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

International audienceDue to the combined effects of advanced variance reduction techniques and enhanced computing power, Monte-Carlo is currently employed to calculate radiation-induced responses in a wide variety of fixed source problems. Its use is also expanding in eigenvalue problems but tends to be restricted to in-core (or more generally in-fissile configuration) integral responses (keff, reactivity coefficients, burn-up effects, etc.). More differential in-core responses or ex-core responses at some (optical) distance from the core remain special cases. The former are usually solved without variance reduction, which limits the range of solvable problems, and the latter by decoupling (either with both decoupled parts Monte-Carlo or mixed deterministic / Monte-Carlo approaches). We evaluate through some sample problems a recent approach that has been developed to calculate differential in- and ex-fissile configuration responses. The ex-core problems focus on a variety of radiation responses around a GEN III PWR core. Comparison is made with an empirical approach involving analog Monte-Carlo in the fissile zone and variance reduction outside. The in-core tests look at the neutron flux and the 96Zr(n,) reaction rate at the lower surface of a control rod in two VERA Core Physics Benchmark problems. Comparison is made with analog Monte-Carlo.This paper consists of two parts: this part I focusing more on methodology and background with the test problems and part II concentrating on comparison with PWR GEN II and III ex-core decoupled results