The Difficulty of Defining the Terms 'Credible' and 'Unlikely' for Nuclear Criticality Safety Purposes

During the last revision of the ANSI/ANS-8.1-2014, extensive efforts were made by the working group (ANS- 8.1) to ensure the statements in the standard clearly reflected their intent. This effort involved the review of each requirement and recommendation in the standard. In some cases, definitions were added to the standard glossary to minimize ambiguity in the standard (e.g., 'process conditions' or 'parameter'). The working group also considered adding definitions for the words 'credible' and 'unlikely' because definitions had been provided to the nuclear criticality safety (NCS) community from the US Department of Energy that caused confusion at some sites. A Nuclear Regulatory Commission document intends for licensees to provide appropriate definitions for these words in their site license. The purpose of this paper is to discuss why the working group ultimately decided not to include definitions for credible and unlikely in the 2014 revision of ANS-8.1