THE PERFORMANCE OF THE 30-GALLON-DRUM NEUTRON MULTIPLICITY COUNTER AT ROCKY FLATS ENVIRONMENTAL TECHNOLOGY SITE*

In the fall of 1994, Rocky Flats Environmental Technology Site (RFETS) committed to offering approximately one metric ton of excess weapons plutonium for safeguards verification by the International Atomic Energy Agency (IAEA). At that time RFETS decided to offer cans of plutonium oxide of varying purity, stored one or two to a package in 10-gal. drums, and contracted with Los Alamos to obtain a multiplicity counter to measure these materials. A multiplicity counter was deemed necessary because of IAEA experience that the standard coincidence counting techniques routinely used in IAEA inspections would fail for plutonium-bearing materials of variable purity. RFETS also thought that such an instrument would be useful for their domestic safeguards needs. The 30-Gallon-Drum Neutron Multiplicity Counter (NMC) was shipped to RFETS and installed early in 1995. The counter was characterized with *52Cf sources before leaving Los Alamos, and this characterization was later corrected to plutonium through measurements made with an identical instrument. Several potential measurement problems were investigated with the 30-Gallon- Drum FJMC before the IAEA’s initial physical inventory verificationat RFETS in December 1995. These investigations were motivated by a concern that the neutron multiplicity counting technique had never been applied to samples packaged in the manner of those offered for IAEA inspection. Selected drums were measured in the 30-Gallon- Drum NMC, and the results were compared to site inventory values derived from calorimeter measurements and gamma-ray isotopic measurements analyzed by GRPAUT or its TRIFID counterpart, EPICS. Los Alamos helped RllETS interpret the multiplicity counting results relative to these data. In this paper we will present assay data for NMC measurements of over 100 10-gaL drums, most of which were verified by the IAEA in December 1995 at RFETS, and discuss the 30-Gallon-Drum NMC’S accuracy and precision relative to site inventory values. We will discuss the expected precision in both the neutron multiplicity assays and the site inventory values and how neutron counting precision and isotopic measurement precision contribute to the overall precision of the assays obtained with this instrument.