CANDU fuel long-term storage and used-fuel integrity

Technologies for long-term dry storage in helium or air have been developed for used CANDU fuel. Used fuel dry storage facilities are presently in operation or under construction at all Canadian nuclear power stations. The evaluation of long-term integrity of stored used fuel started in 1977. Direct examination of fuel stored under water for up to 27 years has indicated that CANDU fuel, defected or not, could be safely stored. The Easily Retrievable Basket and Controlled Environment Experiments have provided information about potential impacts of aging mechanisms during used fuel storage in air for times up to 20 years. The used fuel aging investigation focused on four mechanisms that are believed could have long term impact on used fuel integrity: Stress Corrosion Cracking (SCC), UO2 oxidation, creep and Hydrogen Assisted Cracking (HAC). As a result it was concluded that under normal dry storage conditions it is unlikely that CANDU fuel will suffer significant degradation during a period of 100 years. Additional investigation is still necessary in the areas where there is a higher uncertainty in the prediction of used fuel condition and for the degradation processes that are potentially more aggressive. Additional work is also necessary to evaluate the possible effects of abnormal/accident conditions on the integrity of fuel. In 2004 a new program on used fuel integrity was started. The outlines of this program will be discussed. In the first phase of this project, the focus is on a review of the used fuel characteristics at the beginning of dry storage, and on studies of likely stresses that the fuel will be exposed to during handling and transportation after dry storage. In addition, preservation of relevant information (form and content) about the used fuel during a 100-year period is also being studied. In the first phase most of the investigation is carried out by Nuclear Safety Solutions (NSS). After the first phase additional examination and testing of used fuel is planned. (author)