Passive Active Neutron Radioassay Measurement Uncertainty for Aqueous Sludge Waste

To meet Waste Isolation Pilot Plant waste characterization requirements, a total uncertainty analysis of active-mode measurements of aqueous sludge waste using the Idaho National Engineering and Environmental Laboratory's passive active neutron radioassay system was performed. The uncertainty analysis was based on statistical sampling and verification. PAN system active-mode measurement results were compared to radiochemistry results for 125 waste drums containing aqueous sludge. The radiochemistry data were obtained from core samples of the waste drums. Bias in the PAN system was estimated by regression of the radiochemistry results (assumed to be unbiased) on the PAN measurements. Precision of the PAN measurements was assessed by examination of the variance components associated with the regression. Analysis of the results indicates that a bias correction multiplier of 1.55 should be applied to the PAN aqueous sludge measurements. Without this correction, the PAN measurements are biased low by 35 percent relative to the radiochemistry measurements. With the bias correction, the uncertainty bounds on the expected bias are 0 ± 27%. These bounds meet the Waste Isolation Pilot Plant Quality Assurance Program Plan requirements for radioassay systems.