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HW22-5 - is extremely small(Ua = 1.2 mb for heavy water it...

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11-4 Nuclear Power Chap. 11 disadvantages: .It requires large core to achieve keff > 1. .Reactor-grade graphite (very pure) is moderately expensive. .It is chemically reactive with oxygen; thus, one must use carbon diox- ide or helium as gas coolants, or water in pressuretubes. .Graphite stores energy (Wigner effect) and must periodically be care- fully annealed to remove this stored energy to prevent spontaneous catastrophic release. 4. Why are the blades of a low-pressure turbine larger than those of a high- pressure turbine? Solution: For the samemass flow rate in both turbines, the volume flow rate is necessarily larger in the low-pressure turbine than in the high-pressure turbine. To accom- modate the larger volume flow rate, the volume and blades of the low-pressure turbine must be larger. 5. Why can a heavy-water moderated reactor use a lower enrichment uranium fuel than a light-water moderated reactor? Solution: Because the thermal absorption cross section
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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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