Effect of 241Am buildup during spent fuel cooling on the decay heat of vitrified high-level waste and post-closure safety assessment

Abstract

Long-term interim storage of spent fuel is a key factor affecting the thermal properties of vitrified high-level waste (HLW). In this study, we observed that increasing the spent fuel cooling-off period from 4 to 100 years decreased the decay heat of vitrified HLW and allowed a volume reduction of up to 13% based on the loading of the waste up to the thermal constraint of the repository. Contrastingly, the glass surface temperature 1000 years after disposal and the temperature-dependent glass dissolution rate increased with the cooling-off period due to 241Am buildup from 241Pu within that period. However, these dissolution rates can be regarded as nearly equivalent throughout the lifetime of glass dissolution because the glass temperature approached the host rock temperature and remained at this temperature for most of its lifetime. Consequently, the impact of 241Am buildup was confirmed to be negligible in the safety assessment of geological disposal with an intrinsically large safety margin.Graphical abstractPlots illustrating the effect of spent fuel cooling-off period on the thermal properties of vitrified high-level waste (HLW) and its long-term disposal safety. The left shows the results of waste volume reduction by higher waste loadings in the context of thermal constraint of the bentonite buffer less than 100 °C. The center shows the dissolution rate after disposal depending on the temperature of glass surface. The right is the result of safety assessment for different dissolution rates.