Investigation of reactivity differences in cylinder arrays using various fill geometries with constant mass

The factors affecting criticality safety are well understood by the nuclear engineering industry. Array configurations present multiple challenges to the industry due to the competing effects that are present in array systems - most notably interaction/spacing, moderation, and reflection. However, even the fissile mass geometry has been shown to complicate array systems. This attribute is also seen in filled UF₆ cylinders, thus making it important to understand the reactivity in cylinder arrays. The process of filling UF₆ cylinders results in non-uniform cylinder fill geometries. Due to the non-uniform fill, it becomes necessary to model a specific UF₆ mass, in cylinder arrays, in different ways to ensure understanding of the reactivity of the system. Some of the ways to model the UF₆ include: spreading the UF₆ mass across the cylinder, all UF₆ on the bottom of the cylinder, UF₆ consolidated in the middle of the cylinder, and UF₆ consolidated on the outside of the cylinder. Spreading the UF₆ mass across the cylinder is accomplished by adjusting the UF₆ density to achieve the given UF₆ mass, whereas all the other scenarios use a fixed UF₆ density with varying thicknesses to achieve the given UF₆ mass. A different modeling approach, which helps to capture mixture effects, is to model the UF₆ mass using the nominal UF₆ density along with the void space for a given mass and distribute the mixture throughout the entire cylinder. This paper compares the reactivity of a cylinder array at a constant mass value using three different modeling techniques - completely filling the cylinder using a reduced UF₆ density, filling the cylinder with nominal UF₆ density and void space throughout the cylinder (same amount of void space as in the last technique), and all UF₆ in the bottom of the cylinder (all void space on top of UF₆). The results demonstrate that completely filling the cylinder produces the most reactive UF₆ cylinder array system. (authors)