Methodology for Determination of the Upper Safety Limit for Criticality Calculations for Criticality Safety Analyses

This report considers the methods for determination of an upper safety limit, and incorporating uncertainty and margin into the safety limit, provides comparisons, and recommends a preferred method for determining the Upper Safety Limit (USL). A USL is developed for CSAS25 from SCALE4.4a. The USL is applicable for the CSAS25 control module from the SCALE 4.4a computer code system for use in evaluating nuclear criticality safety of enriched uranium systems. The benchmark calculation results used for this report are documented in Y/DD-896. The statistical evaluation is documented in CCG-380. The 27-group ENDF/B-IV, 44-group ENDF/B-V, and 238-group ENDF/B-V cross-section libraries were used. Numerical methods for applying margins are described, but the determination of appropriate correlating parameters and values for additional margin, applicable to a particular analysis, must be determined as part of a process analysis. As such, this document does not specify final upper subcritical limits as has been done in the past. No correlation between calculation results and neutron energy causing fission was found for the critical experiment results. Analysts using these results are responsible for exercising sound engineering judgment using strong technical arguments to develop ''a margin in k{sub eff} or other correlating parameter that is sufficiently large to ensure that conditions (calculated by this method to be subcritical by this margin) will actually be subcritical.'' Documentation of area of applicability and determination and justification of the appropriate margin in the analyst's evaluation, in conjunction with this report, will constitute the complete Validation Report in accordance with ANSI/ANS-8.1-1998, Section 4.3.6(4)