EVALUATING LONG-TERM SUBSIDENCE ASSOCIATED WITH LOW-LEVEL RADIOACTIVE WASTE SHALLOW BURIAL

Low-level radioactive waste at the United States Department of Energy's Savannah River Site is disposed by shallow trench burial. After a trench is filled, a long-term cover is constructed to prevent surface water infiltration and access. Significant buried waste subsidence can compromise long-term cover integrity, resulting in increased costs and human-health and environmental risks. For the trench considered for this study, up to 17 ft. of subsidence is anticipated, primarily related to waste-container void space and the low density of much of the waste material. An evaluation of long-term subsidence and its implications for long-term cover stability for this in-use trench has been performed, incorporating geotechnical, geologic, and corrosion data. A buried waste container and adjacent soil were excavated to determine corrosion types, rates, and environment. These data were used to develop three scenarios projecting future container structural-strength loss. A parametric study was performed to summarize near- and long-term geologic, seismic, and climatic data, waste-container and trench construction characteristics, and develop a trench geotechnical conceptual model. Using all this information, finite element modeling was performed for both dynamic compaction of the B-25s prior to significant corrosion and final cap construction, and for long-term subsidence (without dynamic compaction) at various degrees of container corrosion. Modeling results match well with field observations for dynamic compaction, and initial results for long-term corrosion related subsidence seem reasonable. These data aid in evaluating long-term cover system maintenance/repair costs and human-health and environmental risks.