Characteristics of Thick Concrete Neutron Reflection

Water is considered as a neutron reflector in nuclear criticality safety evaluations because it is readily available and easily forms a close-fitting shape around fissile material. Concrete is commonly encountered industrial environments and can be a more effective reflector than water. Nuclear criticality safety literature reporting experimental and analytical studies involving concrete as a reflector from the 1950’s to the present was reviewed. Nuclear criticality safety community perspective on concrete reflection has evolved from acknowledgement that concrete is a reflector through recognition that concrete can be a more effective reflector than water to current interest in identifying which constituents are most important. Concrete reflection work that has been done tends to be limited in several ways. Studies usually consider only a few concrete compositions from a relatively large number available. Studies have also tended to consider a limited number of fissile material moderation ranges, typically unmoderated and the moderation for minimum mass or volume criticality, or only very specific material arrangements. Most analytical studies evaluate systems as infinite extent slab arrangements.

To address these limitations and evolving current interest, a comprehensive study using 32 concrete compositions derived from the literature survey was done for the three basic one-dimensional geometries used in radiation transport codes: spherical, infinite length cylindrical, and infinite extent slab geometries using a ²³⁵U [uranium-235] metal/water fissile mixture. The concentration/moderation range over which criticality is possible was spanned, and the change in k_eff [k-effective] per unit change in element atomic number density values, Δk_eff/ΔN, [delta k-effective/delta N] for various concrete composition elements was computed to determine the relative effect of each element on neutron multiplication.

These results will be useful to nuclear criticality safety practitioners in several ways. Some control can be exercised over concrete constituents and the results can be used to guide material selections during project design to minimize positive contributions to k_eff. For existing installations, if the concrete composition is known, the results will permit some ranking of its effectiveness relative to water. For technical studies, conservative conditions that maximize k_eff are generally preferred and the results will help guide constituent selections.