Year
1996
Abstract
A standard active wall coincidence counter (AWCC) was modified and installed for nuclear material measurements at the Institute of Physics and Power Engineering (IPPE) in Obninsk. The AWCC is a very versatile instrument: it can be used either in active mode, to assay 235U mass in highly enriched uranium (HEU) items, or in passive mode, to determine 240Pu effective mass in plutoniumbearing items. For initial work at the Obninsk IPPE, two active and one passive measurement configurations were established. For the first active configuration, the standard AWCC active (with AmLi sources) end plugs were lengthened and modified for optimum thermal-neutron-interrogation measurements of single HEU metal and oxide disks. For the second active configuration, an aluminum carousel was fabricated to have the multiple HEU disk capacity of a single, full 50-cm storage tube. This carousel is designed to be used in the standard AWCC configuration, without the nickel ring, in fast-neutroninterrogation mode. From the standpoints of throughput, measurement precision, and accuracy, this configuration (or a similar one) for the standard 50-cm storage tube is preferable to measurement of single HEU disks. Results of measurements using this configuration are not reported in this paper. Finally, for the third measurement configuration, special graphite end plugs were fabricated for passive (no AmLi sources), fast-neutron-interrogation mode measurements of plutonium disks. The measurement cavity will hold single or multiple disks stacked in one or more 25-cm storage tubes. This configuration can also be used for passive measurements of depleted uranium and UO2, as well as NpO2. In summary, for the inventory of disks stored at Obninsk for the Bolshoi Physics Stand critical assemblies, the AWCC can be used for verification measurements of a large proportion of the uranium and plutonium materials and configurations present. This paper describes results obtained to date and future plans.