INTERCOMPARISON STUDY OF THE SNAPTM GAMMA SPECTROSCOPY ANALYTICAL TECHNIQUE WITH TWO WIPP-CERTIFIED NONDESTRUCTIVE ASSAY SYSTEMS AT LANL

A study was undertaken at the Los Alamos National Laboratory to compare the results of the SNAPTM gamma spectroscopy analytical technique to two WIPP-certified assay systems on drums of real TRU-level waste. A total of sixty-five 55-gal drums originally assayed by either the tomographic gamma scanner (TGS) or the high-efficiency neutron counter (HENC) were also assayed in a blind measurement trial by portable high-purity germanium detectors using the SNAPTM analytical software. The overall SNAPTM bias compared to the TGS system was –9.54% with a relative standard deviation of 13.76%. The overall bias of the SNAPTM technique compared to the HENC was significantly larger: -36.13% with a relative standard deviation of 23.80%. The largest individual discrepancy between SNAPTM and the TGS was -31.56%, while the largest discrepancy compared to the HENC was -89.26%. The possible causes for the large discrepancy between SNAP results and the HENC were investigated carefully because of the size of the bias. Several potential reasons were identified, including the following: 1) potential discrepancies in plutonium isotopics, 2) insufficient modeling information for SNAPTM, 3) less than MDA quantities in the HENC, and 4) a possible bias in the SNAPTM and/or HENC calibration.