Determination and Application of Partial Biases in Criticality Safety Validation

This paper presents a method for determining partial biases and bias uncertainties for application in fission product burnup credit criticality safety analysis. The contribution of each nuclide to the overall system keff bias and the bias uncertainty are determined via the generalized linear least squares method. Where experimental benchmarks are available to validate specific nuclides, sensitivity and uncertainty analysis is used to project biases observed in the benchmarks to biases appropriate for the safety system. Two weighting schemes are proposed to produce an overall bias in the safety system from several single partial biases. Finally, these methods are used to determine partial biases for ¹⁴⁹Sm and ¹⁰³Rh from two experiment series and to apply these biases to a representative used fuel safety system. The biases obtained are compared to bounding estimates, and the sensitivity of the results to relevant assumptions is addressed.