Unformatted text preview: is extremely small (Ua = 1.2 mb) for heavy water, it yields a large thermal utilization factor f, larger than that for any other moderator at the optimum fuel-to-moderator ratio (see Table 10.3). Also, heavy-water moderated reactors have a very large optimum moderator-to-fuel ratio compared to light-water reactors (see Table 10.3). Neutrons, thus, have a much better chance of slowing to thermal energies without encountering 238U and being absorbed while slowing. As a consequence, heavy water reactors have much larger resonance escape probabilities p than do light-water reactors. To compensate for lower values of f and p in light-water reactors, these reactors use enriched uranium to increase the thermal fission factor 1]. As discussed in Section 10.4, only heavy-water and graphite moderated reactors can be made critical with natural uranium. Light-water moderated reactors must use en-riched uranium. July 24, 2002...
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- Spring '06
- Nuclear Fission, Neutron moderator, volume flow rate, Enriched uranium