Utilization of the EPRI fuel reactivity depletion benchmarks in PWR spent fuel pool criticality analysis - 7-04

The Electric Power Research Institute (EPRI) has developed a set of benchmarks that can be used for quantification of the combined nuclide inventory and cross-section bias and uncertainty in computed reactivity decrements. The EPRI benchmarks are a function of enrichment, operating conditions (such as soluble boron concentration, burnable absorbers, and specific power) and rack conditions. The EPRI benchmarks are analyzed here using SCALE 6.1 with the 44 group ENDF/B-V library. The depletion analyses are performed using the TRITON module (with parm=NITAWL) and the criticality calculations are performed with KENO Va. All changes in reactivity with depletion are within 0.3% Δk of the measured benchmarks at 100-hour cooling. From this analysis it is determined that SCALE 6.1 with ENDF/B-V requires only small biases to conservatively account for the reactivity of spent fuel. The conservative k biases are: 0 until 10 GWd/T; then 0.2% for burnups between 10 and 50 GWd/T; and 0.3% for burnups greater than 50 GWd/T. With decay times of 5 years and 15 years, the SCALE depletion is more conservative. The EPRI benchmarks have a maximum uncertainty of 0.643% Δk. This uncertainty needs to be statistically combined with the uncertainties from the rest of the criticality analysis such as the manufacturing tolerances and the criticality uncertainty from the fresh fuel analysis. Historically, for spent fuel pools an uncertainty of 5% of the Δk of depletion was used (Kopp Memo). The historical approach (except at very low burnups) over predicts the net effect of the bias and uncertainty - which is now based on measured data. (authors)