Year
2012
Abstract
Plutonium holdup inventory in gloveboxes are measured by HBAS (passive neutron-coincidence based NDA, Holdup Blender Assay System) for the nuclear material accountancy (NMA) at PCDF. Because the gloveboxes are installed close to one another, we must make a correction for neutron cross-talk between the gloveboxes. Historically, we have used a simple fixed-value for cross-talk correction that assigns a constant number of counts to the cross-talk effect for any given measurement. Because the relative inventory in the various gloveboxes changes according to the facility operation, the use of a fixed cross-talk correction has been shown to be insufficient. In order to address the issue of variable cross-talk contributions to holdup assay values, we developed a dynamic cross-talk correction (DCTC) method, based on the distributed source-term analysis approach, to obtain the actual doubles signal cross-talk between multiple gloveboxes. A response matrix between each detector position and glovebox was determined from an MCNPX model of the area and the HBAS counter and benchmarked against calibration measurements. The measured doubles rates are used in conjunction with MCNPX-derived cross-talk coefficients to determine the count rate at each assay glove box that is attributable to the material in the other gloveboxes in the area. The value of the cross-talk correction is determined for each holdup measurement campaign, reflecting the relative inventory of the various gloveboxes, and thus representing the actual doubles cross-talk at the time of the measurement. We implemented an improved HBAS system using DCTC at the 2011PIT with authorization by inspectorate. The DCTC improves PCDF NMA by eliminating the double-counting of material that stems from cross-talk in the holdup assay data and eliminates this source of bias in the assay results. We have undertaken considerable clean-out and recovery operations. We have also reduced the HBAS vulnerability to cross-talk by adding shielding and optimizing the operating parameters of the system. As a result of these improvements, the present cross-talk contribution to the holdup assay measurements has been determined to be statistically zero by the DCTC analysis. However, we expect that the cross-talk will become statistically important when PCDF returns to operation in the near future. The DCTC methodology can be used to determine the cross-correlation among multiple inventories in small areas and substantially reduce cross-talk-induced biases in assay results.