Characteristics of concrete reflection of ²³⁵U systems

Water reflection is considered in nuclear criticality safety evaluations because it is readily available and easily forms a close-fitting reflector around fissile material. Concrete is commonly encountered in industrial environments and can be a more effective reflector than water. A comprehensive study using 32 concrete compositions derived from an extensive literature survey was done for the three basic one-dimensional geometries used in radiation transport codes (spherical, infinite length cylindrical, and infinite extent slab) using a ²³⁵U metal/water fissile mixture. The concentration/moderation range over which criticality is possible was spanned, and the change in k_eff per unit change in element atomic number density for various concrete composition elements was computed to determine the relative effect of each element on neutron multiplication. The results show that: -1) the most effective reflector is not the same for each geometry; -2) in general, for each geometry, Δk_eff/ΔN has similar values for ²³⁵U concentrations >0.200 g/cm³; and -3) for ²³⁵U concentrations < 0.200 g/cm³, the value of Δk_eff/ΔN diminishes toward zero