Calibration parameters from Monte Carlo simulations for neutron coincidence assay of MOX (mixed oxide) fuel elements: A substitute for standards

Results from application of a calculational model for the two- parameter (singles and doubles) passive neutron coincidence assay of finished Fast Breeder Reactor (FBR) subassemblies are compared with calibration measurements. Two assay instruments are considered; the Universal Fast Breeder Reactor Subassembly Counter (UFBC) and the Capsule Counter installed at the Japanese Plutonium Fuel Production Facility (PFPF). In the case of US Fast Flux Test Facility (FFTF) fuel, the absolute ratio of calculations to measurements for the multiplication-corrected coincidence calibration constant is +1.1 /+-/ 1.0% (average of four subassemblies) for the UFBC and /minus/1.3 /+-/ 0.6% (average of five subassemblies) for the Capsule Counter. For initial measurements of Japanese fuel in the Capsule Counter, the absolute ratio is /minus/1.0 /+-/ 0.7% for three JOYO subassemblies and +0.8 /+-/ 0.7% for one MONJU subassembly. Calculations of relative effects such as the change in coincidence response from, for example, subassembly can thickness of U enrichment are more accurate (better than 0.5%) than absolute calibration parameters. This very good accuracy offers more effective and less costly inspector verification of finished FBR fuel elements by reducing reliance on physical standards to expand the cross-calibration databases. 11 refs., 8 figs., 5 tabs.