WASTE CRATE ASSAY SYSTEM (WCAS) : ASSAY SOLUTIONS FOR VERY LARGE REMOTE HANDLED CRATES

An advanced passive neutron counter has been designed and fabricated to measure the plutonium content in large remote handled (RH) waste crates. The waste crate assay system (WCAS) was developed under an agreement between Los Alamos National Laboratory, Japan Nuclear Fuel Limited (JNFL), and BNFL Instruments Inc. (BII) to measure the plutonium content in the waste generated in the Rokkasho reprocessing facility. The primary goal of the design was to produce an assay system for large waste containers. The system also includes 200-L drum pallet assay capability. The measurements are based on neutron-time correlation counting of the passive neutron emissions from the 240Pu, and the plutonium isotopic ratios are used to calculate the total plutonium. The system is designed for both RH waste and low-activity plutonium waste. The system permits the measurement of the singles, doubles, and triples rates and the multiplicity mode analysis is used together with the 'add-a-source' method to correct for the matrix materials in the crates. In the multiplicity analysis, the efficiency for counting the neutrons emitted from the crate is directly calculated from the three measured rates. For improved detectability limits, advanced methods have been incorporated in the WCAS-A to reduce the cosmic-ray neutron backgrounds. These methods include statistical filters and truncation of high-multiplicity events. The paper describes the WCAS-A design, performance, and calibration