UO2 and MOX fuel behaviour in long term dry storage

Full text: On April 27, 2002, Germany's new nuclear energy act, which replaces the act dated 1959, came into force. Instead of promoting the nuclear energy use its purpose is an orderly termination on the basis of an average plant operational life time of 32 years. Focal points related to fuel assemblies are as follows: - As of July 1, 2005 the delivery of spent fuel elements for reprocessing will be prohibited and nuclear waste disposal restricted to a final repository. - The operators of nuclear power plants will be requested to construct on-site storage facilities and to keep spent fuel assemblies until a final repository is available. Since the new on-site installations will be mainly of the dry type, dry cask storage will gain more and more importance. A central requirement on dry cask storage systems is to meet safety functions. For fuel assemblies, this means integrity of the assembly structure and exclusion of systematic cladding failures for the envisaged storage period. This paper mainly deals with cladding integrity assessment, since the behaviour of fuel assembly structure is largely determined by the mechanical design and has shown to undergo no adverse timely changes. The cladding behaviour, however, is dependent on material properties and irradiation as well as boundary conditions of reactor operation and storage. Cladding integrity is usually assessed by means of creep laws and rupture behaviour, which can be obtained under various experimental conditions. A stress limit of 120 N/mm2 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 determined by short-time creep-burst experiments on high burnup PWR fuel rods with commercial Zry cladding. Burnup was up to 64 MWd/kgU and oxide layer thickness up to 100 μm. 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, an unnecessary conservatism with regard to the experimental verification, for example short term testing at high stress levels or the definition of a constant strain capability neglecting temperature benefits, has to be replaced by a more realistic methodology. Therefore an extensive program is in progress on M5 in order to derive more predictive creep laws and rupture criteria for irradiated cladding. On the other hand, variations of the storage period in the range of 20 up to 100 years do not affect integrity assessment methodologies. (author)