Studies of the effects of chelates on the leaching of radionuclides from cement.

Abstract

To evaluate the release of some important radionuclides from cement waste forms, a variety of studies have been completed. Leach rates and equilibrium concentrations of some complexing agents and radionuclides were measured, and the effect of a simulated waste form on these values was determined. Complexing agent concentrations were evaluated. Tests in deionized water showed ethylenediaminetetraacetic acid (EDTA) was leached from a simulated waste form more rapidly than nitrilotriacetic acid (NTA) which was released more rapidly than citric acid. Equilibrium concentrations were measured in deionized water, saturated sodium chloride, 24% magnesium chloride and Q-brine which is a simulated groundwater based on magnesium chloride. At equilibrium, the EDTA release fraction was independent of the amount of EDTA in the cement. The citric acid concentration was constant at less than 10⁻⁵ M. Finally, NTA concentrations and release fractions were both dependent on the cement loading. To evaluate the effect of complexing agents on some radionuclides, leach rates of cesium and cobalt and equilibrium concentrations of cobalt and uranium were studied. The effects of EDTA, NTA and citric acid on release rates of cesium and cobalt were studied, and the effects of several complexing agents on the equilibrium concentrations of cobalt and uranium were examined. Complexing agents had no effect on cesium leach rates. In water and in sodium chloride, cobalt and uranium concentrations in the presence of cement did not vary with any complexing agent. In Q-brine in the presence of cement, tartaric acid, EDTA and NTA increased the soluble cobalt concentration two orders of magnitude, and citric acid had a much smaller effect. Uranium concentrations in Q-brine were increased in EDTA, tartaric acid and citric acid when cement was in the system.