Use of EPRI Depletion Benchmarks for Transport Criticality Burnup Credit

EPRI sponsored the generation of depletion reactivity benchmarks that can be used to validate burnup credit for spent fuel casks. These benchmarks are measurement based using 44 cycles of flux maps to infer the depletion reactivity. The benchmarks have been analyzed using SCALE 6.1 and ENDF/B-VII. The agreement between the 11 benchmarks at burnups from 10 to 60 GWd/T and three cooling times with SCALE calculated values is between -0.0026 and +0.0028 in delta k. The uncertainty in the benchmarks is 0.0064 in delta k. The depletion reactivity bias and uncertainty is much less than the 0.015 delta k uncertainty (approximate since burnup dependent) for just the isotopic content uncertainty for 28 isotopes recently released as part of the US NRC’s ISG-8 Rev. 3. The EPRI benchmarks have a number of advantages for criticality analysis when compared to chemical assay based validation: 1) The benchmarks cover the change in reactivity from all isotopes, not just the 28 where assay data is available. This increased knowledge of reactivity allows for more cost effective cask designs. 2) The benchmark analysis closely matches the criticality safety analysis. The depletion analysis models for the benchmarks can be nearly identical to the depletion models for the cask criticality. The chemical assay depletion models must be tailored to pin details rather than assembly average conditions creating a modeling disconnect between the validation models and the criticality safety models. 3) The uncertainty in the chemical assays is so large that modeling insufficiencies are difficult to see. The uncertainty in the EPRI benchmarks is much smaller, which makes seeing the impact of modeling changes possible. 4) Analysis of the EPRI benchmarks is simple by design. This simplicity makes it more likely that the criticality analyst will do their own validation rather than a specialist which is needed to match the chemical assays. 5) Fewer low capacity casks would be needed to accommodate the existing spent fuel inventory.