AN INDUSTRIAL APPLICATION OF THE BURNUP CREDIT INCLUDING ACTINIDES AND FISSION PRODUCTS FOR PWR UO_2 USED FUEL TRANSPORTATION

摘要

Criticality safety analyses related to the transportation of PWR uranium oxide (UO_2) used fuel assemblies are usually performed under the fresh fuel assumption. This leads to huge safety margins, as the negative reactivity worth due to the irradiation of the assemblies is not taken into account. Since 1987, AREVA TN has been using a limited burnup credit method based on the sole consideration of major actinides and the use of a uniform axial burnup profile corresponding to the mean burnup at the 50 least-irradiated centimetres of the assembly. This method is quite limited today for some applications, due mainly to the increase of the fuel enrichment in PWR UO_2 fuel assemblies. The strategy of AREVA TN to deal with high enriched PWR UO_2 fuel assemblies and also to limit the increase of the neutron poison content in the new baskets' designs is to take benefit from the reactivity reserve, which can be gained by considering main fission products and less penalizing axial burnup profiles instead of uniform axial burnup profiles. The Burnup Credit (BUC) calculation route for PWR UO_2 used fuel is based on the connection of the depletion code DARWIN (developed by the CEA~1) and the Criticality Safety Package CRISTAL VI (developed by the CEA and the IRSN~2 in partnership with AREVA & EDF~3). French BUC experimental programs have been also separately performed in Cadarache and in Valduc in order to validate respectively the APOLLO2-DARWIN2 depletion code and the CRISTAL VI Criticality Safety package. This paper presents the BUC methodology including actinides and fission products but also the approach used for the criticality safety assessment of representative transport cask loaded with PWR UO_2 used fuel assemblies approved for the first time by the French Safety Competent Authority.