Source Term Characterization for the Maxey Flats Low-Level Radioactive Waste Disposal Site

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Abstract
The results of source term characterization studies for the Maxey Flats low-level radioactive waste disposal site show that because of the long residence time of water accumulations in the trenches, prolonged leaching and microbial degradation of waste materials occur continuously, leading to leachate formation. As a result of such interactions for extended time periods, the resultant trench leachates exhibit significant modifications in terms of inorganic, organic, and radionuclide constituents and acquire geochemical properties that are unique, compared to ambient groundwater. The leachates generally exhibit varying degrees of anoxia characterized by negative redox potentials, low dissolved oxygen and sulfate concentrations, high alkalinity, and high ammonia concentrations. The enrichments, to varying degrees, of inorganic, organic, and radionuclide constituents associated with fuel cycle and non-fuel cycle low-level wastes reflect the nature of the leaching process itself and of the waste materials. Elevated concentrations of Na+, K+, Fetotal, Mntotal, Cl−, dissolved organic and inorganic carbon, and several organic compounds as well as radionuclides, such as 3H, 241Am, 60Co, 134Cs, 137Cs, 90Sr, 238Pu, and 239,240Pu are a consequence of waste leaching. Some of the waste-derived organic compounds present in the trenches, such as chelating agents and several carboxylic acids, are strong complexing agents and have the potential to form stable radionuclide complexes and thus enhance nuclide mobility. The consequences of past disposal practices as reflected in the problems associated with the burial of unsegregated, poorly packaged, and unstabilized wastes at the Maxey Flats disposal site indicate the significance of waste segregation, improved stabilization, and proper packaging. In accordance with current disposal procedures, stabilized and packaged waste not only ensures trench stability but also decreases the rate and extent of leaching and microbial degradation of waste materials.