Nonfissionable isotopes

Unenriched uranium—which contains more than 99 percent of the nonfissionable isotope U-238—undergoes a chain reaction only if it is mixed with a moderator to slow down the neutrons. Uranium in ore is mixed with other substances that impede the reaction and has no moderator to slow down the neutrons, so no chain reaction occurs. 75- Nuclear fission is a poor prospect for powering automobiles primarily because of the massive shielding that would be required to protect the occupants and others from the radioactivity and the problem of radioactive waste disposal. 

In the United States, reactors designed for commercial power production use uranium oxide, UsOs, that is enriched with the relatively scarce fissionable U-235 isotope. Because the supply of U-235 is limited, a new type of reactor known as the breeder reactor has been developed. Breeder reactors produce additional fissionable material at the same time that the fission reaction is occurring. In a breeder reactor, excess neutrons convert nonfissionable isotopes, such as U-238 or Th-232, to fissionable isotopes, Pu-239 or U-233, as shown below ... 

The possibility of shortages in the known supply of has led to the development of breeder reactors, which can manufacture more fuel than they use. A breeder reactor is designed not only to generate electrical power but also to maximize neutron capture in the core by fU. The fuel of a typical breeder reactor consists of the abundant but nonfissionable isotope mixed with 92U or y u, which produce neutrons when they undergo fission. Some of these neutrons are absorbed by 92U to form U.This unstable uranium isotope soon leads, after two steps of beta emission, to I Pu. 

Uraniuin-235 is capable of sustaining a chain reaction, but it makes up only 0.7% of all naturally occurring uranium. Therefore, it is not a very satisfactory source of nuclear fuel (Fig. 20.16). An alternative is the plutonium isotope, Pu, produced from the most abundant uranium isotope (Equations 20.4 to 20.6). Pu has a long half-life (24,360 years) and is fissionable. It has been used in the production of atomic bombs and is also used in some nuclear power plants to generate electrical energy. It is made in a breeder reactor, the name given to a device whose purpose is to produce fissionable fuel from nonfissionable isotopes. 

Z = 92—can be produced via nuclear bombardment in a particle accelerator. 6. Nuclear fission is a nuclear reaction in which a large nucleus splits into two smaller nuclei. A nuclear power plant uses the energy released in a fission reaction to generate electricity. Nuclear fusion is a nuclear reaction in which two small nuclei combine to form a larger nucleus. Solar energy is an example of a fusion reaction. 7. A chain reaction has one of its own reactants as a product, allowing the original reaction to continue. The minimum quantity of matter necessary for a chain reaction to continue is its critical mass. A breeder reactor is a nuclear reactor in which fissionable fuel is produced from nonfissionable isotopes. 

The most common use of uranium is to convert the rare isotope U-235, which is naturally fissionable, into plutonium through neutron capture. Plutonium, through controlled fission, is used in nuclear reactors to produce energy, heat, and electricity. Breeder reactors convert the more abundant, but nonfissionable, uranium-238 into the more useful and fissionable plutonium-239, which can be used for the generation of electricity in nuclear power plants or to make nuclear weapons. 

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. 

Natural uranium consists of different isotopes of uranium. Natural uranium is 0.7% U-235 and 99.3% U-238. Uranium-238 is nonfissionable, and therefore naturally occurring uranium must be enriched to a concentration of approximately 4% to be used as fuel for nuclear reactors or 90% for weapons-grade uranium. Yellow cake is shipped to conversion plants... 

One of the fascinating features of fission power is the breeding of fission fuel from nonfissionable uranium-238. Breeding occurs when small amounts of fissionable isotopes are mixed with uranium-238 in a reactor. Fission liberates neutrons that convert the relatively abundant nonfissionable uranium-238 to uranium-239, which beta-decays to neptunium-239, which in turn beta-decays to fissionable plutonium-239. So in addition to the abundant energy produced, fission fuel is bred from relatively abundant uranium-238 in the process. 

That is the advantage of fission. Its drawback is the deadly radioactivity it generates, particles whose mass, from one type of reactor, is almost equal to the mass of the fuel consumed. Waste from a fission reactor typically requires thousands of years before it breaks down into biologically safe levels. Fission reactors are also relatively inefficient. They can use but a single isotope (atoms of an element that have the same number of protons but a different number of neutrons) of uranium, U-235, which makes up less than 1 percent of natural uranium ore. (More than 99 percent of natural uranium is nonfissionable U-238.) So-called fast breeder reactors might overcome the supply limitation by breeding fissionable fuel from U-238. But the fuel it produces from the uranium is plutonium, the same stuff that was inside the Nagasaki bomb—not an ideal by-product in a politically unstable world. 

In Section 26-2 we discussed the stability of nuclei with even numbers of protons and even numbers of neutrons. We should not be surprised to learn that both and can be excited to fissionable states by slow neutrons much more easily than because they are less stable. It is so difficult to cause fission in 23Su that this isotope is said to be nonfissionable. ... 

In this manner the nonfissionable uranium-238 is transmuted into the fissionable isotope pIutonium-239 (Figure 23.13). 

There is no critical mass in a fusion bomb, and the force of the explosion is limited only by the quantity of reactants present. Thermonuclear bombs are described as being cleaner than atomic bombs because the only radioactive isotopes they produce are tritium, which is a weak /S-particle emitter (ti = 12.5 yr), and the products of the fission starter. Their damaging effects on the environment can be aggravated, however, by incorporating in the construction some nonfissionable material such as cobalt. Upon bombardment by neutrons, cobalt-59 is converted to cobalt-60, which is a very strong 7-ray emitter with a half-life of 5.2 yr. The presence of radioactive cobalt isotopes in the debris or fallout from a thermonuclear explosion would be fatal to those who survived the initial blast. 

The fourth term on the right, X/, iV/,0/,0, is new, and represents additional consumption of neutrons by nonfissionable higher isotopes such as U, Pu, and Pu, which were not present in the reference design. 

One of the most important applications of Graham s law is the enrichment of nuclear reactor fuel separating nonfissionable, more abundant from fissionable to increase the proportion of in the mixture. The two isotopes have identical chemical properties, so they are separated by differences in physical properties—the effusion rates of their gaseous compounds. Uranium ore is converted to gaseous UFg (a mixture of and UFg), which is pumped through... 

Breeder reactors convert nonfissionable into fissionable Pu. The material used for fission is a combination of U-235 (which undergoes fission in a chain reaction) and the more common U-238 isotope. Excess neutrons from the U-235 fission are absorbed by the U-238, converting it to the fissionable plutonium isotope Pu-239. The chemical and physical properties of Pu-239 make it very difficult and expensive to handle and process. 

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