuels (see Nuclear REACTORS, CHEMICAL reprocessing). In the United States reprocessing is carried out only for fuel from naval reactors. In the nuclear programs of some other countries, especially France and Japan, reprocessing is routine. 

Spent Fuel Treatment. Spent fuel assembhes from nuclear power reactors are highly radioactive because they contain fission products. Relatively few options are available for the treatment of spent fuel. The tubes and the fuel matrix provide considerable containment against attack and release of nucHdes. To minimi2e the volume of spent fuel that must be shipped or disposed of, consoHdation of rods in assembhes into compact bundles of fuel rods has been successfully tested. Alternatively, intact assembhes can be encased in metal containers. 

During the operation of nuclear power reactors, which are fuelled with ceramic UO2 fuel rods, the fission of the nuclei leads to die formation of fission products which are isotopes of elements in all of tire Groups of the Periodic Table. The major fission products, present in 1-10% abundance, fall into five groups divided according to the chemical interaction of each product with the fuel ... 

Nuclear power reactors cause the transmutation of chemicals (uranium and plutonium) to fission products using neutrons as the catalyst to produce heat. Fossil furnaces use the chemical reaction of carbon and oxygen to produce CO2 and other wastes to produce heat. There is only one reaction and one purpose for nuclear power reactors there is one reaction but many puiposes for fossil-burning furnaces there are myriad chemical processes and purposes. 

Nuclear power production involves bringing fissionable material together to react nuclearly, removing the heat, converting the heat to steam to drive a turbogenerator. and managing the wastes. 

Nevertheless, WASH-740 indicated the worst-case hazard of nuclear power, and the need for detailed PSAs of nuclear power plants. Since plants proposed for construction are sufficiently like the hypothetical plant in terms of fission products subsequent scoping analyses are not needed before the detailed PSA. 

The nucleus of an atom consists of protons and neutrons that are bound together by a nuclear force. Neutrons and protons are rearranged in a nuclear reaction in a manner somewhat akin to rearrang ing atoms in a chemical reaction. The nuclear reaction liberating energy in a nuclear power plant is called nuclear fission. The word fission is derived from fissure, which means a crack or a separation. A nucleus is separated (fissioned) into two major parts by bombardment with a neutron. 

The fear of accidents like Chernobyl, and the high cost of nuclear waste disposal, halted nuclear power plant construction in the United States m the 1980s, and in most ol the rest ol the world by the 1990s. Because nuclear fusion does not present the waste disposal problem of fission reactors, there is hope that fusion will be the primary energy source late in the twenty-first centuiy as the supplies of natural gas and petroleum dwindle. 

Fermi began to assemble a nuclear pile in a squash court under the football stands at the University of Chicago. This was really the first nuclear power reactor, in which a controlled, self-sustaining series of fission processes occurred. The controls consisted of cadmium rods inserted to absorb neutrons and keep the reactor from going... 

The energy from nuclear fission is released mainly as kinetic energy of the new, smaller nuclei and neutrons that are produced. This kinetic energy is essentially heat, which is used to boil water to generate steam that turns turbines to drive electrical generators. In a nuclear power plant, the electrical generation area is essentially the same as in a plant that burns fossil fuels to boil the water. 

A nuclear power plant generates electricity in a manner similar to a fossil fuel plant. The fundamental difference is the source of heat to create the steam that turns the turbine-generator. A fossil plant relies on the combustion of natural resources (coal, oil) to create steam. A nuclear reactor creates steam with the heat produced from a controlled chain reaction of nuclear fission (the splitting of atoms). 

Uranium is used as the primai-y source of nuclear energy in a nuclear reactor, although one-third to one-half of the power will be produced from plutonium before the power plant is refueled. Plutonium is created during the uranium fission cycle, and after being created will also fission, contributing heat to make steam in the nuclear power plant. These two nuclear fuels are discussed separately in order to explore their similarities and differences. Mixed oxide fuel, a combination of uranium and recovered plutonium, also has limited application in nuclear fuel, and will be briefly discussed. 

Plutonium-239 is a fissile element, and vvill split into fragments when struck by a neutron in the nuclear reactor. This makes Pu-239 similar to U-235, able to produce heat and sustain a controlled nuclear reaction inside the nuclear reactor. Nuclear power plants derive over one-third of their power output from the fission of Pu-239. Most of the uranium inside nuclear fuel is U-238. Only a small fraction is the fissile U-235. Over the life cycle of the nuclear fuel, the U-238 changes into Pu-239, which continues to provide nuclear energy to generate electricity. 

The main drawback to nuclear power is the production of radioactive waste. Spent fuel from a nuclear reactor is considered a high-level radioactive waste, and remains radioactive for a veiy long time. Spent fuel consists of fission products from the U-235 and Pu-239 fission process, and also from unspent U-238, Pu-240, and other heavy metals produced during the fuel cycle. That is why special programs exist for the handling and disposal of nuclear waste. 

HLW comprises most of the radioactivity associated with nuclear waste. Because that designation can cover radioactive waste from more than one source, the term spent nuclear fuel (SNF) will be used to discuss HLW originating from commercial nuclear reactors. LLW comprises nearly 90 percent of the volume of nuclear waste but little of the radioactivity. Nuclear power reactors produce SNF and most of the nation s LLW, although there are approximately 20,000 different sources of LLW. The name SNF is a bit of a misnomer because it implies that there is no useful material left in the fuel, when in fact some fissionable material is left in it. 

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