Impact of Operating Parameters on Extended BWR Burnup Credit (NUREG/CR-7240)

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Publication Information

Manuscript Completed: June 2017
Date Published: January 2018

Prepared by:
Brian J. Ade
William (B. J.) Marshall
Germina Ilas
Benjamin R. Betzler
Stephen M. Bowman

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

Mourad Aissa, NRC Project Manager

NRC Job Code V6452

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

Availability Notice

Abstract

The technical basis for extended boiling water reactor (BWR) burnup credit beyond peak reactivity for spent nuclear fuel (SNF) transportation and dry storage cask systems is under evaluation in a research program being conducted by Oak Ridge National Laboratory (ORNL) under contract with the NRC Office of Research. NUREG/CR-7158, Review and Prioritization of Technical Issues Related to Burnup Credit for BWR Fuel, identified and ranked parameters of importance. The highest ranking parameters (axial coolant density distributions, control blade usage, and axial burnup profiles) were studied in NUREG/CR-7224, Axial Moderator Density Distributions, Control Blade Usage, and Axial Burnup Distributions for Extended BWR Burnup Credit. This report studies several parameters of medium importance, including fuel temperature, operating history, specific power, and bypass water density. A summary of the effect of each of these parameters is provided in this report.

In addition, the correlation of various operating parameters is studied here. Because BWRs use control blades during operation, there can be significant changes to the local axial power shape, coolant density profile, and other parameters when the blades are inserted. Simultaneously using limiting conditions for all parameters may be unrealistic. When the control blades are inserted deeply into the reactor (a limiting condition), the power is reduced, and the void fraction decreases (a less limiting condition). This study identifies the impacts of using assembly-specific conditions for the control blade history, coolant density profile, burnup profile, and fuel temperature profile. Cask reactivity is reduced by using assembly-specific operating conditions versus combining limiting conditions for the individual parameters of interest, but the magnitude of the reactivity reduction varies based on each assembly and its operating conditions. Finally, a summary of the present studies is presented within the context of previous studies. The conclusions summarize the technical basis for extended BWR burnup credit as a result of the past and present studies.