Evaluation of the worth of non-hydrogen constituents in hydrogenous moderators on criticality safety applications

This paper serves to address a current limitation in moderator characterization in criticality safety applications. The standard practice evaluates only the concentration of hydrogen assuming the contributions of secondary elements to be negligible. This paper quantifies the relationship between selected secondary moderating elements added to a hydrogenous moderating medium and their impact on criticality safety calculations and safety limits. A regression model is developed based on three parameters in a common safety basis calculation: quantity of moderator (in atoms/barn-cm), quantity of fissile material (in grams ²³⁵U), and various reflective environments. Results show that a simple equation, linear with respect to moderator density and quadratic with respect to mass of ²³⁵U, may be generated for a given reflector environment to accurately predict the effective multiplication factor to within 2.5% for moderating materials that are predominately (>50%) hydrogenous in composition.