LONG-TERM SUBSIDENCE ASSOCIATED WITH LOW-LEVEL RADIOACTIVE WASTE SHALLOW BURIAL - FIELD EVALUATION, MODELING, PREDICTION, AND MONITORING

Low-level radioactive waste at United States Department of Energy sites, including the Savannah River Site, is disposed by shallow burial. After a disposal trench is filled, a long-term cover is constructed to prevent surface water infiltration and human access. Significant buried waste subsidence can compromise long-term cover integrity, resulting in increased costs and human-health and environmental risks. For the disposal system evaluated in this study, up to 15 ft. of subsidence is anticipated, primarily related to waste-container void space and low-density waste. 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 waste containers 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. Ongoing efforts include field corrosion monitoring to narrow-down the future time frame for container structural-strength loss. These data aid in evaluating long-term cover system maintenance/repair costs and human-health and environmental risks.