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Keisuke Okumura, Kojiro Nishina
Nuclear Science and Engineering | Volume 102 | Number 4 | August 1989 | Pages 381-390
Technical Paper | doi.org/10.13182/NSE89-A23649
Articles are hosted by Taylor and Francis Online.
By cell calculation with the SRAC code system, void reactivity is evaluated for a high conversion light water reactor tight lattice, with an emphasis on the breakdown of the void effect into component nuclides, nuclear reactions, and energy groups. The analysis is restricted to infinite lattices and deals with the consequence of neutron energy spectrum shifts caused by void.In a preliminary parameter survey over various fissile plutonium enrichments, a 7.5 % enrichment is found approximately to border the negative and the positive coefficients, when the moderator channel volume to fuel volume Vm/Vf is fixed at a typical value of 0.53. With this combination of the enrichment and Vm/Vf values fixed, the reactivity effect for an incremental void increase is analyzed in detail at low-void conditions (0 to 10%) and at high-void conditions (95 to 100%).At low-void conditions, the 238U contribution is negative by the capture increase in the kilo-electron-volt range, whereas the 240Pu and 242Pu contributions proved to be positive by the capture decrease in the 0.1- to 10-eV range. At high-void conditions, on the other hand, 239Pu makes a positive contribution, originating from (a) the fission increase in the 50-eV to 1-MeV range dominating over the fission decrease in the 10- to 50-eV range, and (b) the lower capture-to-fission ratio above 10 keV. Such a positive contribution of 239Pu is in contrast to the negative contribution of 235U in a highly voided pressurized water reactor lattice. Americium-241 generated by the decay of 241 Pu makes a positive contribution in both low- and high-void conditions. The breakdown of the void effect clearly illustrates the physical mechanism.
