Comparison of Irradiated TRISO Fuel Radioactivity from Multiple Advanced Reactor Designs

The SCALE code system was used to model, deplete, and compare several different tri-structural isotropic (TRISO)–fueled reactor designs: a helium-cooled prismatic reactor, a helium-cooled pebble-bed reactor (PBR), and a fluoride-lithium-beryllium (FLIBE) molten-salt-cooled PBR. The purpose of this comparison was to understand how differences in the reactor designs affect the radioactivity of the fuel after discharge and whether those differences are significant. First, the various reactor designs were built and depleted in the TRITON module for each design and fuel enrichment. Then, the TRITON outputs were used to create burnup-dependent reactor libraries. These libraries were then used by ORIGEN to determine the activities of discharged fuel for each reactor, which were compared to generic Westinghouse 17 × 17 fuel.

Overall, the results showed that short-term activities are dominated by reactors with higher operating powers, and the reactor type, initial fuel enrichment, and maximum burnup are of only secondary importance. Although this analysis only focuses on activities in Becquerels, these dependencies are consistent with the expected behavior of decay heat. However, analysis of long-term time periods post irradiation shows that the reactor type and maximum burnup have strong impacts on the activities; initial fuel enrichment has a secondary impact while operating power is inconsequential.

These results would be useful for analyses, such as dose assessment and modeling in postrelease scenarios, normal fuel handling operations, and spent fuel transport, storage, and disposal. Of particular interest, the results in this technical note show that analyses that focus on spent nuclear fuel of advanced reactors need to consider each parameter carefully. Unsurprisingly, if the correct operating power is not used in short-term analyses, the results will not be correct. Perhaps unexpectedly, however, if the correct reactor type is not used, then the long-term results will also be incorrect, especially for areas such as permanent disposal. Even though this technical note focuses on the total activity of nuclear fuel, it provides initial results on the effects of various input parameters and also provides a framework to extend the work into other analyses of spent fuel from advanced reactors, especially those employing TRISO fuel.