CONSISTENT CRITICALITY SAFETY APPROACH TO NUCLEAR SITES, EXTERNAL TRANSPORT, AND FINAL DISPOSAL

摘要

A general issue and some selected topics discussed in international projects (ISO, IAEA, OECD) are presented. Long and broad experience in criticality safety is applied to find consistent safety principles for nuclear sites, external transport and final disposal. Support of standards and regulations is a main purpose. An ISO 1709 (criticality safety principles) draft proposes expansion of the scope to transport. Research on the basis for the IAEA Transport Regulations provides insights. Those Regulations are based on criticality safety principles. A general requirement for prevention of criticality during transport is combined with a prescriptive set of requirements on how to get regulatory approval. "Confinement system" was introduced in 1996 to separate subcriticality from radioactive material containment specifications. The much older intention was to provide technical support, but not safety, for the interphases between sites, transport and emergency. The application has changed unintentionally and significantly. A solution is proposed. The principle for determination of a threshold fissile nuclide mass limit for fissile material is used to support acceptability of rounding a Criticality Safety Index value down to zero, similar to determination of the Transport Index. Temperature effects on nuclear properties need to be accounted for. A -40 °C to 800 °C temperature range outside a package applies to transport. Burnup credit benchmarks involve high temperatures (depletion) and interpretation of operating reactor measurements for typical water pools (20 °C to 120 °C). KRITZ critical experiments between 20 °C and 250 °C are currently being evaluated.