Criticality and uncertainty assessment of assembly misloading in BWR transportation cask

Abstract

Misloading of a spent fuel assembly could occur during the loading process of transportation casks due to inaccurate burnup records or mistaken assembly identification. Therefore, criticality safety analysis is important for such conditions to ensure that the cask remains subcritical with a sufficient margin. Investigations of assembly misload in PWR spent fuel cask has been conducted in previous studies, a similar analysis is still needed for BWR casks because of the differences between them, such as operational conditions, assembly size, gadolinium rods presence, enrichment variation, and others. Consequently, a quantitative computational criticality safety analysis is conducted in this work for a variety of misloading conditions in a BWR burnup credit shipping cask (GBC-68). The uncertainty in keff estimation due to both nuclear data and statistical sampling has been quantified to ensure that keff uncertainty margin is within criticality bounds. The analysis has been carried out using different modules within the SCALE code system, namely, TRITON, KENO-V.a, and TSUNAMI-3D. The results demonstrate that BWR misloading has different trend than PWR because of the presence of gadolinium rods. Two sets of nuclides have been considered in this analysis to show the credit of the fission products: major actinides and major actinides plus fission products. The peak reactivity burnup is calculated to be 24 GWD/MTU. Assembly misload results illustrate that misloaded assemblies with burnup less than the peak reactivity burnup insert more reactivity than the fresh misloaded assemblies, which is the opposite for PWR where misloaded fresh assemblies insert more reactivity than the burned assemblies. The uncertainty analysis demonstrates that reactivity uncertainty is within 5% for most of the cases considered in this analysis. The estimation of probability of occurrence of misloading accidents is out of scope of this work and it is left as a future work.