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Determination of Bias and Bias Uncertainty for Criticality Safety Computational Methods (NUREG/CR-7311, ORNL/TM-2024/3)

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Publication Information

Manuscript Completed: March 2025
Date Published: April 2025

Prepared by:
J. B. Clarity (Currently at Pacific Northwest National Laboratory)
W. J. Marshall
D. E. Mueller
S. S. Powers
B. T. Rearden
S. M. Bowman

Oak Ridge National Laboratory
Oak Ridge, TN 37831

Andrew Barto, NRC Project Manager

Prepared for:
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

Nuclear criticality safety evaluations must demonstrate that operations are subcritical under both normal and credible abnormal conditions, and such evaluations often rely upon computational techniques to determine the neutron multiplication factor for complex three-dimensional systems. Validation of the computer codes and data used to model these systems establishes their suitability for specific applications. The validation activity also determines the computational bias and the uncertainty in that bias that is relevant to the application. The bias is
developed from calculations of known laboratory critical experiments that are similar to the intended application of interest. This report describes techniques that can be used by criticality safety analysts to perform the validation activity, including determination of calculational bias, bias uncertainty, and the application of those values to develop limits that can be applied in safety analyses. This report builds upon earlier works in the criticality safety validation area and incorporates modern analytical techniques developed over the last twenty years, as well as lessons learned from observations of previous validation efforts.

Page Last Reviewed/Updated Thursday, May 01, 2025