RD&D-Programme 2004 - SKB

The fuel, the canister, the buffer and the rock all contribute towards retarding any escaping
radionuclides in the event a canister is breached. The fuel in itself is very stable in the
environment prevailing at a depth of hundreds of metres in the crystalline bedrock. Many of
the most hazardous radionuclides have such low solubility in groundwater that this renders them
inaccessible for further dispersal. Even if they should be breached, both the copper canister
and the cast iron insert impair the ingress of groundwater and the escape of radionuclides. The
buffer retards both ingress of water to a damaged canister and escape of radionuclides. The
groundwater moves slowly in the fracture system in the rock, and many radionuclides have a
strong tendency to adhere to fracture surfaces in the rock.
The canister and buffer materials have been selected based on the principle that they should
be naturally occurring and stable in an environment similar to that in deep Swedish bedrock.
The buffer clay SKB wants to use in the deep repository was formed around 100 million years
ago, after which it remained in an environment resembling Swedish groundwater for about
50 million years. Copper is an extremely durable metal in the chemical environment that exists
in Sweden’s deep groundwater.
Spent nuclear fuel emits radiation which is converted to heat in a repository. Temperatures
above approximately 100°C can cause changes in the canister and buffer whose long-term
consequences can be difficult to predict. The repository is therefore designed to avoid high
temperatures. This is accomplished by interim storage of the fuel in Clab for 30–40 years to
allow the radiation to decline, by restricting the quantity of fuel in each canister, and by not
spacing the canisters too closely in the repository.
The system of barriers must also be passive, i.e. it must work effectively in its natural state
without human intervention and without input of energy or materials.
In summary, the principles behind SKB’s safety philosophy are as follows:
• The repository shall be situated in a long-term stable environment that is protected from
both societal changes and long-term climatic changes.
• The repository shall be situated in bedrock that can be assumed to be of no economic
interest to future generations.
• The spent fuel shall be surrounded in the repository by multiple barriers.
• The barriers shall primarily isolate the fuel.
• If the isolation should be breached, the barriers shall retard any escaping radionuclides.
• Engineered barriers shall consist of naturally occurring materials that are long-term stable
in the repository environment.
• The repository shall be designed so that high temperatures are avoided.
• The barriers shall work passively, i.e. without human intervention and without input of
energy or materials.
Accounting prior to permit applications
Each of the two applications foreseen in Figure 3 requires an assessment and account of the
long-term safety of the deep repository system. The requirement of a safety assessment for
the permit application for the deep repository is self-evident. The permit application for the
encapsulation plant will also require an account of long-term safety, since it must be shown
in the application that a deep repository with the sealed canisters that will be delivered from
the encapsulation plant meets the requirements on long-term safety stipulated by the Swedish
regulatory authorities.
372 RD&D-Programme 2004