ENRICHMENT AND URANIUM MASS FROM NMIS FOR HEU METAL
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Recently, the Nuclear Materials Identification System (NMIS) was employed to verify both the mass and enrichment of a number of high-enriched uranium (HEU) metal items stored at the Oak Ridge Y-12 Plant. NMIS was applied for this measurement as an active neutron interrogation system; the measurement performed was similar in nature to a pulsed neutron measurement in that the distribution of detector counts following source emission was accumulated and subjected to analysis. In order to develop a calibration “surface” versus mass and enrichment, Monte Carlo models of the item geometry were employed to calculate the distribution for a matrix of independently varying masses and enrichments spanning the nominal declared mass and enrichment of the actual items. Each calculated distribution was then decomposed into its moments, and an empirical model of each moment as a function of mass and enrichment was developed. These models were then simultaneously analytically inverted to yield a nonlinear calibration surface that predicted mass and enrichment given the low-order moments of a measured distribution. The uncertainty in the calibration was estimated using standard squared-error minimization and propagation-of-error techniques, and the bias (and its uncertainty) in the calibration was evaluated using measurements of several randomly selected samples. No single mass or enrichment verified using this method deviated from declaration by more than 5.0%. The typical, i.e., root-mean-squared, error in mass was only 1.6% of the declaration, and the typical error in enrichment was only 1.5% of the declaration. As a result, a finding previously issued for this storage area by the Department of Energy, Oak Ridge Operations, (DOE-ORO) was resolved. INTRODUCTION In 1999, DOE-ORO issued a finding for a particular storage area at the Oak Ridge Y-12 Plant; to be resolved, this finding required that the mass and enrichment of a number of HEU metal items stored in the area be verified. However, the Oak Ridge Y-12 Plant Nuclear Materials Control and Accountability (NMC&A) Organization determined that it could not, by standard techniques, safely verify the mass and enrichment of HEU metal items in this particular storage area due to the significant potential for the spread of airborne contamination. Ordinarily, the mass of these items would have been measured using a scale, and their enrichment would have been estimated using passive gamma spectrometry. However, the former required the items to be removed from their storage containers, and the latter was similarly difficult to implement unless the items were removed from their storage containers. Unfortunately, no hood that could be transported to the storage area was available, and radiological control dictated that a fixed hood outside the area could not be used extensively without risking the spread of airborne contamination to the rest of the facility. In any event, the only suitable fixed hood in the same facility was at that time dedicated to higher priority tasks. Consequently, the Oak Ridge National Laboratory was asked if it would be possible to use the Nuclear Materials Identification System (NMIS) to verify the mass and enrichment of these items in-situ, i.e., without removing them from their containers and without removing the containers from the storage area. Chiang, et al., describes the operational aspects of these verification measurements. Prior to this, NMIS had never been used to estimate the enrichment of uranium metal. After performing a series of measurements on ten samples selected at random from the storage area, the subsequently described method to simultaneously estimate both the mass and enrichment of these HEU metal items was developed.