Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

The total energy released by the nuclear fission of a uranium atom is more than200 MeV: Under the right conditions, this reaction can be perpetuated if the fissionneutrons interact with other U-235 atoms, causing additional fissions andleading to a self-sustaining nuclear chain reaction (Fig. 19-5). The probability offission (called the fission cross-section) with U-235 is greatly enhanced as neutronsslow down or thermalize. The neutrons emitted from the fission are very energetic(calledftst neutrons), and are slowed (moderated) to thermal energies (-0.025 eV)as they scatter in water in the reactor core. Good moderators are low-Z materialsthat slow the neutrons without absorbing a significant fraction of them. Water isthe most commonly used moderator, although other materials, such as graphite(used in the reactors at the Chernobyl plant in the Ukraine) and heavy watereHzO), are also used.Some neutrons are absorbed by nonfissionable material in the reactor, whileothers are moderated and absorbed by U-235 atoms and induce additional fissions.The ratio of the number of fissions in one generation to the number in theprevious generation is called the multiplication ftctor. When the number of fissionsper generation is constant, the multiplication factor is 1 and the reactor issaid to be critical. When the multiplication factor is greater than 1, the rate of thechain reaction increases, at which time the reactor is said to be supercritical. If themultiplication factor is less than 1 (i.e., more neutrons being absorbed than pro-FIGURE 19-5. Schematic of a nuclear chain reaction. The neutrons (shown as smallblackened circles) are not drawn to scale with respect to the uranium atoms.