Strategies for Application of Isotopic Uncertainties in Burnup Credit

Uncertainties in the predicted isotopic concentrations in spent nuclear fuel represent one of the largest
sources of overall uncertainty in criticality calculations that use burnup credit. The methods used to
propagate the uncertainties in the calculated nuclide concentrations to the uncertainty in the predicted
neutron multiplication factor (keff) of the system can have a significant effect on the uncertainty in the
safety margin in criticality calculations and ultimately affect the potential capacity of spent fuel transport
and storage casks employing burnup credit. Methods that can provide a more accurate and realistic
estimate of the uncertainty may enable increased spent fuel cask capacity and fewer casks needing to be
transported, thereby reducing regulatory burden on licensee while maintaining safety for transporting
spent fuel. This report surveys several different best-estimate strategies for considering the effects of
nuclide uncertainties in burnup-credit analyses. The potential benefits of these strategies are illustrated
for a prototypical burnup-credit cask design. The subcritical margin estimated using best-estimate
methods is discussed in comparison to the margin estimated using conventional bounding methods of
uncertainty propagation. To quantify the comparison, each of the strategies for estimating uncertainty has
been performed using a common database of spent fuel isotopic assay measurements for pressurizedlight-
water reactor fuels and predicted nuclide concentrations obtained using the current version of the
SCALE code system. The experimental database applied in this study has been significantly expanded to
include new high-enrichment and high-burnup spent fuel assay data recently published for a wide range
of important burnup-credit actinides and fission products. Expanded rare earth fission-product
measurements performed at the Khlopin Radium Institute in Russia that contain the only known publiclyavailable
measurement for 103Rh have also been included.