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An Afinnative Action/Equal Opportunity Employer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither The Regents of the Unfversity OfCalifornia, the United States Government nor anyagency thereoJnorany of theiremployees, makesanywarranty,erpressorimplied,orassumesany legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spec@ commercial product, process, or service by trade name, trademark, manufacturer, or othenuise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by The Regents of the University of Cnlifomia, the United States Government, or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of The Regents of the University of California, the United States Government, orany agency thereof. The Los AlamosNational Laborato y stronglysupportsacademic freedomandaresearcher'sright to publish; therefore, theLaboratoyasan institution doesnotendorse the viewpoint of a publication or guamntee its technical correctness.. .

An Active Well Coincidence Counter (AWCC) has been modified to measure variable enrichment uranium hexafluoride (UF6) in storage bottles. An active assay technique was used to measure the 235U content because of the small quantity (nominal loading of 2 kg UF6) and nonuniform distribution of UF6 in the storage bottles. A new insert was designed for the AWCC composed of graphite containing four americium-lithium sources. Monte Carlo calculations were used to design the insert and to calibrate the detector. Benchmark measurements and calculations were performed using uranium oxide samples. The Monte Carlo generated calibration curves benchmarked to uranium oxide resulted in assay values that agreed within 2 to 3% of destructive assay values. In addition to UF6, the detector was also calibrated for HEU ingots, billets, and alloy scrap using the standard Mode 1 end-plug configuration.