Isotopic Models for Commercial SNF Burnup Credit

Disposal Criticality Analysis Methodology Topical Report1 describes a methodology for performing postclosure criticality analyses within the repository at Yucca Mountain, Nevada. An important component of the postclosure criticality analysis is the calculation of conservative isotopic concentrations for spent nuclear fuel. This report documents the isotopic calculation methodology. The isotopic calculation methodology is shown to be conservative based upon current data for pressurized water reactor and boiling water reactor spent nuclear fuel. The isotopic calculation methodology is to be used for conservatively calculating isotopic concentrations for criticality calculations for commercial spent nuclear fuel waste forms. The
isotopic calculation methodology uses the SAS2H control module of the SCALE code system to apply the transition matrix method along with a nuclear data library to solve the transmutation and radioactive decay equations that describe the isotopic changes as fuel is irradiated in a reactor. Bounding parameters are chosen to ensure that the calculated reactivity of spent nuclear fuel is conservative. Radiochemical assay data or commercial reactor critical data are used to test the bounding parameter set to ensure that they produce conservative reactivity results for the enrichment and burnup ranges. The isotopic calculation methodology also provides the isotopic component of the critical limit, as required in Disposal Criticality Analysis Methodology Topical Report1. The use of conservative, bounding parameters ensures that the ΔkISO term of the critical limit equation is zero, indicating that no additional isotopic term is needed.