Acceleration and Real Variance Reduction in Continuous-Energy Monte Carlo Whole-Core Calculation via p-CMFD Feedback

In the three-dimensional (3-D) continuous-energy whole-core reactor analysis, the partial current–based coarse mesh finite difference (p-CMFD) feedback was applied to the Monte Carlo (MC) k-eigenvalue problem simulation for both inactive and active iterations (cycles). To reduce the stochastic errors in the p-CMFD parameters and their biases due to the ratio-type estimators, the first-in-first-out (FIFO) accumulation scheme was introduced in the MC/p-CMFD procedure. The MC/p-CMFD procedure was tested on a typical pressurized water reactor 3-D continuous-energy whole-core problem while varying the FIFO queue lengths and the results were compared with the conventional power iteration. The Shannon entropy analysis showed that MC/p-CMFD accelerates the convergence of the fission source distributions and mitigates the spatial clustering phenomenon. The real variance analysis also showed that MC/p-CMFD reduces the interiteration correlation, leading to the most real variance reduction in the local MC tallies at the optimum queue length (L = 5). It was also shown that a nontrivial bias was introduced by the p-CMFD feedback, especially for the global tally (k_eff) with L = 1. However, the bias decreased as the tally bin size became smaller and it was effectively reduced by increasing the queue length (L ≥ 5). In conclusion, the MC/p-CMFD procedure showed promising capability for 3-D continuous-energy whole-core reactor analysis by MC simulation.