THE FACILITATION OF CRITICALITY SAFETY ASSESSMENTS FOR FUEL ASSEMBLIES - Presentation

Packages for the international shipment of fissile nuclear fuel cycle materials require multilateral approval in respect of criticality safety. This involves not only certification and validation by the Competent Authority of the country of origin but also approval by each country through, or into which, the consignment is to be transported. Although the regulations in TS-R-1 which govern criticality safety are unambiguous, the interpretation of these regulations and the assumptions which form the basis of the assessment may differ significantly between the applicant and the various Competent Authorities. As a result, gaining a full set of approvals can sometimes be a lengthy and expensive business. The World Nuclear Transport Institute, WNTI, believes that the approval process can be made more efficient. An improved approach to criticality safety case preparation would be to use consistent methodologies and more realistic assumptions based on reliable data. This could lead to significant reductions in costs and timescales for both applicants and regulators. WNTI has established a working group of criticality experts from its member companies to explore ways to improve the preparation of criticality safety cases. To begin with the working group has concentrated on fuel cycle materials, with the aim of identifying ways to facilitate consistency, reduce the effort and shorten the time involved in obtaining approvals. The main features of the WNTI study have been presented elsewhere [1, 2]. The initial focus has been on new and spent fuel assemblies because often criticality safety cases for these materials present technical challenges. Fuel lattice expansion and fuel pin cladding failure under impact accident conditions have been studied and the principal issues and conclusions reported in [1,2]. Recently other criticality methodologies for important topics relevant to the criticality safety case for new and spent fuel elements, namely enrichment mapping, water ingress, burn-up credit, the deformation of internal components of the package and safety margins have been reviewed. The findings are described below.