Fissile isotopes

A variation of the classical fuel cycle is the breeder cycle. Special breeder reactors are used to convert fertile isotopes iato fissile isotopes, which creates more fuel than is burned (see Nuclear reactors, reactor types). There are two viable breeder cycles U/ Pu, and Th/ U. The thorium fuels were, however, not ia use as of 1995. A breeder economy implies the existence of both breeder reactors that generate and nonbreeder reactors that consume the fissile material. The breeder reactor fuel cycle has been partially implemented ia France and the U.K. 

Uses of Plutonium. The fissile isotope Pu had its first use in fission weapons, beginning with the Trinity test at Alamogordo, New Mexico, on July 16, 1945, followed soon thereafter by the "Litde Boy" bomb dropped on Nagasaki on August 9, 1945. Its weapons use was extended as triggers for thermonuclear weapons. This isotope is produced in and consumed as fuel in breeder reactors. The short-Hved isotope Tu has been used in radioisotope electrical generators in unmanned space sateUites, lunar and interplanetary spaceships, heart pacemakers, and (as Tu—Be alloy) neutron sources (23). 

A unique problem arises when reducing the fissile isotope The amount of that can be reduced is limited by its critical mass. In these cases, where the charge must be kept relatively small, calcium becomes the preferred reductant, and iodine is often used as a reaction booster. This method was introduced by Baker in 1946 (54). Researchers at Los Alamos National Laboratory have recently introduced a laser-initiated modification to this reduction process that offers several advantages (55). A carbon dioxide laser is used to initiate the reaction between UF and calcium metal. This new method does not requite induction heating in a closed bomb, nor does it utilize iodine as a booster. This promising technology has been demonstrated on a 200 g scale. 

Large quantities of fissile isotopes, U and U, should be handled and stored appropriately to avoid a criticahty hazard. Clear and relatively simple precautions, such as dividing quantities so that the minimum critical mass is avoided, following adniinisttative controls, using neutron poisons, and avoiding critical configurations (or shapes), must be followed to prevent an extremely treacherous explosion (246). 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

Naturally occurring uranium contains a very small percentage (0.7%) of the fissile isotope to be used either in nuclear weapons, which require a very... 

Tn a typical fast breeder, most of the fnel is 238I (90 to 9.3%). 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 23 Pu and 241Pu, although a small portion of 235U call also be present. Normally, the fissile isotopes are located in a central core region that is surrounded by the fertile isotopes in the blanket region. This is illustrated in Fig. 30. 

When the fuel is initially loaded into die reactor, the core region will typically contain from 10 to 15% fissile isotopes with the remainder being ijSU. Essentially all of the blanket will be 238U. As energy is extracted from the fissile isotopes, they become depleted (the initial plutonium is gradually used up), However, in a breeder reactor, new plutonium will be formed in die cure and blanket regions faster Ilian it is consumed. Additionally, undesirable fission products are formed which must ultimately be removed. This process is schematically illustrated in Fig. 31. The before chart... 

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. 

Bohr used the reaction cross-sections that Meitner had measured in 1937 for the three processes to deduce that the highly fissile isotope of uranium must be 235U, and not 238U. 

A. Uranium-235 and plutonium-239 are the two fissile isotopes used for nuclear power. 238U is the most common uranium isotope. 238Pu is used as a heat source for energy in space probes and some pacemakers. 

Content of fissile isotopes of SNF in NPU robust models (in average by RC)... 

SNF type Type of NPU Content of fissile isotopes, g Enriehment, %... 

Minimum of Safeguards. (a) Processes that are incapable of producing fissile material in a pure form, or with a high enrichment of fissile isotopes, or with a high level of decontamination may require little or no added safeguards except for protection from sabotage. 

There are three fast-flux reactors proposed for development the sodium cooled, the gas cooled, and the lead cooled. The fission cross sections for fast neutrons (high-energy spectrum neutrons) for all of the fissile actinides are nearly the same so the fast-flux reactors use all of the fissile actinides as fuel. The fast-flux isotopic fission cross sections are smaller than for thermal neutrons so the fraction of fissile isotopes (e.g., 235u 239pu, range of... 

Fluorine is used in the nuclear industries of many countries to make uranium hexafluoride for enrichment of uranium in the fissile isotope ... 

At the end of irradiation in such reactors, fuel consists of a mixture of thorium, uranium containing fissile isotopes, and fission products. Figure 3.33 showed a fuel-cycle flow sheet for an HTGR. The Thorex process has been developed for recovering the uranium and thorium from such fuel cycles, freeing them from fission products and separating them from each other. The Thorex process will be described in this section. When the fuel being irradiated contains appreciable the plutonium thus formed requires that a combination of the Thorex and Purex processes be used. 

In the most common type of fission counter, the interior surface of the detector is coated with a fissile isotope (Fig. 14.4). When fission takes place, one of the fission fragments (denoted as FFj in Fig. 14.4) is emitted toward the center of the counter and is detected. The other (FF2) stops in the fissile deposit or the wall of the counter. The counting rate of a fission counter is proportional... 

The sensitivity of a fission counter decreases with exposure because of the depletion of the fissile isotope (the same phenomenon as depletion of boron... 

---