Actinide-Only Burnup Credit for Pressurized Water Reactor Spent Nuclear Fuel - II: Validation

The calculation of isotopic concentrations in spent nuclear fuel (SNF) assemblies and the subcritical multiplication factor of SNF packages are two of the essential requirements of the actinide-only burnup credit methodology. To justify the accuracy of the computed values, the code systems used to perform the calculations must be validated. Here, the techniques used for actinide-only burnup credit isotopic and criticality validation are presented and demonstrated.

Fifty-four chemical assays are included in the isotopic validation benchmark set. To perform the validation, the samples are analyzed to obtain isotopic concentrations for each of the isotopes included in the methodology (²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, and ²⁴¹Am). Correction factors are computed based on the measured and calculated values, which are then used to conservatively bias computed isotopic concentrations.

For the criticality validation, 57 critical experiments are included in the benchmark set. The set is composed of 21 UO₂ and 36 mixed-oxide experiments, which are analyzed to determine the bias and corresponding uncertainty, ultimately resulting in an upper safety limit. This limit represents the maximum computed k_eff value that would be considered subcritical.