Performance considerations for used BWR fuel in dry storage and transportation

Used nuclear fuel is currently stored onsite at 35 boiling water reactors (BWRs) in the United States. Some of this BWR fuel has been transferred to dry-storage canisters that are stored onsite, but the vast majority still remains in the spent fuel pools. When considering the state of used fuel management in the United States, it is clear that in the near term, more BWR fuel will be transferred to dry-storage canisters with the understanding that it will eventually be transported offsite (preferably in the same dry-storage canisters used for onsite storage). The objective of this paper is to evaluate the performance of used GNF fuel at exposures between 40 GWd/MTU and 55 GWd/MTU, under both dry-storage and transportation conditions. Metallography shows that the hydrogen content increases with exposure and hydride reorientation can occur. However, the potential detrimental effect of hydrogen may be partially mitigated by the presence of a zirconium liner, which creates a hydrogen depleted zone near the liner during cooling. The stress states for both dry-storage and transportation conditions are evaluated. The stress state under normal dry-storage conditions is such that only time-dependent failure mechanisms are active (creep and slow-crack growth). However, the time-dependent nature of these failure mechanisms results in depressurization that will limit the failure size (if failure actually occurs) - maintaining the confinement function of the cladding. For transportation, the effects of bending on the axial stress were considered. It is argued that hydrides in BWR fuel will have limited effects on axial fatigue meaning the O'Donnell-Langer fatigue limit is applicable for evaluating fatigue of GNF fuel in transportation.