Nuclear fission plutonium fuel

Approximately 25—30% of a reactor s fuel is removed and replaced during plaimed refueling outages, which normally occur every 12 to 18 months. Spent fuel is highly radioactive because it contains by-products from nuclear fission created during reactor operation. A characteristic of these radioactive materials is that they gradually decay, losing their radioactive properties at a set rate. Each radioactive component has a different rate of decay known as its half-life, which is the time it takes for a material to lose half of its radioactivity. The radioactive components in spent nuclear fuel include cobalt-60 (5-yr half-Hfe), cesium-137 (30-yr half-Hfe), and plutonium-239 (24,400-yr half-Hfe). 

Not all of the Pu-239 will fission during the fuel cycle in a nuclear reactor. Some of the plutonium will not experience neutron bombardment sufficient to cause fission. Other plutonium atoms will absorb one or more neutrons and become higher numbered isotopes of plutonium, such as Pu-240, Pu-241, etc. Plutonium comprises just over 1 percent of nuclear reactor spent fuel—the fuel removed from the... 

Induced nuclear fission is fission caused by bombarding a heavy nucleus with neutrons (Fig. 17.23). The nucleus breaks into two fragments when struck by a projectile. Nuclei that can undergo induced fission are called fissionable. For most nuclei, fission takes place only if the impinging neutrons travel so rapidly that they can smash into the nucleus and drive it apart with the shock of impact uranium-238 undergoes fission in this way. Fissile nuclei, however, are nuclei that can be nudged into breaking apart even by slow neutrons. They include uranium-235, uranium-233, and plutonium-239—the fuels of nuclear power plants. 

Nuclear fission power plants were at one time thought to be the answer to diminishing fossil fuels. Although the enriched uranium fuel was also limited, an advanced nuclear reactor called breeders would be able to produce more radioactive fuel, in the form of plutonium, than consumed. This would make plutonium fuel renewable. Although plutonium has been called one of the most toxic elements known, it is similar to other radioactive materials and requires careful handling since it can remain radioactive for thousands of years. 

Fissile materials are defined as materials that are fissionable by nentrons with zero kinetic energy. In nuclear engineering, a fissile material is one that is capable of snstaining a chain reaction of nuclear fission Nuclear power reactors are mainly fueled with manium, the heaviest element that occurs in natnre in more than trace qnantities. The principal nuclear energy soiuces are maninm-235, plutonium-239, uranium-233 and thorium. 

The most common use of plutonium is as a fuel in nuclear reactors to produce electricity or as a source for the critical mass required to sustain a fission chain reaction to produce nuclear weapons. Plutonium also is used to convert nonfissionable uranium-238 into the isotope capable of sustaining a controlled nuclear chain reaction in nuclear power plants. It takes only 10 pounds of plutonium-239 to reach a critical mass and cause a nuclear explosion, as compared with about 33 pounds of fissionable, but scarce, uranium-235. 

Uranium-235 is the most important uranium isotope for nuclear fuel. Uranium-238, although not fissionable itself, can be converted into the fissionable plutonium-239 in a breeder reactor by the following nuclear reaction ... 

Deriving electrical energy from nuclear fission produces almost no atmospheric pollutants, such as carbon dioxide, sulfur oxides, nitrogen oxides, heavy metals, and airborne particulates. Although not discussed in the text, there is also an abundant supply of fuel for nuclear fission reactors in the form of plutonium-239, which can be manufactured from uranium-238. Use the keyword Breeder Reactor on your Internet search engine to learn about how this is so. 

