Margins for Uncertainty in the Predicted Spent Fuel Isotopic Inventories for BWR Burnup Credit (NUREG/CR-7251,ORNL/TM-2018/782)

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Publication Information

Manuscript Completed: June 2018
Date Published: December 2018

Prepared by:
I. Gauld
U. Mertyurek

Oak Ridge National Laboratory
Bethel Valley Road
Oak Ridge, TN 37831

Mourad Aissa, NRC Project Manager

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

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US Nuclear Regulatory Commission staff guidance on burnup credit for spent fuel storage and transportation is currently limited to spent fuel assemblies from pressurized water reactors. This report describes research to develop a technical basis to expand burnup credit to boiling water reactors (BWRs). One of the largest components of uncertainty in burnup credit analyses is the predicted isotopic inventories of spent fuel as applied to the criticality safety analysis application model. The analysis of BWR fuel inventories is challenging due to the complexity of BWR assembly designs, the lack of publicly available radiochemical assay measurements, and limited access to documentation on fuel design and operating conditions. This study has compiled and evaluated experimental data on measured nuclide concentrations in commercial spent fuel for more than 75 fuel samples that cover a wide range of modern assembly designs and operating conditions. These data were applied to predict the net effect of isotopic uncertainties on the effective neutron multiplication factor for a representative spent nuclear fuel storage system. The experimental data, uncertainty analysis methodology, and results for a dry storage cask application system are described. The uncertainty analysis methodology presented in this report is independent of the depletion analysis code and the application model and can be easily adopted to estimate margins of uncertainty for other codes, nuclear data libraries, and application models. The results are only applicable to BWR burnup credit beyond peak reactivity where any initial gadolinium present in the fuel has been fully depleted.

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