Application of spectral summing to indeterminate suspect low-level drums at Los Alamos National Laboratory

An analytical technique developed by Pajarito Scientific Corporation (PSC), utilizing spectral summing of spectra from groups of drums of similar waste type, is being employed by the Waste Disposition Project - Low Level Waste Disposal (WDP-LLWD) Group at Los Alamos National Laboratory (LANL). This technique has been used to disposition low-level radioactive waste that has dropped out of the transuranic (TRU) category and has no place to go unless it can be proven to be LLW and not TRU. The TRU program at LANL run by Mobile Characterization Services (MCS) employs two High Efficiency Neutron Counters (HENC) with built-in gamma assay systems to assay radioactive waste for shipment and disposal as TRU waste at the Waste Isolation Pilot Plant (WIPP) at Carlsbad, New Mexico. As well as being certified for WIPP assays, the HENC systems can also be used for low-level waste assays for disposal at LANL or off-site disposal facilities, such as the Nevada Test Site (NTS). Some of the waste processed through the HENC systems cannot be confinned TRU due to the absence of detected TRU alpha emitters above the TRU cutoff of 100 nCi/g. This waste becomes suspect low-level waste (SLLW). In many cases, the waste alsomore » can't be classified as LLW because the minimum detectable activity (MDA) of TRU radio nuclides is above the 100 nCi/g level. These wastes that do not have enough detectable TRU activity to be classified as TRU waste and have too high a MDA to be classified as LLW enter a radioactive waste characterization indetenninate status that prevents their dispositioning as either TRU waste or LLW. Spectral summing allows an experienced ganuna spectroscopy analyst to add the HENC gamma spectra of a number of similar waste items together to form a consolidated (summed) spectrum. This summed spectrum contains the assay results of the group of items rather than the individual item, and gamma peaks that were not discernable in the individual spectra can become quantifiable in the summed spectrum and the MDA for group sum is reduced. The group of waste items can then be properly classified as LLW based on the summed spectrum and valid assay values can be assigned for disposal. This technique has been successfully applied to a set of 52 debris drums - with individual MDA > 100 nCi/g - with a resulting group total TRU alpha activity concentration below 40nCi/g. Further application of the technique at LANL to other waste drums that are measured on a WIPP certified HENC system is planned and good candidate drum sets are being evaluated as indeterminate situations develop.« less