Diffusivities of Some Dissolved Constituents in Compacted Wet Bentonite Clay-MX80 and the Impact on Radionuclide Migration in the Buffer

摘要

The diffusivities measured by various investigations of several species in compacted bentonite clay have been compiled and analysed. Small anions diffuse slower than an uncharged molecule such as methane. Large anions move orders of magnitude slower still. The actinides Th, U, Pu, Np and Am are considerably retarded by sorption effects. Their movement can be explained by pore diffusion with retardation. Cs, Sr and Pa 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. The flowrate out from the backfill to the flowing water can be compared to the inflow into the backfill due to leaching. Using the previously obtained diffusivities it is found that a 0.375 m thick backfill will eventually let through all I exp 129 , Tc exp 99 , Ra exp 226 , Pa exp 231 , U exp 234 , U exp 235 , U exp 238 and Np exp 237 . The maximum release rate for Cs exp 137 , Sr exp 90 , Pu exp 239 , Pu exp 240 and Am exp 243 will decrease by one to three orders of magnitude compared to the leach rate. Am exp 241 will decay to insignificance in the backfill. (Atomindex citation 15:010632)