RD&D-Programme 2004 - SKB

17.2.24 Radionuclide transport – advection
The main function of the buffer is to guarantee that diffusion is the dominant transport mechanism
around the canisters. With an MX-80 buffer with a water-saturated density of 2,000 kg/m 3 ,
the transport capacity for diffusion is at least 10,000 times higher than that for advection.
17.2.25 Radionuclide transport – diffusion
The transport of radionuclides through the buffer is mediated by different diffusion mechanisms.
It has been established that certain cations can have high diffusivities (be transported more
efficiently). One possible explanation for this phenomenon is the theory of surface diffusion.
The process is handled in the safety assessment by assigning higher diffusivity values to
caesium, strontium and radium.
When bentonite has such a high density that the electrical double layers between two planes
overlap, a phenomenon known as anion exclusion occurs. Anions cannot penetrate into the
interlamellar pores due to the electrostatic forces between the negatively charged surfaces and
the anion. Anion exclusion significantly reduces the porosity available for diffusion of anions.
The effect of anion exclusion becomes less at high salinities, and in crushed rock/bentonite
mixtures it is negligible.
Conclusions in RD&D 2001 and its review
SKI questions whether it is correct to describe the surface diffusion process by postulating
higher diffusivity values for caesium, strontium and radium, see also section 17.2.26.
Newfound knowledge since RD&D 2001
A number of laboratory tests have been carried out to investigate in detail various processes
that influence ion diffusion in bentonite clay. Anion diffusion in bentonite clay compacted to
different dry densities has been investigated /17-21/. The results indicate that anion diffusion
in bentonite clay consists of two processes, a fast one and a slow one. The explanation that has
been proposed is that the rapid diffusion process is diffusion between the bentonite layers, while
the slow one is diffusion between the bentonite particles, where the anions to some degree sorb
to edge positions on the montmorillonite and/or to other minerals in the bentonite.
Diffusion in bentonite clay has been investigated in two in situ experiments in the Äspö HRL:
Chemlab and Lot. Chemlab is a down-the-hole laboratory probe in which diffusion tests have
been conducted with cations (Cs + , Sr 2+ and Co 2+ ) and anions (I – and TcO 4– ) in bentonite and with
natural groundwater. The water was taken from a fracture in the same borehole in which the test
was performed /17-22/. The redox-sensitive pertechnate ion (TcO 4– ) diffused unreduced, despite
the fact that it should have been reduced thermodynamically and precipitated as TcO 2·nH 2 O
/17-23/. The technetium activity was much higher at certain places, however. The activity peaks
are attributed to iron-bearing minerals where Tc(VII) has been reduced to Tc(IV) and precipitated.
The cations Sr 2+ , Cs + and Co 2+ , as well as the anion I – , behaved in the same way as in a
laboratory test.
In the Lot tests /17-24/, cylindrical bentonite blocks were placed around a copper tube in a
four-metre deep vertical borehole. The hole was then sealed and the bentonite blocks were left
in the hole for one year, five years or a much longer time than five years. When the bentonite is
water-saturated, the copper tube is heated to simulate the energy that is liberated by radioactive
decay in the spent nuclear fuel. Bentonite that has been doped with radioactive caesium and
cobalt was placed in one of the lowermost blocks. When the test was concluded, the activitycontaining
bentonite block was taken to a radiological laboratory, where the activity distribution
in the block was determined. The activity distribution was as expected from laboratory studies
for both the cations.
224 RD&D-Programme 2004