UO2 and MOX fuel behaviour in long term dry storage

Germany's new energy act, dated 2002, stipulates prohibition of reprocessing as of 1 July 2005 and nuclear waste disposal restricted to a final repository. To avoid large-scale transportation, nuclear plant operators shall construct on-site storage facilities, mainly for dry cask storage, to keep spent fuel assemblies until a final repository is available. This paper deals with cladding integrity assessment, which is usually assessed by means of creep laws and rupture criteria. A stress limit of 120 MPa and a maximum strain of 1% are used in Germany to avoid systematic cladding degradation. The strain calculations are based on a creep formula of non-irradiated fast creeping cladding, whereas the straining capability is derived from similar irradiated material. The straining capability was originally determined by short-time creep-burst experiments on high burnup PWR fuel rods with commercial Zry cladding. The experiments revealed a high straining capability of about 2% and no degradation with regard to increased oxide thickness and hydrogen content of the cladding. The straining capability has been also evaluated for M5 cladding by creep tests on irradiated material and strains of at least 1% without defect are guaranteed for a large range of temperatures. Future fuel cycle developments are featured by an even higher discharge burnup and intensified use of MOX fuel. As a result, rod inner pressures and storage temperatures will increase and a considerable percentage of rods will exceed the 1%-strain limit if calculated by creep laws based on non-irradiated material. To cope with this more demanding situation, unnecessary conservatism has to be replaced by a more realistic methodology. Extensive programmes are in progress on commercial materials in order to derive more predictive creep laws and rupture criteria for irradiated cladding.