Computational Breakdown of the High Conversion Light Water Reactor Infinite Lattice Void Reactivity into Contributing Nuclides and Energy Groups

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 V_m/V_f is fixed at a typical value of 0.53. With this combination of the enrichment and V_m/V_f 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.