UPDATE ON REGULATORY ISSUES ASSOCIATED WITH CRITICALITY SAFETY OF SPENT FUEL CASKS CONSIDERING WATER INLEAKAGE

Criticality safety requirements in the U.S. specify that a package must be designed and the contents limited so that a single package is subcritical with water in the containment system. Along with this provision, the regulations state that a single package must be subcritical under normal and accident conditions. The regulations allow certain exceptions to these requirements. In May 2003, U.S. Nuclear Regulatory Commission (NRC) staff issued revised technical review guidance regarding the demonstration of subcriticality of a single package under accident conditions. This guidance was issued to address questions regarding the structural integrity of cladding for high burnup fuels, and the possible reconfiguration of the fissile material under accident conditions. For certain cases, the guidance allows the evaluation under accident conditions to be performed without water in the containment system, provided a physical test has demonstrated that the system is water-tight. The guidance specifies that the package must still be shown to be subcritical with water in the containment system with the fuel in the undamaged condition, and considering damage to the packaging. NRC has not approved any spent fuel cask designs that are not subcritical with water in the containment system. However, applicants have indicated that package designs are being developed that may rely on the absence of water for criticality safety. Because of the safety significance of any departure from past practice, the staff is preparing to request Commission guidance regarding this technical issue. This paper will describe recent activities as well as the status of this regulatory issue in the U.S.