Sensitivity-Guided Nuclide Rankings of Non-LWR Inventory for Storage and Transport Applications

Reactor operations produce a range of radionuclides, and fuel storage and transportation package applications span multiple analytical areas, including decay heat, radiation shielding, and criticality safety. Analysis of these systems is dominated by unique sets of primary nuclides and is based on physical properties such as half-life, decay energy, and cross section. NUREG/CR-6700 ranks nuclide contributions to light-water reactor (LWR) spent nuclear fuel decay heat, shielding, and criticality analyses to assess the importance of these nuclides for each analytical field. A forthcoming NUREG/CR includes a similar study for five representative non-LWR reactor designs. Non-LWR systems have different specific powers, final burnups, initial enrichments, and fuel forms than LWRs. Fuel is sometimes mobile, with several designs implementing liquid fuel or flowing pebbles. Although the underlying physics of fission is the same and the same physical data are important for analysis, determining the important nuclides in these reactor systems is warranted. In terms of isotopic concentration and distribution, typical LWR spent nuclear fuel differs significantly from high-burnup fuel or fuel with online refueling and noble metal plating. Analysis will determine if meaningful changes occur to the ranking of primary nuclides. NUREG/CR-6700 criticality safety rankings were determined by tallying absorption reaction rates, which weight capture and fission equally while ignoring scattering contributions. An effort to examine rankings by applying sensitivity and uncertainty (S/U) methods is described here. The expectation is not that S/U provides a more accurate assessment of primary contributors but rather that S/U provides importance rankings from a different perspective. Sensitivity coefficient calculations are limited by higher computational uncertainties and have magnitude and direction, offsetting when integrated into a total sensitivity value. Light elements (e.g., FLiBe) and fissile species have slightly elevated rank. Otherwise, important nuclides are generally consistent.