Review of results for the OECD/NEA phase VII benchmark: Study of spent fuel compositions for long-term disposal

This paper summarizes the problem specification and results for the Phase VII Benchmark - Study of Spent Fuel Compositions for Long-Term Disposal (Ref. 1) organized by the Organisation for Co-operation and Development Nuclear Energy Agency Expert Group on Burn-up Credit Criticality Safety. The Phase VII benchmark was developed to study the ability of computer codes and associated nuclear data to predict spent fuel isotopic compositions and corresponding effective neutron multiplication factor (k_eff) values in a cask configuration over the time duration relevant to spent nuclear fuel disposal. The benchmark was divided into two sets of calculations: (1) decay calculations out to 1,000,000 years for provided pressurized-water-reactor UO₂ discharged fuel compositions and (2) burnup credit criticality calculations for a representative cask model at selected time steps. Contributions from 15 organizations in 10 countries were submitted to the Phase VII benchmark exercise. This paper provides a description of the Phase VII benchmark and detailed comparisons of the participants' isotopic compositions and k _eff values that were calculated with a diverse set of computer codes and nuclear data libraries. Differences observed in the calculated time-dependent nuclide densities are attributed to different decay data or code-specific numerical approximations. The variability of the k_eff results is consistent with the evaluated uncertainty associated with cross-section data.