Sorption and diffusion of I-125 and Sr-90 in a mixture of bentonite and crushed granite backfill of a radioactive waste repository

Abstract

Sorption and diffusion are important processes in the transport of radionuclides through mixtures of bentonite and crushed granite. Sorption was investigated by batch tests? and the diffusion of I-125 and Sr-90 in compacted mixtures was investigated using through-diffusion tests. The distribution coefficients of Sr-90 onto the mixtures of bentonite and crushed granite depended largely on the proportion of bentonite. The distribution coefficient of Sr-90 on the bentonite decreased with decreasing the solution-to-bentonite ratio. It was found in desorption tests that Sr-90 was sorbed via an ion exchange reaction. When the dry density for the mixture of 30% bentonite and 70% crushed granite was 1.4 Mg/m(3), the apparent diffusion coefficients of I-125 and Sr-90 were 1.4 X 10(-10) and 6.7 x 10(-12) m(2)/sec, respectively. The corresponding diffusion coefficients for the dry density of 2.0 Mg/m(3) were 7.9 X 10(-11) and 3.4 X 10(-12) m(2)/sec for I-125 and Sr-90, respectively. At the high dry density, the contributions of surface diffusion and pore diffusion to Sr-90 diffusion were comparable in magnitude. At this higher dry density, the diffusion of I-125 through the pores was reduced by the mechanism of ion exclusion. The diffusion of radionuclides in the bentonite/crushed granite mixture was dominated by the bentonite. This could be explained by the concept of effective clay dry density. When the effective clay dry density of a mixture was equal to the dry density of pure bentonite? then the diffusion coefficient for the mixture was close to that for pure bentonite.