Trends in dry storage cask loading at operating reactor sites

Spent nuclear fuel (SNF) is presently being stored at reactor sites awaiting shipment offsite. Analyses conducted for the US Department of Energy (DOE) to evaluate potential strategies for removing SNF from reactor sites use detailed, agent-based models of SNF storage and transportation. Often these models include assumptions to facilitate analysis. Some assumptions might rely on expert judgement, and others are implicitly made when analysis is performed. One common, implicit assumption is that there is no preferred time of year to load casks. Another assumption is that operating reactor sites can only load a fixed number of casks per year. An examination of cask loading dates might improve the quality of these and other assumptions and provide further insight into industry practice. Investigators reviewed dates listed in cask registration letters submitted to the US Nuclear Regulatory Commission. The cask loading dates were then evaluated to identify patterns that would shed light on the validity of assumptions. First, the typical number of casks loaded in a campaign was examined. Next, the time between cask loadings was examined. Seasonal variations were evaluated, along with variations that appear to occur as a result of plant outage cycles. Finally, some larger loading campaigns were noted. Loading campaign size and the relationship between loading campaigns and outages followed notable trends. For smaller sites with fewer reactors, a relatively large degree of scheduling flexibility appears to be present. However, for sites having more reactors, less flexibility is observed, and the relationship between outages and dry cask loading is clearly apparent. It is clear that fewer casks are loaded during the spring and fall, when outages are more likely. Furthermore, cask loadings are reduced during the winter months.