Leaching Behavior of Cesium-137 in Cement-waste Composites

To assess the safety for disposal of a radioactive waste-cement composite, the leaching of 137Cs from a waste composite into a surrounding fluid has been studied. Leaching tests were carried out in accordance with a method recommended by IAEA. The leachability was measured as functions of waste to cement ratio (Wa/C), concentration of the salt (Na2SO4), temperature of leachant and curing time of specimens. The fraction of 137Cs leached from a specimen of Portland cement is 0.3–0.4 at the leaching time of 100 days, whereas the corresponding value for a specimen of slag cement is 0.1–0.15. The leachability depends on the structure of cement composite which is influenced by such factors as Wa/C, salt concentration of the wastes, and curing time of the specimens. The leach coefficient of 137Cs increases with increasing porosity of the cement composite. At lower temperature, curing time of the specimens significantly affects the leachability, probably due to insufficient curing. A curing time of ca. 50 days before the leaching test is enough to eliminate the apparent effect of temperature.