Benchmarks for Quantifying Fuel Reactivity Depletion Uncertainty

Analytical methods, described in this report, are used to
systematically determine experimental fuel sub-batch
reactivities as a function of burnup. Fuel sub-batch reactivities
are inferred using more than 600 in-core pressurized water
reactor (PWR) flux maps taken during 44 cycles of operation
at the Catawba and McGuire nuclear power plants. The
analytical methods systematically search for fuel sub-batch
reactivities that minimize differences between measured and
computed reaction rates, using Studsvik Scandpower’s
CASMO and SIMULATE-3 reactor analysis tools. More
than eight million SIMULATE-3 core calculations are used to
reduce one million measured reaction rate signals to a set of
2500 experimental fuel sub-batch reactivities over the range of
0 to 55 gigawatt-days per metric ton (GWd/T) burnup.
Experimental biases derived for the CASMO lattice physics code
were used to develop a series of experimental benchmarks that can be
used to quantify reactivity decrement biases and uncertainties of
other code systems used in spent-fuel pool (SFP) and cask criticality
analyses. Specification of 11 experimental lattice benchmarks,
covering a range of enrichments, burnable absorber loading, boron
concentration, and lattice types are documented in this report.
Numerous tests are used to demonstrate that experimental reactivity
burnup decrements are insensitive to the specific lattice physics codes
and neutron cross-section libraries used to analyze the flux map data.
Experimental results also demonstrate that CASMO hot full power
(HFP) reactivity burnup decrement biases are less than 250 pcm over
the burnup range from 0 to 55 GWd/T, and corresponding 2σ
uncertainties are less than 250 pcm. The TSUNAMI tools of Oak
Ridge National Laboratory’s SCALE 6 package were used to extend
HFP results to cold conditions, and cold reactivity burnup decrement
uncertainties increased to approximately 600 pcm.
This report provides a basis for quantification of combined nuclide
inventory and cross-section uncertainties in computed reactivity
burnup decrements. Results support the Kopp Memo 5% reactivity
decrement uncertainty assumption, often applied in SFP criticality
analysis, which is shown to be both valid and conservative for
CASMO-based fuel depletion analyses.