Evaluation and design of drained low-level radioactive disposal sites. Final report

Low-level disposal in shallow trenches has been the subject of much critical assessment in recent years. Historically most trenches have been located in fairly permeable settings and any liquid waste stored has migrated at rates limited mainly by hydraulic effects and the ion exchange capacity of underlying soil minerals. Attempts to minimize such seepage by choosing sites in very impermeable settings lead to overflow and surface runoff, whenever the trench cap is breached by subsidence or erosion. The work described in this report was directed to an optimum compromise situation where less reliance is placed on cap permanence, any ground seepage is directed and controlled, and the amount of waste leaching that would occur is minimized by keeping the soil surrounding the waste at only residual moisture levels at all times. Measurements have been conducted to determine these residual levels for some representative soils, to estimate the impact on waste migration of mainly unsaturated flow conditions, and to generate a conceptual design of a disposal facility which would provide adequate drainage to keep the waste from being exposed to continuous leaching by standing water. An attempt has also been made to quantify the reduced source terms under such periodic, unsaturated flow conditions, but those tests have not been conclusive to date. For low-permeability soils the waste should be placed about 1 ft. above the saturated layer formed by suction forces immediately above the gravel layer. Since most disposal sites, even in humid regions of the United States, are exposed only to intermittent rainfall and as most trench designs incorporate some gravel base for drainage, the results of this project have broader applications in assessing actual migration conditions in shallow trench disposal sites. Similar considerations may also apply to disposal of hazardous wastes