Just a word or two: Nuclear criticality - 14376

What's in a name? The purpose is to demonstrate a need for better definitions, how to achieve that and how to use them. A controversial definition, nuclear criticality, and implementation of available directives for preparation of standards have been selected for illustration. The definitions of bias and a few other terms are discussed briefly. Reliable communication, supported by established standards, is essential to (nuclear) criticality safety. Sufficiently accurate, without necessarily being scientific, and consistent definitions are required. A term is the name of a concept (a general idea or understanding of something). A definition is a link between a term and a concept. A concept may be complex but the term and the concept description should be exchangeable in the text they are used. They have identical meanings. Established standards include ANSI/ANS and ISO consensus standards, the IAEA Safety Glossary and the IAEA Transport Regulations. The report LA-11627-MS and the associated ANS Glossary of Terms in Nuclear Science and Technology are sources for the consensus standards. In the spring of 2015, ISO approved a new terminology standard for the nuclear fuel cycle (excluding reactors). In response to an ISO member state proposal, a definition of nuclear criticality, based on the IAEA Safety Glossary, was added at the very last moment by the high-level (not criticality safety only) working group preparing the new standard. The ISO criticality safety working group, after some discussion, did not provide any comment on a nuclear criticality definition to the terminology working group. As a concept, nuclear criticality has a very precise 'reactor physics' meaning. Criticality, as a term related to fission chain reactions, is used in some contexts to cover less precise meanings ('loosely') or as a constituent of a combined expression for rather different concepts. Criticality may thus be loosely used to mean 'criticality or something worse', a criticality accident, a different concept with is own definition. The new definition proposed by the ISO member state was based on such use. What does fraction of a critical mass, e.g. 40%, mean if the critical mass has an unspecified k_eff value larger than unity? Bias, typically an estimated deviation from the correct value in a measured or a calculated value, is sometimes used to mean the opposite concept, bias correction. That is a direct cause of safety concern since the conflicting concepts may be used in the same document or evaluation. (author)