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Fissile and Fertile Materials

In discussing nuclear reactors we must distinguish between two classes of fissionable materials. A fissile material is one that will undergo fission when bombarded by neutrons of any energy. The isotope uranium-235 is a fissile material. A fertile material is one that will capture a neutron, and transmute by radioactive decay into a fissile material. Uranium-238 is a fertile material. Fertile isotopes may also undergo fission directly, but only if impacted by a highenergy neutron, typically in the MeV range. Thus fissile and fertile materials together are defined as fissionable materials. Fertile materials by themselves, however, are not capable of sustaining a chain reaction. Uranium-235 is the only naturally occurring fissile material. Moreover, it constitutes only 0.7% of natural uranium. Except for trace amounts of other isotopes, uranium-238 constitutes the
remaining 99.3% of natural uranium. By capturing a neutron, uranium-238 becomes radioactive and decays to plutonium-239:



If a neutron of any energy strikes plutonium-239, there is a strong probability that it will cause fission. Thus it is a fissile isotope. Plutonium-239 itself is radioactive. However its half-life of 24.4 thousand years is plenty long enough that it can be stored and used as a reactor fuel. There is a smaller probability that the plutonium will simply capture the neutron, resulting in the reaction



Plutonium-240, however, is again a fertile material. If it captures a second neutron it will become plutonium-241, a fissile material. In addition to uranium-238, a second fertile material occurring in nature is thorium-232. Upon capturing a neutron it undergoes decay as follows:




yielding the fissile material uranium-233. This reaction is of particu
lar interest for sustaining nuclear energy over the very long term since the earth’s crust contains substantially more thorium than uranium. Fissile materials can be produced by including the parent fertile material in a reactor core. Returning to Fig. 1.2, we see that if more than two neutrons are produced per fission—and the number is about 2.4 for uranium-235—then there is the possibility of utilizing one neutron to sustain the chain reaction, and more than one to convert fertile to fissile material. If this process creates more fissile material than it destroys, the reactor is said to be a breeder; it breeds more fissile material than it consumes. Since most power reactors are fueled by natural or partially enriched uranium, there is a bountiful supply of uranium-238 in the reactor for conversion to plutonium. However, as subsequent chapters will detail, to sustain breeding the designer must prevent a large fraction of the fission neutrons from being absorbed in nonfissile materials or from leaking from the reactor. This is a major challenge. Most reactors burn more fissile material than they create. Because half-lives, cross sections, and other properties of fissile and fertile isotopes are ubiquitous to reactor theory, the following unambiguous shorthand frequently is used for their designation.

Their properties are designated by the last digits of their atomic charge, and atomic mass:
Thus properties of fissionable element




are simply designated sub- or superscripts ‘‘ec.’’ For example




One question remains: Where do the neutrons come from to initiate a chain reaction? Some neutrons occur naturally, as the result of very high-energy cosmic rays colliding with nuclei and causing neutrons to be ejected. If no other source were present these would trigger a chain reaction. Invariably, a stronger and more reliable source is desirable. Although there are a number of possibilities, probably the most widely used is the radium beryllium source. It combines the alpha decay of a naturally occurring radium isotope










to provide the needed neutrons.


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