Modeled Uncertainty Components For The High Efficiency Neutron Counters At The Savannah River Site

Prior to disposal at the Waste Isolation Pilot Plant (WIPP), packages must be non-destructively assayed to verify the contents meet the WIPP acceptance criteria. As part of the acceptance criteria, twice the assay uncertainty is added to the fissile content of the package, which must be below the appropriate limit. Therefore, the assay uncertainty directly limits the quantity of fissile material that can be loaded. Experts from multiple Department of Energy laboratories have studied various measurement techniques that will give the lowest assay uncertainty on the plutonium content of Criticality Control Overpacks (CCOs) produced at the Savannah River Site.Neutron coincidence counting was chosen as the preferred method for assay of the material. Two High-Efficiency Neutron Counters (HENCs) have been procured for these measurements. The majority of the material consists of relatively pure plutonium oxide. However, a portion of the material consists of scrap oxide containing low-Z elemental impurities, which interact with alpha particles to create additional neutrons. As a result, these impure items tend to give an erroneously high reading on the HENCs.Monte Carlo modeling was performed to break down the total measurement uncertainty into its individual components, such as source distribution and location, chemical form, and multiplication. In particular, (alpha, n) neutrons induce additional fissions in the plutonium, resulting in a positive bias and large random uncertainty. Because the energy of these neutrons does not match the fission spectrum, these biases persist even when using multiplicity or other analysis techniques.Plutonium-bearing materials have been sorted into families based on their elemental impurities. Working standards will be made for each family in order to determine the precision and accuracy of the HENC assay. The combination of families and working standards has been chosen to minimize the HENC uncertainty, and thus maximize plutonium loading of the CCOs.