Laboratory studies of the diffusive transport of super(137)Cs and super(60)Co through potential waste repository soils

Tests using reconstituted samples have been performed to assess the diffusive transport of super(137)Cs and super(60)Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion (D sub(e)) and sorption coefficients (K sub(d)) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D sub(e) values obtained in this study were of the order of 10 super(-6) cm super(2) s super(-1) for both species, higher than previously published data. The K sub(d) values from the diffusion and batch sorption tests were in reasonable agreement for super(137)Cs, but an order of magnitude different for super(60)Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.