Diffusivities of Some Constituents in Compacted Wet Bentonite Clay and the Impact on Radionuclide Migration in the Buffer

The diffusivities measured by various investigators of several species in compacted bentonite clay have been compiled and analyzed. Small anions diffuse slower than an uncharged molecule such as methane. Large anions move orders of magnitude slower still. The actinides thorium, uranium, plutonium, neptunium, and americium are considerably retarded by sorption effects. Their movement can be explained by pore diffusion with retardation. Cesium, strontium, and protactinium move considerably faster than can be explained by these effects. The faster mobility is probably due to surface migration. A simplified model is presented by which the importance of the backfill barrier in retarding the radionuclides can be assessed. It is based on the computation of the evolution of the concentration profile of the diffusing nuclide in the backfill. Using this model, the flow rate out from the backfill to the flowing water can be compared to the inflow into the backfill due to leaching. A second model treats nuclides with solubility limitations in a similar way. A diagram is presented where the maximum outflow or concentration of a nuclide from the backfill can be determined as a function of barrier thickness and nuclide diffusivity and decay constant. Using the experimentally obtained diffusivities, it is found that a 0.375-m-thick backfill will eventually let through practically all ¹²⁹I, ⁹⁹Tc, ²²⁶Ra, ²³¹Pa, ²³⁴U, ²³⁵U, ²³⁸U, and ²³⁷Np. The maximum release rate for ¹³⁷Cs, ⁹⁰Sr, ²³⁹Pu, ²⁴⁰Pu, and ²⁴³Am will decrease by one to three orders of magnitude compared to the leach rate. Americium-241 will decay to insignificance in the backfill.