Field Demonstration of in Situ Treatment of Buried Low-Level Radioactive Solid Waste with Caustic Soda and Soda Ash to Immobilize exp 90 Sr

摘要

A low-level radioactive solid waste disposal trench was injected on four occasions with solutions of caustic soda, soda ash, caustic soda, and lime/soda ash, respectively. Because investigations had indicated that exp 90 Sr could be coprecipitated with soil calcium carbonate by treatment with soda ash, this demonstration was undertaken as a test of its technical feasibility. After concentrations of exp 90 Sr and water hardness decreased within the intratrench monitoring wells; one well at the foot of the trench decreased from over 100 to a persistent level of less than 10 kBq of exp 90 Sr per liter. Recharge of exp 90 Sr from the trench to a sump immediately below was reduced by about 90%. Water hardness and exp 90 Sr concentrations were strongly correlated through time within each monitoring well, indicating that exp 90 Sr behaved as a tracer for soil calcium and magnesium. The disappearance of exp 90 Sr from the trench water, therefore, was an in situ water softening. Soil samples retrieved from the trench indicated that as much as 98% of the total exp 90 Sr was present as a coprecipitate with calcium carbonate. The hydrologic characterization of this trench indicated an average void space of 41% and an average trench-wall hydraulic conductivity of 3.4 x 10 exp -7 m/s. Sampling of the trench's discharge contamination plume indicated that it had resulted from a combination of subsurface seepage and bathtub overflow during infrequent periods of intense precipitation. A generic assessment of soda ash treatment indicated that treatment would be most effective for soils of high cation exchange capacity with either low (< 20%) or high (> 80%) basic cation saturation of that cation exchange capacity. (ERA citation 09:018255)