Variance reduction adjustment in Monte Carlo TRIPOLI-4® neutron gamma coupled calculations

International audienceThis paper presents a diagnosis tool for the users of the TRIPOLI-4 Monte Carlo transport code. The aim is to facilitate the adjustment of the variance reduction scheme when dealing with coupled neutron-photon simulations. Up to now, neutron and photon importance functions have been generated independently by TRIPOLI-4 when transport biasing for both particles was necessary, leading to further adjustments by the user after running a first coupled simulation. The idea is here to combine in a fast step the TRIPOLI-4 photon importance map on the one hand, and the photon production data used by the simulation on the other hand, so as to provide at once a better knowledge of important points of neutron phase space with respect to photon tallies. The highest neutron importance mesh cells are then derived and optionally displayed on geometry graphs. The user's intervention consists then in different possible options defining the neutron variance reduction scheme of the TRIPOLI-4 coupled simulation, in the light of the previous diagnosis results. Finally, coupled neutron-photon simulations involving polyethylene and stainless steel slabs are investigated: an example of use of this diagnosis tool is provided and figures of merit related to photon tallies are then compared