Investigation of Burnup Credit Modeling Issues Associated with BWR Fuel

This report investigates various calculational modeling issues associated with boilingwater-
reactor (BWR) fuel depletion relevant to burnup credit. To date, most of the efforts in
burnup-credit studies in the United States have focused on issues related to pressurized-waterreactor
(PWR) fuel. However, requirements for the permanent disposal of BWR fuel have
necessitated the development of methods for predicting the spent fuel contents for such fuels.
Concomitant with such analyses, validation is also necessary. This report provides a summary of
initial efforts to better understand and validate away-from-reactor spent fuel analysis methods for
BWR fuel. These efforts include: assessment of SAS2H for BWR depletion calculations by
code-to-code comparisons with HELIOS, investigation of SAS2H modeling issues and depletion
assumptions, and finally, analysis of the sensitivity of three-dimensional criticality calculations
to depletion assumptions.
The one-dimensional assembly model approximation within SAS2H appears to yield
consistent results such that a reasonable bias and uncertainty could be determined in the
estimation of assembly-averaged isotopic concentrations. In general, SAS2H overpredicts
nuclide concentrations relative to HELIOS, with the significant exception of 235U. The underestimation
of 235U is shown to be associated with the single fuel enrichment limitation in SAS2H
and increases as a function of burnup. Finally, the effects of variations in the depletion
parameters on the calculated reactivity were observed to be consistent with those shown in a
previous study of PWR depletion modeling.