United States Nuclear Regulatory Commission - Protecting People and the Environment

Technical Basis for Peak Reactivity Burnup Credit for BWR Spent Nuclear Fuel in Storage and Transportation Systems (NUREG/CR-7194)

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Publication Information

Manuscript Completed: December 2014
Date Published: April 2015

Prepared by:
William (B.J.) Marshall
Brian J. Ade
Stephen M. Bowman
Ian C. Gauld
Germina Ilas
Ugur Mertyurek
Georgeta Radulescu

Oak Ridge National Laboratory
Managed by UT-Battelle, LLC
Oak Ridge, TN 37831-6170

M. Aissa, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Criticality safety analyses of pressurized-water-reactor (PWR) spent nuclear fuel (SNF) assemblies in storage and transportation casks frequently take credit for reactivity reduction during depletion. This credit is commonly referred to as "burnup credit" (BUC) as outlined in U.S. Nuclear Regulatory Commission (NRC) Interim Staff Guidance (ISG) 8, Revision 3, "Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transportation and Storage Casks." However, such credit for boiling-water reactor (BWR) SNF is not addressed in ISG 8. The focus of this report is to document studies performed to provide a technical basis for the application of BUC in storage and transportation casks using BWR peak reactivity methods.

Most BWR fuel assemblies contain gadolinium oxide (Gd2O3, or gadolinium) burnable absorber in some fuel rods. The gadolinium absorber depletes more rapidly than the fuel during the initial part of its irradiation, which causes the fuel assembly reactivity to increase and reach a maximum value at an assembly average burnup typically less than 20 gigawatt days per metric ton of uranium (GWd/MTU). Then the reactivity decreases for the remainder of fuel assembly irradiation. Criticality analyses of BWR spent fuel pools (SFPs) typically employ what are known as peak reactivity methods to account for this behavior in the SNF. Some peak reactivity methods correlate the peak reactivity value in storage to the infinite multiplication factor (kinf) in the standard cold core geometry (SCCG). The SCCG is an infinite planar array of fuel assemblies in reactor geometry, typically a 6-inch pitch, flooded with full density water at 20°C. The peak reactivity methods are also sometimes referred to as "gadolinium credit."

This report reviews the most commonly used peak reactivity methods in SFP analyses for BWR SNF to provide technical background for potential application to storage and transportation casks, including (1) an examination of the fuel assembly lattice design and operating parameters that affect the burnup and reactivity of the peak reactivity in storage and transportation configurations, (2) validation of these reactivity calculations, and (3) validation of the depleted isotopic inventories in BWR SNF at burnups associated with peak reactivity. Each of these three areas is investigated in detail in this report. This report is focused on peak reactivity, so it applies to fuel assemblies with average burnups of approximately 20 GWd/MTU or less. Burnup credit for BWR fuel assemblies with typical discharge burnups will be addressed in future reports planned over the next few years.

The following parameters have been studied, and their impact on the reactivity of BWR SNF has been quantified for determining potentially limiting conditions: initial fuel composition, number and loading of gadolinium pins, control blade insertion (referred to as "rodded" vs. "unrodded" conditions), moderator void fraction (unrodded and rodded), fuel temperature, specific power, and operating history. The depletion parameters used and the nuclides credited in an analysis will depend on the methodology developed and implemented by the applicant.

A suitable number of critical experiments has been identified to support validation of peak reactivity analysis of BWR SNF. All experiments identified in this report are low-enriched uranium (LEU), water-moderated pin array experiments. Penalty factors have been developed for the unvalidated transuranic, gadolinium, and fission product nuclides included in generic BUC cask (GBC)-68 models used in this report. The sum of these three factors is less than 0.5% Δk. The physics of BWR fuel depletion are well understood, reliable, and predictable in their effects on discharged fuel reactivity near peak reactivity. This study confirms that a conservative set of analysis conditions can be identified and implemented to allow criticality safety analysis of BWR SNF for peak reactivity BUC in storage and transportation casks.

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