Actinide-Only Burnup Credit for Pressurized Water Reactor Spent Nuclear Fuel - I: Methodology Overview

A conservative methodology is presented that would allow taking credit for burnup in the criticality safety analysis of spent nuclear fuel (SNF) packages. The method is based on the assumption that the isotopic concentration in the SNF and cross sections of each isotope for which credit is taken must be supported by validation experiments. The method allows credit for the changes in the 234U, 235U, 236U, 238U, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 241Am concentration with burnup. No credit for fission product neutron absorbers is taken. The methodology consists of five major steps:

1. Validate a computer code system to calculate isotopic concentrations of SNF created during burnup in the reactor core and subsequent decay. Chemical assay benchmarks are used for this purpose, in conjunction with a method for assessing the calculational bias and uncertainty for each isotope.

2. Validate a computer code system to predict the subcritical multiplication factor keff of an SNF package by use of UO2 and UO2/PuO2 critical experiments. The method uses an upper safety limit on keff (which can be a function of trending parameters) to ensure that the calculated keff when increased for the bias and uncertainty is <0.95.

3. Establish conditions for the SNF (depletion analysis) and package (criticality analysis) that bound keff. Bounding axial and horizontal profiles must be established to ensure that the "end effect" and "horizontal effect" are accounted for conservatively.

4. Use the validated codes and bounding conditions to generate package-loading criteria (burnup credit loading curves). Burnup credit loading curves show the minimum burnup required for a given initial enrichment.

5. Verify by measurement that SNF assemblies meet the package-loading criteria, and confirm proper assembly selection prior to loading.