Parametric analysis of handbook metal-water critical mass curves with MCNP

The purpose of this paper is to present and discuss the methodology, results, and conclusions of an analysis of spherical, homogenous metal-water mixtures in MCNP-6.2 with ENDF/BVII.1 data. The parametric analysis reproduced curves of critical fissile mass as a function of metal-water mixture density (i.e., concentration, H/X) that have been published in criticality safety handbooks. Curves are provided for highly-enriched uranium (HEU) and plutonium-239. Furthermore, the sensitivity of the effective neutron multiplication factor to changes in mass or cross-section data is plotted and discussed. Further evaluation of these curves and the data used to generate them reveals that the reactivity worth of incremental unit of mass in systems that are critical or nearly critical is worth less than the average reactivity worth per unit of existing mass in the system. This also means that, in order to obtain a given margin in k_eff (e.g., 5% or a USL of 0.95), a greater reduction in the corresponding mass limit must be made (e.g., a ∼25% reduction from the critical mass value)