To demonstrate the criticality safety of a fissile containers storage, many calculations can be performed with different assumptions. The nature more or less bounding of these calculation assumptions implies or not a reduction of exploitation constraints: assuming fissile material at the optimum of moderation surrounded by water with variable density in calculation implies no restriction for water presence in exploitation (flooding, fire extinguishing, presence of moderating materials,…), assuming room walls thickness equals to 60 cm in calculation implies no thickness guaranty for room walls in exploitation or assuming no tank or container walls in the calculation storage also implies no thickness guaranty for tank or container walls in exploitation… One of many assumptions, which is quite attractive in terms of exploitation constraints reduction, is to model the storage as a 2D-infinite triangular pitch array of elementary cells in air. Such a calculation model implies 1) no containers arrangement guaranty (opposed to a square pitch array) and 2) no maximum number of containers to be respected in the storage. In criticality studies, it is usual to model a 2D-infinite containers storage by applying a mirror boundary condition on each face of an elementary cell, which can be 1) a circular section cell (cylindrical cell, bounding model) or 2) a hexagonal section cell (hexagonal prismatic cell, realistic model). The purpose of this paper is to present and quantify the effects of the choice of the elementary cell to model a 2D-infinite containers storage model (cylindrical section or hexagonal section, as shown in Figure 1) on criticality calculation results. The case studied is a storage of 150 grams Pu-containers in air when containers walls are not modeled and when the pitch between containers increases. The Monte-Carlo calculations performed to out point the effects are run with standard deviations of 0.001 or 0.002 according to the objective of this study.
1 Commissariat à l’Energie Atomique (CEA), BP 28 - 91191 Gif sur Yvette Cedex, France
2 Institut de Radioprotection et Sûreté Nucléaire (IRSN), BP 17 - 92262 Fontenay aux Roses Cedex, France