On integrating Monte Carlo calculations in and around near-critical configurations I. Methodology

Monte Carlo with variance reduction (VR) is employed to calculate radiation-induced responses in a variety of fixed source problems. In eigenvalue problems, Monte Carlo is used without VR (apart from implicit capture) for in-core integral responses (keff, reactivity coefficients, burn-up effects, etc.). More differential in- or ex-core responses are either treated in the same way thus limiting the range of solvable problems, or by decoupling. We evaluate an all-Monte Carlo approach developed to calculate differential in- or ex-core responses without decoupling. The ex-core problems involve responses in a GEN III PWR. The in-core problems study the neutron flux and 96Zr(n,γ) rate at the surface of a control rod in the VERA Benchmark. Comparison is made with an empirical approach involving analog Monte Carlo in-core with VR ex-core. The paper consists of two parts: part I tests the methodology; part II compares with PWR GEN II and III ex-core decoupled results.