An Extension of the Validation of SCALE (SAS2H) Isotopic Predictions of PWR Spent Fuel

Isotopic characterization of spent fuel via depletion and decay calculations is necessary for
determination of source terms for subsequent system analyses involving heat transfer, radiation
shielding, isotopic migration, etc. Unlike fresh fuel assumptions typically employed in the criticality
safety analysis of spent fuel configurations, burnup credit applications also rely on depletion and
decay calculations to predict the isotopic composition of spent fuel. These isotopics are used in
subsequent criticality calculations to assess the reduced worth of spent fuel. To validate the codes
and data used in depletion approaches, experimental measurements are compared with numerical
predictions for relevant spent fuel samples. Such comparisons have been performed in earlier work
at the Oak Ridge National Laboratory (ORNL). This report describes additional independent
measurements and corresponding calculations, which supplement the results of the earlier work. The
current work includes measured isotopic data from 19 spent fuel samples obtained from the Italian
Trino Vercelles pressurized-water reactor (PWR) and the U.S. Turkey Point Unit 3 PWR.
In addition, an approach to determine biases and uncertainties between calculated and
measured isotopic concentrations is discussed, together with a method to statistically combine these
terms to obtain a conservative estimate of spent fuel isotopic concentrations. Results are presented
based on the combination of measured-to-calculated ratios for earlier work and the current analyses.
The results described herein represent an extension to a new reactor design not included in
the earlier work, and spent fuel samples with enrichment as high as 3.9 wt % 235U. Results for the
current work are found to be, for the most part, consistent with the findings of the earlier work. This
consistency was observed for results obtained from each of two different cross-section libraries and
suggests that the estimated biases determined for each of the isotopes in the earlier work are
reasonably good estimates, as the additional measurement/calculated ratios resulting from the current
work tend to confirm these estimates.