Utilization of the EPRI Depletion Benchmarks for Burnup Credit Validation

Pressurized water reactor (PWR) burnup credit validation is
demonstrated using the benchmarks for quantifying fuel reactivity
decrements, published as Benchmarks for Quantifying Fuel Reactivity
Depletion Uncertainty, Electric Power Research Institute (EPRI)
report 1022909. This demonstration uses the depletion module
TRITON (Transport Rigor Implemented with Time-Dependent
Operation for Neutronic Depletion) available in the SCALE 6.1
(Standardized Computer Analyses for Licensing Evaluations) code
system, followed by criticality calculations using KENO-V.a and
MCNP (Monte Carlo N-Particle Transport Code System).
The difference between predicted depletion reactivity and
benchmark’s depletion reactivity is a bias for the criticality
calculations. The uncertainty in the benchmarks is the depletion
reactivity uncertainty. This depletion bias and uncertainty is used
with the bias and uncertainty from fresh UO2 critical experiments to
determine the criticality safety limits on the neutron multiplication
factor, keff.
The analysis shows that SCALE 6.1 with the ENDF/B-VII
238-group cross-section library supports the use of a depletion bias of
only 0.0015 in Δk at peak reactivity after discharge from the core.
This peak reactivity occurs after 100 hours of cooling. If credit is
taken for more cooling, the bias should be increased to 0.0025.
Reliance on the ENDF/B-V cross-section library produces larger
disagreement with the benchmarks. Using MCNP for the criticality
calculations rather than KENO-V.a produces essentially the same
results if the same ENDF/B cross-section library is used.
The analysis covers numerous combinations of depletion and
criticality options. Since prediction of the Δk of depletion is within a
few tenths of a percent in k, it is possible to discern the impact of
small modeling changes on the accuracy of the predictions of the
changes in k. The uncertainty associated with the chemical assays is
too large to allow discriminating among many methods. In all cases,
the historical uncertainty of 5% of the Δk of depletion is shown to be
conservative for fuel with burnup greater than 30 gigawatt-days per
metric ton of uranium (GWD/MTU).