The Use of Staorage Tank Holdup Measurements to Reduce Inventory Differences in an Ion Exchange Process

Inventory differences (ID) in an ion exchange process area have plagued the Los Alamos National Laboratory for years. The problem has always been attributed to plutonium precipitation in banks of horizontally oriented storage tanks; however, efforts to maintain the precipitates at low enough or even stable levels failed. Factoring tank holdup measurements into the end-ofmonth inventory balance would probably solve the ID problem; however, the authors were advised that gamma-based holdup measurements would yield very poor quality holdup estimates because of difficulties in determining transmission corrections and tank \"cross talk.\" When the ID problem became particularly troublesome in the spring of 1985, the authors evaluated two different gamma-based measurement techniques for estimating tank holdup. Not only did holdup estimates made by the two techniques agree, but plutonium recovered during intensive tank cleanout confirmed that the holdup measurements were of sufficient accuracy to be used for material balance adjustments. The measurement method chosen for routine use is somewhat unique since it is calibrated using tank cleanout data and requires no transmission corrections. The holdup measurements are made on a monthly basis and have dramatically reduced end-of-month inventory differences. This paper will present both a description of the measurement methodology and the inventory difference improvements.