Modelling of Fission Gas Release in Fast Reactor Fuels in SEALER-One

Sammanfattning

This thesis implements and evaluates a semi-empirical fission gas release model for UN and UO2 inthe TRANSURANUS fuel performance code, applied to the SEALER-One lead-cooled fast reactor byBlykalla under steady-state and severe unprotected transient conditions.UN shows substantially lower FGR than UO2 in all cases with the semi-empirical model. Having beenin the reactor the same amount of time and under identical conditions at steady state, the mechanisticmodel predicts 1.5% FGR for UN at 12% FIMA versus 21% for UO2 at 16% FIMA. Under a combinedunprotected loss-of-heat-sink, loss-of-flow, and transient over-power event, the mechanistic modelgives 1.5% for UN and 11% for UO2. The semi-empirical Wallenius model predicts 8.9% for UNunder the transient but fails for UO2, aborting at 10% FIMA with 66% FGR due to a feedback loopdriven by the temperature increase in UO2 during irradiation. This behaviour demonstrates a broaderlimitation of steady-state empirical FGR correlations. When applied directly to transients, they respondinstantaneously to the current temperature and burnup and therefore do not account for the finite timerequired for fission gas to diffuse from individual grains to the free volume. Reducing the fission gasmigration barrier from 3.0 eV to 2.0 eV in the Wallenius model, representing oxygen impurity effects,increases UN FGR by approximately a factor of 30 at 20% FIMA. Comparison against ROADRUNNERpredictions using the Storms and Rogozkin correlations places the Wallenius model as a consistentlower bound for low temperatures, with the Rogozkin correlation diverging at higher temperatures. Arecommendation for future work is to re-fit the Wallenius correlation to the ROADRUNNER data.The Wallenius semi-empirical correlation should not be used as a predictive transient FGR modelin its current form for either UN or UO2, although the issue is most severe for UO2 in the presentimplementation. UN is confirmed as a thermally stable fuel candidate with significantly lower FGRcompared to UO2