Methodologies for Treatment of Spectral Effects at Core-Reflector Interfaces in Fast Neutron Systems

The existence of space, angle and energy neutron flux distribution transients is a well known phenomenon which makes the calculation of fast reactors difficult when the core is surrounded by reflectors, particularly steel reflectors. The current interest for fast neutron reactors as TRU burners of relatively low power and small core size (high leakage) motivates the search for an accurate deterministic treatment of neutron reflection. Indeed, without due care taken, the treatment of reflector effects can introduce significant uncertainties in Keff and power distribution calculated values. We report here further investigations that we have performed with the deterministic code system ERANOS2.0 and with the continuous energy Monte Carlo code TRIPOLI-4, in order to understand these effects and converge to a satisfactory calculation scheme for ERANOS: our final objective is to define a recommended deterministic calculation procedure for an accurate treatment of fast reactors neutron balance in the presence of reflectors. Our work shows that satisfactory results can be achieved by optimization of the macrocell option already implemented in the ECCO cell code of ERANOS, the agreement with the Monte Carlo calculation becoming satisfactory even when using a reduced number of energy groups.