A Novel Approach to Realistic Conservatism in Nuclear Criticality Safety Analysis

Abstract

The standard approach in nuclear criticality safety analysis is to rely quite heavily—and in some cases exclusively—on passive controls, such as assuming all worst-case conditions are by default attained. This means assumptions are made such as no poison, optimum moderation, and pure fissile actinide content at the maximum mass with optimum full reflection. What is clearly attainable is something less than any of these extremal conditions, but how can one rely on a limit based on assuming less than the worst case without some controls ensuring those assumptions are not challenged? This technical note discusses various options for approaching a defendable realistic technical basis for safety analysis by associating probabilities with conservative assumptions.

Keywords:

• Nuclear criticality safety
• probabilistic safety analysis
• regulatory compliance
• qualitative risk assessment

Acknowledgments

This work is partially paid through a joint faculty appointment between North Carolina State University and Oak Ridge National Laboratory in coordination with the Office of Defense Nuclear Nonproliferation Research and Development of the National Nuclear Security Administration–sponsored Consortium for Nonproliferation Enabling Capabilities by the U.S. Department of Energy National Nuclear Security Administration under award number DE-NA0002576.

Notes

^a With a past difference, this assumes the assay information was incorrect in the ratio values credited for analysis.

^b Here, the definition of “incredible” is a probability value per year p such that p < 10⁻⁶.