RD&D-Programme 2004 - SKB

Programme
SKB does not consider sorption in bentonite to be a prioritized research area. However, existing
data will be updated prior to each new safety report.
17.2.27 Speciation of radionuclides
The speciation of radionuclides is of importance for sorption and diffusion in the buffer. It
is influenced by what speciation the nuclide had at the boundary to the buffer, i.e. inside the
canister, but also by the chemical conditions in the buffer.
The speciation process is discussed in section 15.2.14 and the evolution of the chemical
environment in the buffer in section 17.2.23. In SR-Can, the same types of speciation
calculations are carried out for e.g. the buffer environment as in SR 97.
17.2.28 Radionuclide transport – colloid transport through bentonite
The buffer is supposed to prevent colloids with radionuclides from escaping from a damaged
canister.
Conclusions in RD&D 2001 and its review
In SKI’s opinion, SKB should investigate the importance for the safety assessment of the fact
that organic colloids can diffuse through the buffer, as well as whether the same phenomenon
applies to inorganic colloids.
Newfound knowledge since RD&D 2001
Diffusion of organic colloids in bentonite has been studied in a doctoral thesis /17-28/. The
purpose of the thesis was to obtain knowledge of the sorption and diffusion of organic colloids
in compacted bentonite, and to investigate whether bentonite is an effective barrier to colloids.
The transport properties of radionuclides in bentonite may be altered in the presence of colloids
due to the fact that speciation and sorption processes are altered. It was found that humic acids,
unlike bentonite colloids, are stable in granitic groundwaters. Despite the fact that humic acids
give rise to charged and relatively large aggregates, they diffuse easily through compacted
bentonite (diffusivity D e about 10 –11 m/s). The experiments show that humic acid diffuses in the
entire pore volume, i.e. both intralamellarly and in the spaces between particles. The transport of
Eu(III) and Co(II) in bentonite increases significantly in the presence of humic colloids, while
the transport of Sr(II) is unaffected.
Programme
It is important that colloids that are formed inside the canister be filtered in the bentonite.
According to /17-28/, it is clear that organic colloids are not filtered by the bentonite in the same
way as inorganic colloids. Understanding of the properties of colloids in bentonite will require
further research.
226 RD&D-Programme 2004