The effect of measurement bias on nuclear criticality safety calculations for Wipp TRUPACT-II shipments

Current nuclear criticality safety limit requirements for transporting TRUP ACT-II waste containers to the U,S, Department of Energy's Waste Isolation Pilot Plant (WIPP) specify that the Pu-239 fissile gram equivalent (FGE) pl us two times its measurement error must be less than or equal to 325 g for a payload of fourteen 55-gal drums. The authorized method for calculating a TR UPACT-II FGE measurement error value is to take the square root of the sum of the squared error values for the individual containers (often called root-sum-squares or simply RSS). However, to the extent that the individua l drum measurements contain common bias effects (e,g., due to common calibr ation or other adjustment factors), the corresponding measurement errors ar e correlated, and simple RSS calculations will underestimate the true error in the TRUPACT-II FGE value. The RSS calculations assume independence, while common bins effects can ind uce strong correlations between the errors ill measurements, Significant bi as effects can occur when the matrix characteristics for a particular waste type are not fully accounted for in the measurement process. Depending on the relative size of the bins error compared to precision error, the true m easurement error can be greater than twice that calculated by RSS. In such cases, the FGE shipping requirement may not be met. To avoid underestimatin g the error, bins components should be estimated and propagated separately (combined only at the final step in the TR UPACT-II FGE calculation), or th e effect of bias on covariance between measurements must be calculated Thes e covariance terms then need to be included in the final uncertainty calcul ations.