In a typical fast breeder nuclear reactor, most of the fuel is 238U (90 to 93%). The remainder of the fuel is in the form of fissile isotopes, which sustain the fission process. The majority of these fissile isotopes are in the form of 239Pu and 241Pu, although a small portion of 235U can also be present. Because the fast breeder converts die fertile isotope 238 U into the fissile isotope 239Pu, no enrichment plant is necessary. The fast breeder serves as its own enrichment plant. The need for electricity for supplemental uses in the fuel cycle process is thus reduced. Several of the early hquid-metal-cooled fast reactors used plutonium fuels. The reactor Clementine, first operated in the Unired States in 1949. utilized plutonium metal, as did the BR-1 and BR.-2 reactors in the former Soviet Union in 1955 and 1956, respectively. The BR-5 in the former Soviet Union, put into operation in 1959. utilized plutonium oxide and carbide. The reactor Rapsodie first operated in France in 1967 utilized uranium and plutonium oxides. 

Atomic bomb— An explosive weapon which uses uranium-235 or plutonium as fuel. Its tremendous destructive power is produced by energy released from the "splitting of atoms" or nuclear fission. Also called A-bomb, atom bomb, or fission bomb. 

The concept of a nuclear fuel cycle is an old one, almost dating back to the concept of controlled nuclear fission to generate electricity. At the time of the development of the first nuclear power plants, it was generally taken for granted that fuel from power reactors would be reprocessed and that the recovered uranium and plutonium would be recycled. 

The production of plutonium fuel (plutonium-239) is a fascinating story. When nuclear reactors were first built, they all used uranium-235 as a fuel. Of the three naturally occurring isotopes of uranium, only uranium-235 will undergo fission. 

Plutonium is also used as a fuel in nuclear power plants and in making nuclear weapons (atomic bombs). The isotope used for this purpose is plutonium-239. It is used because it will undergo nuclear fission. Very few isotopes will undergo nuclear fission. Two isotopes of uranium, uranium-233 and uranium-235, are among these. But uranium-233 does not occur at all in nature and uranium-235 occurs in only very small amounts. 

The extended radiation time for the domestic fuel increases the quantity of fission products and the higher actinides. Pure plutonium product poses nuclear weapons proliferation risk and is the primary reason reprocessing is not practiced in the United States. The modified PUREX process has been practiced on an industrial scale in Europe and supports the production of mixed uranium-plutonium fuel. Blended UO2 and PUO2 powder is compacted and sinter to form the mixed oxide (MOX) fuel pellets much like the enriched UO2 fuel. Natural and depleted uranium can be used to prepare MOX fuel and is the demonstrated option to recover fuel values from spent fuel. 

In an energy system based on breeder reactors, it is necessary to process the used fuel elements to recover the fertile elements and discard the waste products. The difficulty with this approach lies in the handling of the uranium, plutonium and highly radioactive nuclear fission products. 

Many people, including environmentalists, regard nuclear fission as a highly undesirable method of energy production. Many fission products such as strontium-90 are dangerous radioactive isotopes with long half-lives. Plutonium-239, used as a nuclear fuel and produced in breeder reactors, is one of the most toxic substances known. It is an alpha emitter with a half-hfe of 24,400 yr. 

In addition to two other isotopes can be used as fuel in nuclear fission reactors. These are plutonium-239, Pu, produced by absorption of neutrons in and produced by absorption of neutrons in natural thorium. The reactions by which these isotopes are made are as follows ... 

In Section 2.5, we discussed the production of energy by nuclear fission, and the reprocessing of nuclear fuels. We described how short-lived radioactive products decay during pond storage, and how uranium is converted into [U02][N03]2 and, finally, UFg. One of the complicating factors in this process is that the fuel to be reprocessed contains plutonium and fission products in addition to uranium. Two dilferent solvent extraction processes are needed to elfect separation. 

Problems relevant in the long-term are the radiotoxicity of the fuel and the long-term risk related to a final repository, which can be steered by an adequate choice of fuel cycle and reactor type. Moreover, it is possible to transmute very long-lived actinides and fission products into less toxic or stable nuclei by means of specific nuclear reactions. Following figure summarises these options for the back-end in the case of the uranium-plutonium fuel cycle. 

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