Implications Of Large Total Measurement Uncertainty In Policing The Criticality Safety Limit

The fissile gram equivalent (FGE) mass assigned to an item via nondestructive assay and its corresponding total measurement uncertainty (TMU) estimate are often used to determine if the item contents fall below an administrative criticality mass limit for that class of items. Overly simplistic models of the TMU or incorrect application of these uncertainties can result in improper dispositioning of items exceeding both the administrative and actual criticality safety mass limit for the item. This incorrect assessment results from the erroneous assumption that the assay result from which the uncertainty is derived is the same as the true content of the container. The traditional uncertainty analysis applied to nondestructive assay provides an uncertainty that represents the range of assay results consistent with the true mass value, whereas the uncertainty needed should represent the range of true mass values that are consistent with the assay result. For small relative TMU, the impact is insignificant because as the assay value and true (but unknown) values should be “close,” but as uncertainties become larger (e.g., greater than 20%) the potential to mischaracterize an item relative to the criticality safety limits becomes non-negligible. This paper examines the potential impact of large TMU on the proper dispositioning of the item.