METHODOLOGY FOR DETERMINATION OF RADIOASSAY PROPERTIES FORRH-TRU WASTE

摘要

The Battelle Columbus Laboratories Decommissioning Project (BCLDP) decontamination anddecommissioning activities will generate approximately 25 cubic meters of remote-handledtransuranic (RH-TRU) wastes, which are planned for ultimate disposal at the Waste IsolationPilot Plant (WIPP). For WIPP disposal, the isotopic inventory of the waste must becharacterized to demonstrate compliance with applicable limits. The BCLDP has developed amethodology that combines limited assay, knowledge of the processes generating the waste, anda dose/weight-to-curie evaluation to determine the isotopic characterization of the waste.The BCLDP methodology determines the isotopic content for an identified TRU waste stream bya combination of (1) representative waste stream sample analyses, (2) application of Oak RidgeIsotope Generation and Depletion (ORIGEN2) code values for isotopes expected in spent nuclearfuel but not represented by the sample analyses, and (3) assessment of cesium (Cs)-137 contentof a payload container based on external radiation field measurements and calculation of TRUisotopic content using a ratio of radionuclides based on known Cs-137 content. The measuredisotopic distribution (gamma emitters) of a waste stream is combined with values determinedthrough the conservative application of the ORIGEN2 to account for all isotopes. External doserates for 1 millicurie of an isotopic mix are modeled for different packaging configurations.Field sort charts permit determining the quantity of radionuclides in each waste package frommeasured weights and dose rates. The inventory of Cs-137 is determined for each drum basedon external radiation field measurement using a high-purity germanium detector. Cs-137 is usedas an indicator of TRU content of a drum based on the ratio of radionuclides. The methodologydeterminations are verified on an approved, periodic basis by gamma and/or alpha spectroscopy.The methodology includes the conservative estimation of measurement errors and assumptionsused to determine a total uncertainty that is bounding for the methodology.This methodology is consistent with worker safety principles and is optimal for the BCLDP, asmall quantity site with limited waste and resources. In addition, given the small fraction of RHTRUwastes to be disposed of at the WIPP, and its insignificant impact on the long-termperformance of the repository, the use of sophisticated assay techniques for RH-TRU waste isneither warranted nor necessary. The BCLDP methodology is a simple system that meets theintent of the governing regulations for the WIPP.