Implementation of Dynamic Cross-Talk Correction (DCTC) for MOX Holdup Assay Measurements among Multiple Gloveboxes

摘要

Plutonium holdup inventory in gloveboxes are measured by HBAS (passive neutron-coincidencebased 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 neutroncross-talk between the gloveboxes. Historically, we have used a simple fixed-value for cross-talkcorrection 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 issueof variable cross-talk contributions to holdup assay values, we developed a dynamic cross-talkcorrection (DCTC) method, based on the distributed source-term analysis approach, to obtain theactual doubles signal cross-talk between multiple gloveboxes.A response matrix between each detector position and glovebox was determined from an MCNPXmodel of the area and the HBAS counter and benchmarked against calibration measurements. Themeasured doubles rates are used in conjunction with MCNPX-derived cross-talk coefficients todetermine the count rate at each assay glove box that is attributable to the material in the othergloveboxes in the area. The value of the cross-talk correction is determined for each holdupmeasurement campaign, reflecting the relative inventory of the various gloveboxes, and thusrepresenting the actual doubles cross-talk at the time of the measurement.We implemented an improved HBAS system using DCTC at the 2011PIT with authorization byinspectorate. The DCTC improves PCDF NMA by eliminating the double-counting of material thatstems 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 HBASvulnerability 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 assaymeasurements has been determined to be statistically zero by the DCTC analysis. However, we expectthat the cross-talk will become statistically important when PCDF returns to operation in the nearfuture. The DCTC methodology can be used to determine the cross-correlation among multipleinventories in small areas and substantially reduce cross-talk-induced biases in assay results.