Extensive study of the heterogeneous repartition of the moderation when both the fissile mass and the moderation are controlled

This article presents an extensive study of the calculations performed in the configuration where the criticality safety is achieved by both controlling the mass of fissile material and the moderation (for example water) of a single unit. This case often occurs when the control of the fissile mass alone is not sufficient to economically or practically operate a process. This method is often used for the fuel fabrication where an important quantity of powder need to be handled but may also be met in other nuclear facilities and transportation. In this context, from a calculation point of view, a homogeneous distribution of the moderation within the fissile material is generally not a penalizing configuration. So a heterogeneous repartition of the moderation is then considered: it currently consists in a given part of the fissile material uniformly moderated by the entire quantity of the moderator, this system being surrounded by the rest of the dry fissile material and eventually another reflector (water, concrete, lead, etc.). This paper will firstly briefly discuss how to calculate safety limits for this kind of configuration, in the past and nowadays, using state-of-the-art algorithms. Then, new results will be presented for this kind of configuration with various enrichments and densities. These calculations show that the penalizing configuration depends on the maximum dry density of the fissile material considered. The new results obtained with high maximum dry densities are more restrictive than those previously presented.