Reducing Uncertainty Allowance in Core Limits Calculations

To provide adequate margin between the operating state and the core safety limits, allowance for calculational uncertainties is incorporated in the reactor core operating limits. In practice, these uncertainties are estimated by comparing calculations of the limits with benchmark calculations and/or measurements. The resulting uncertainty estimates include the uncertainties of the benchmark and are overly conservative when the benchmark uncertainties are large. An exact statistical method for subtracting the benchmark uncertainty and determining reduced best-estimate calculational uncertainties is derived. The method is applied to the determination of the upper tolerance (confidence) limit on the calculational uncertainty required in core limits analyses.