Comparison Calculations of Subcritical Limits in ANSI/ANS-8.1

The American National Standards Institute (ANSI) standard, “Nuclear Criticality Safety in Operations with Fissionable Materials Outside of Reactors” (ANSI/ANS-8.1-2014), provides analysts and practitioners with subcritical limits (SCLs) for a variety of uranium and plutonium compounds and mixtures. Currently, the standard is being reviewed to evaluate the impact of changes to nuclear data and transport codes and to assess whether the listed values require a revision. In addition, the ANS-8.1 working group is assessing whether to include additional uranium compounds with enrichments ranging up to 20 wt% 235U to assist with the development of new advanced reactor concepts and designs, as well as the use of high-assay low-enriched uranium (HALEU) in the current commercial reactor fleet. The purpose of this work is to analyze the values listed in the ANS-8.1 standard with the Evaluated Nuclear Data File (ENDF)/B-VII.1 nuclear data library and the KENO transport code in the SCALE code package. Eigenvalues of additional uranium compounds not found in the standard are also presented. These values are intended to provide analysts and practitioners with guidance regarding the SCLs for use in nuclear safety applications.