Year
1996
Abstract
Any measurement capable of directly estimating the fissile material content of a nuclear assembly has obvious utility in safeguards, control and accountability, and criticality safety applications. Such a measurement applied to the fissile mass assay of spent nuclear fuel is clearly usefi.d as a method of obtaining bumup credit for storage pool and shipping cask loading. The feasibility of applying 252Cf-source-driven fi-equency-analysis to perform fissile mass assay of spent nuclear fhel assemblies has been evaluated using MCNP-DSP, an analog Monte Carlo transport code to simulate source-driven measurements. This feasibility study employed models of an isolated Westinghouse 17x 17 pressurized water reactor fhel assembly in a 1500-ppm berated water storage pool. In the models, the fhel bumup profile was represented using seven axial bumup zones, each with isotopics estimated by the PDQ code. Four different fuel assemblies with average bumups ilom fi-eshto 32 GWd/MTU were modeled and analyzed. Analysis of the fiel assemblies was simulated by inducing fission in the fuel using a 252Cf source adjacent to the assembly and correlating source fissions with the response of a bank of 3He detectors adjacent to the assembly opposite the source. This analysis was performed at seven different axial positions on each of the four assemblies, and the source-detector cross-spectrum signature was calculated for each of these 28 simulated measurements. The magnitude of the crossspectrum signature follows a smooth upward trend with increasing fissile material (235U and 239Pu) content, and the signature is independent of the concentration of spontaneously fissioning isotopes (e.g., 2MCm)and (a,n) sources. Furthermore, the cross-spectrum signature is highly sensitive to changes in fissile material content. This feasibility study indicated that the signature would increase -100% in response to an increase of only 0.1 g/cm3 of fissile material.