High-Level Waste Glass Field Burial Tests at CRNL. The Effect of Geochemical Kinetics on the Release and Migration of Fission Products in a Sandy Aquifer

摘要

In 1960 June, 25 nepheline syenite-based glass hemispheres containing the fission products sup 137 Cs, sup 90 Sr, sup 144 Ce and sup 106 Ru were buried below the water table in fluvial sand at the Chalk River Nuclear Laboratories of Atomic Energy of Canada Limited. Soil and groundwater concentrations of sup 90 Sr and sup 137 Cs have been determined since then and the data have been interpreted using kinetically limited migration models to deduce the leaching history of the glass for these burial conditions. The leaching history derived from the field data is compared to laboratory leaching of samples from a glass hemisphere retrieved in 1978, and also to pre-burial laboratory leaching of identical hemispheres. The time dependence of the leach rates observed for the buried specimens suggests that leaching is being inhibited by the formation of a protective surface layer, although no direct observation of this layer has been made. Using an average leach rate of 5.6 x 10 exp -14 kg/(m exp 2 .s) derived from the field data for the period 1966 to 1977, it is estimated that it would require approximately 20 million years to dissolve the glass hemispheres. The effect of the kinetic limitations of the fission-product/fluvial-sand interactions is discussed with respect to the migration of sup 90 Sr and sup 137 Cs over a 20-a time scale. It is concluded that kinetically limited sorption by oxyhydroxides rather than equilibrium ion exchange controls the long-term migration of exp 90 Cr; the action of the oxyhydroxides immobilizes the sup 90 Sr on the longer time scale. Cesium is initially rapidly bound to the micaceous fraction of the sand. On a longer time scale, slow remobilization of sup 137 Cs in particulate form is observed and is believed to be related to bacterial action. (Atomindex citation 16:030186)