NBL Safeguards Measurement Evaluation Program: Uncertainty Estimation in Nuclear Material Accountability Measurements

The NBL Measurement Evaluation (ME) program is designed to provide independent verification of internal quality control as practiced in safeguards analytical laboratories making routine accountability measurements of uranium and plutonium bearing materials. Program participants analyze well-characterized test samples and the results of those analyses are evaluated for accuracy and precision. The average relative deviation of a set of (method/material specific) measurement results with respect to the characterized value is taken as an indicator of accuracy and the standard deviation of the relative deviations is taken as an indicator of precision. Whether the measurements are sufficiently accurate and precise is determined through comparison with the appropriate International Target Values (ITVs) for random [u(r)] and systematic [u(s)] uncertainty components. More recently, increased emphasis is being placed on expressing and using measurement uncertainty in accordance with the Guide to the Expression of Uncertainty in Measurement (GUM). This is done by combining all sources of uncertainty (e.g., uncertainties in characterized values, measurements, and any corrections for systematic effects) that affect each measurement result and incorporating the uncertainty in the interpretation of the results. Some of the potential implications of adopting the GUM method for evaluating uncertainties in nuclear safeguard measurements will be discussed with specific reference to the currently used ITVs and in the contemplated revision of those values.