RD&D-Programme 2004 - SKB

Originally there is also air in the buffer, which can leave the buffer by dissolving in the pore
water. This process is called gas dissolution. After water saturation, gas transport can occur if a
canister is damaged, leading to hydrogen formation in the canister.
On absorbing water, the buffer and backfill swell, whereby a swelling pressure is built up. The
swelling pressure is different in the buffer and backfill, which therefore interact mechanically.
The swelling pressure is decisive for the mechanical interaction between canister and buffer,
which can cause the canister to move in the buffer. On heating, the pore water in particular can
expand due to thermal expansion.
The chemical evolution in buffer and backfill is determined by a number of transport and
reaction processes. Solutes in the water can be transported by advection and diffusion. In the
buffer, advection occurs almost exclusively during the water saturation process, after which
diffusion dominates. By means of osmosis, the salinity of the groundwater in particular can
affect the physical properties of the buffer. By means of ion exchange and sorption, the buffer’s
original content of charge-compensating counterions can be replaced by other ionic species.
Chemical transformation of the buffer’s swelling minerals can occur, leading to altered buffer
properties. Other minerals undergo various dissolution and precipitation reactions in the buffer.
On swelling, the buffer penetrates out into the fractures in the surrounding rock, where it can
form colloids which can be carried away by the groundwater. This can lead to gradual erosion
of the buffer. The clay can be transformed by radiation effects and the pore water can be
decomposed by radiolysis. Finally, microbial processes might possibly occur in the buffer.
After water saturation, radionuclide transport is expected to take place in the buffer exclusively
by diffusion in the pores of the buffer, and possibly also on the surfaces of the clay particles.
Neither advection nor colloid transport is expected in a saturated buffer. Radionuclides can be
sorbed to the surfaces of the clay particles. A crucial factor for this is the chemical form of the
radionuclide, which is determined by the chemical environment in the buffer via the process of
speciation. Together with the transport conditions, the rate of radioactive decay determines to
what extent radionuclides from a broken canister will decay before reaching the outer boundary
of the buffer.
The research programme for the various processes in the buffer is dealt with in the following
sections. Many processes in the buffer are coupled and need to be studied integrated. Such
studies are described in sections 17.2.12 and 17.2.23, which deal with the evolution of the
buffer under unsaturated and saturated conditions, respectively.
17.2.2 Radiation attenuation/heat generation
Gamma and neutron radiation from the canister are attenuated in the buffer. The magnitude of
the attenuation is dependent above all on the density and water content of the buffer. The result
is a radiation field in the buffer that can lead to radiolysis of water and have a marginal impact
on the montmorillonite. The radiation that is not attenuated in the buffer reaches out into the
near-field rock. Our understanding of this process is deemed to be good enough for the needs
of the safety assessment.
17.2.3 Heat transport
In a water-saturated buffer, heat is transported by conduction with well-known heat conduction
properties. After swelling, at full water saturation, the buffer is in direct contact with both
canister and rock and heat transfer takes place by conduction.
The heat transport in the buffer during the saturation process is more complicated, since the
buffer’s thermal conductivity is dependent on its water content and density.
The heat transfer between the canister and the buffer is also more complicated, since there is a
gas-filled gap between these materials during the water saturation phase. The gap will be filled
RD&D-Programme 2004 201