United States Nuclear Regulatory Commission - Protecting People and the Environment

Burnup Credit PIRT Report (NUREG/CR-6764)

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Publication Information

Manuscript Completed: December 2001
Date Published: May 2002

Prepared by:
G. H. Bidinger, R. J. Cacciapouti, J. M. Conde,
P. Cousinou, T. Doering, P. Grimm, H-R Hwang,
W. J. Lee, D. Marloye, R. L. Murray, J-C Neuber,
M. B. Raap, J. Saptya, D. A. Thomas, S. Turner,
R. E. Wilson, B. Boyack, D. J. Diamond

Brookhaven National Laboratory
P.O. Box 5000
Upton, NY 11973-5000

R. Y. Lee, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6587

Availability Notice


When fuel is irradiated in a reactor, the isotopic composition, and therefore the reactivity, of the fuel changes with burnup. Nevertheless, until recently, the NRC's approval of the criticality safety evaluations for commercial spent fuel in casks had been based on analyzing the spent fuel as though it were unirradiated. This "fresh fuel" assumption provided a well-defined, straightforward, and bounding approach that eliminated the need for information on the reactor operating conditions and reactor power history and the core history of individual fuel assemblies. However, while being bounding, the fresh fuel assumption leads to overly conservative cask designs. To address this conservatism, the NRC issued the Spent Fuel Project Office Interim Staff Guidance - 8 Rev. l based on information available at the time. ISG8R1 allows limited bumup credit (BUC) in the criticality safety analyses of Pressurized Water Reactor (PWR) spent fuel in transport and storage casks. In the same time frame of ISG8R1, the NRC initiated a research program to: clarify guidance on acceptable technical approaches to BUC; develop approaches for expanding the range of BUC; and address regulatory needs for safe, simple, and cost-effective implementation of BUC. As an element in this research program, the NRC commissioned the formation of a Phenomena Identification and Ranking Table (PIRT) panel. Two objectives of the PIRT panel were to 1) identify and rank phenomena, processes, and parameters, that influence the effective neutron multiplication factor (keff) for irradiated nuclear fuel in transport and storage configurations, and 2) assess the worth of experimental measurement facility classes for validation of calculational methods. Results of the panel's findings are presented in this report.

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