Abstract
Processing of irradiated UAl targets by NaOH dissolution for the extraction of 99Mo results in the creation of high-activity highly enriched uranium (HEU)–bearing residues that must be quantified. The HEU residues generated from this process are assayed using an Active Well Coincidence Counter (AWCC) that has been modified to accommodate >1 Sv/h gamma-ray exposure rates to provide a 235U mass value for each residues container. The HEU residues include a significant amount of residual moderator from the dissolution process. Fission product decay heat drives off much—but not all—of the hydrogen from the dissolution, leaving an unknown moderator content in the residues. The moderator content impacts the induced fission rate in the HEU, detection efficiency, and die-away time of the emitted neutrons and requires implementation of a moderator correction factor. Three moderator correction techniques have been implemented: an active coincidence ring ratio method, a conventional 252Cf Add-A-Source (AAS) measurement, and an AAS ring ratio measurement. The 252Cf AAS and the AAS ring ratio methods are potentially much faster, but they require additional sample handling and precise positioning of the residues container and the 252Cf source. This study was performed using a combination of Monte Carlo N-Particle (MCNP) [1] simulation and measurements. The impact of the moderator content on the AWCC measurement and the relative performance of the three moderator correction methods are presented.
