Year
2016
Abstract
The inventory of special nuclear material remaining in equipment after a facility has been cleaned-out is called holdup. When possible, in-situ nondestructive assay of holdup is typically performed using simple portable gamma-spectroscopy systems. This is because they are often the lowest cost acceptable solution, in part because they may be deployed most conveniently by operators working in difficult measurement conditions. The established practices for holdup measurements have not substantially changed for many years1,2,3,4). A study of the portable holdup detection process reveals that in fact the process comprises two separate but related functions. The first function comprises the location of the presence of a fissile material deposit. The second is the quantification of that deposit. Each of these functions has differing performance requirements and to accomplish optimal quantification performance and total measurement uncertainty, each must be considered separately. Fissile deposit presence sensing requires a scanning approach combined with a properly designed detector geometry. This then allows profiling of the deposit geometry so that a relevant model can be generated. With a properly understood model, the deposit can be properly quantified through one or more fixed and known geometry measurements. This approach requires that the scans become part of the reportable record5). By understanding and addressing both these components6), the measurement and quantification process can be optimized, in terms of process throughput, total measurement uncertainty6) and compliance with facility quality standards. In this paper we consider the advantages of alternative detector configurations, alternative data acquisition protocols, and alternative inversion algorithms, compared to the familiar HMS-4 approach, to address these challenges in the case of holdup in a gaseous diffusion enrichment plant setting. These approaches are applicable to other scenarios and measurement applications.