Creep properties of non-irradiated Zr1Nb cladding tubes under normal and abnormal storage conditions

The Czech Republic has, for its long-term storage of WWER-440 spent fuel, opted for the dry storage alternative and uses the CASTOR 440/84 casks for this purpose. The maximum burnup of the spent fuel stored under a protective helium atmosphere in these casks is limited to 40 MW · d/kg U and the cladding temperature should not exceed 350 deg. C. This temperature limit was established as a result of corrosion cracking analyses. However, with regard to the low pressure of the fission gases it can be anticipated that corrosion cracking will not be a critical mechanism of the cladding degradation. Thus, the damage is creep - controlled, as it was also indicated in the Russian paper and the temperature of 350 deg. C is unnecessarily conservative. This was a reason, why CEZ, a.s. (the Czech Power Utility), supported by the Ministry of Industry and Trade and the State Office for Nuclear Safety (SONS), initiated an experimental study of Zr1Nb creep behaviour. This paper summarises experimental results of thermal creep of non-irradiated Zr1Nb cladding tubes after recrystallization annealing, which modelled the creep behaviour of dry stored WWER spent fuel under normal and accident conditions. Determination of the storage limiting temperatures, at which there will be no creep caused damage over 40 and 70 year of storage period, taking into account the irradiation and 'cooling' history of fuel, is also discussed. Zircaloy-4 specimens, exposed to creep similarly as Zr1Nb ones, served to verify the tests methodology and the evaluation applied. (author)