Assay of Impure Plutonium Oxide wtih the Large Neutron Multiplicity Counter for IAEA Verification of Excess Weapons Material at the Rocky Flats Environmental Technology Site

The large neutron multiplicity counter (LNMC), also known as the 30-gal.-drum neutron multiplicity counter, has now been used successfully by the International Atomic Energy Agency (IAEA) for the Initial Physical Inventory Verification (IPIV) and the first annual Physical Inventory Verification (PIV) of excess weapons plutonium at the Rocky Flats Environmental Technology Site (RFETS). These excess plutonium oxide materials contain a variety of impurities. They are stroed two cans to a 10-gal.-drum. The drums contain from 1.3 to 4.0 kg of plutonium. The isotopic declarations vary from can to can but the material averages 6%240Pu. During the IPIV, 94 samples were measured in the LNMC; 19 were measured during the PIV. The assays for all but a single drum agreed to within three standard deviations of the declared value. This problematic drum could not be measured by the LNMC because of its unusually high neutron emission rate. In this paper we will report on the overall performance of the LNMC in these inspections.