MEASURING IN-PROCESS INVENTORY BY SPECTROSCOPIC IMAGING

A commercial, scintillator-based gamma-ray spectroscopic imaging system has measured the plutonium in-process inventory in a continuous-feed, multi-stage cascade dissolver designed for difficult matrices such as residues, ash and impure oxide inputs to nitrate aqueous recovery. Safeguards requires frequent, time-consuming and resourceintensive cleanouts of the high-throughput dissolver to account for plutonium. This is because propagation of variance in the measured quantities of feed and product causes an ever-increasing uncertainty in the relatively constant in-process inventory determined “by difference.” Analysis of the two-dimensional spectroscopic image data obtained in a short, nonintrusive measurement gives the inventory quantity with a fixed uncertainty regardless of the period of dissolver operation. The experimental results with the compact spectroscopic imager indicate the following: • On-line measurements of plutonium inventory in the static (valved-off) or dynamic (active solid feed and solution withdrawal) dissolver system are straightforward to perform with little process impact. • Measurements of an on-line plutonium reference solution before and throughout dissolver operation show uniform response of the imager to 239Pu over time and despite large changes in background. • Repeated quantitative measurements of the static dissolver system show evidence of continued dissolution of solids in the heated system. • Applications of the present imager for accountability of in-process inventory will require repeated measurements of the static, heated dissolver, which still saves time and costs compared to cleanout. • Potential improvements with detectors of medium energy resolution include measuring the effects of undissolved solids rather than repeating static measurements to assure more complete dissolution.