RD&D-Programme 2004 - SKB

17 Buffer
The main purpose of the buffer is to prevent flowing water from the rock from coming into
contact with the canister and the spent fuel. To do this, the buffer must meet the following
requirements:
• The buffer’s hydraulic conductivity must be so low that any transport of corrodants and
radionuclides is dominated by diffusion.
• The buffer must retain its dimensions.
• The buffer must have a self-healing capacity, so that no permanent cracks form.
• The properties of the buffer must be physically and chemically stable on a long time scale.
It is also desirable that the buffer should prevent microbial activity on or near the surface of the
canister.
Nor may the buffer have any properties that might have an adverse effect on the other barriers.
To meet these requirements:
• The buffer’s gas permeability must be sufficient to allow the large quantities of gas that
may be formed in a damaged canister to pass through. This gas passage must not lead to
the formation of permanent gas-permeable channels or cavities in the buffer.
• The buffer’s swelling pressure must be high enough to provide good contact with the host
rock and the canister, but not higher than what the canister and host rock can withstand.
• The buffer’s deformability must be great enough to absorb rock movements without the
canisters being damaged, but small enough to hold the canisters in position.
• The buffer’s heat conduction properties must be such that the heat from the canisters does
not lead to unacceptable physical and chemical changes in the buffer.
• The buffer must not contain anything that has an adverse effect on the performance of the
other barriers.
The buffer should also be able to filter out colloidal particles.
SKB has previously chosen a natural sodium bentonite of the Wyoming type as a reference
material for the buffer. MX-80 is a trade name for a blend of different horizons of natural clay
from Wyoming or South Dakota in the USA. MX-80 specifies a given grade and grain size of
dried and ground bentonite. However, studies in recent years of alternative buffer materials have
shown that there are a number of sodium and calcium bentonites on the market that are very
capable of meeting SKB’s requirements, see further section 17.1.2.
Based on completed Investigations, SKB has concluded that a buffer consisting of MX-80
should have a density of 1,900–2,100 kg/m 3 after water saturation.
The chosen material has the following properties that relate to the above requirements:
Hydraulic conductivity and ion diffusion
The main function of the buffer is to guarantee that diffusion is the dominant transport
mechanism around the canisters. With a bentonite buffer with a density of 2,000 kg/m 3 in the
water-saturated state, the transport capacity for diffusion is at least 10,000 times higher than
that for advection.
Bentonite limits the release of radionuclides from a defective canister. However, the effect
is dependent on the properties of the individual nuclide (diffusivity, sorption coefficient and
half-life) as well as the geometry of the near field (the defect in the canister, transport pathways
in the rock).
RD&D-Programme 2004 193