Development and Comparison of Monte Carlo Techniques Implemented in the MORET4 Code for the Calculation of Loosely Coupled Systems

The performances of the Monte Carlo method and the easiness of its use enable to calculate more and more complex configurations. In some cases, when there is little coupling of neutrons between the different fissile media of the system, a careful check of neutron simulation is needed. Within the framework of nuclear criticality, the use of the current codes and the reliability of their results suffer restrictions. In fact, as was stressed by Whitesides in a famous article [1], the calculated keff becomes erroneous under certain circumstances (often under-estimating the reactivity of the system). In this paper, one shows some aspects of the difficulty encountered when calculating weakly coupled systems. New developments have been performed in the M0RET4 code [2] to improve the calculation of keff in geometries including such weakly coupled fissile units.