Bentonite in nuclear waste disposal: A review of research in support of the Basalt Waste Isolation Project

Abstract

The Basalt Waste Isolation Project is investigating the feasibility and safety of a high-level nuclear waste repository in the Columbia River basalts. A mixture of crushed basalt and sodium bentonite is under investigation as packing material around individual waste containers. A 75:25 basalt/bentonite mixture, with a density of 2.1 g/cm 3, is the BWIP reference packing material. The hydraulic conductivity of highly compacted bentonite ranges from 4 to 7·10 −13 cm/s, while that of a basalt/bentonite mixture is 1–4·10 −10 cm/s. The diffusion coefficient for water in basalt/bentonite mixtures is approximately 2·10 −5 cm 2/s at 90°C. The swelling pressure of packing material under repository conditions is expected to be less than 2.5 MPa. The thermal conductivity of bentonite is in the range of 0.3–1.6 W/mK. Bentonite subjected to dry heating for nearly a year undergoes only reversible dehydration at temperatures to 370°C. Under hydrothermal conditions montmorillonite in the bentonite undergoes minor ion exchange at 200°C, but significant dissolution and reprecipitation at 300°C. Basalt/bentonite mixtures undergo small-scale dissolution of the basalt mesostasis and ion exchange in the montmorillonite at 150°C. At temperatures of 300°C and higher, in-situ alteration of the mesostasis and partial illitization of the montmorillonite occur. Water vapor at temperatures greater than 200°C irreversibly degrades the swelling ability of bentonite. Sodium montmorillonite shows no evidence of damage under high gamma doses. Extremely high doses of alpha radiation can lead to structural damage in the clay. Initial corrosion rates for steel and copper in contact with saturated packing material are several tens of micrometers per year. Such rates are sensitive to local temperature, moisture, redox conditions, and the formation of magnetite and clay-rich surface coatings. Bentonite strongly sorbs Sr, Cs and Am from solution, and is a less efficient sorber of U and Np. Sorption by basalt/bentonite mixtures is strongly dependent on redox state, with large sorption capacities reported for U, Np and Tc under reducing conditions. Desorption of U, Np, Se and Tc is much less efficient than initial sorption. The sorption of radionuclides by colloidal bentonite may have a significant effect on waste package performance.