Multiple barriers helps to ensure that no nuclear waste will be released to living nature
Spent nuclear fuel will be stored in copper canisters inside Olkiluoto bedrock, in the depth of about 400-450 meters. The principle behind the final disposal is to use multiple release barriers that help to ensure that no nuclear waste will be released to living nature or is accessible to people.
Posiva's final disposal plans are based on the KBS-3 concept, developed by SKB (Swedish Nuclear Fuel and Waste Management Co), the company responsible for nuclear waste management in Sweden. The solution is based on the multiple barriers principle. Radioactive substances are contained within several overlapping protective barriers so that no deficiency in one barrier and no predictable geological or other change will endanger the isolation.
State of Matter of the Fuel: The ceramic state of the fuel forms the first release barrier in itself. The uranium within the gas-tight metal rods is solid and dissolves in water only slowly, which slows down the rate of release of radioactive substances.
Final Disposal Canister: The fuel is packed in a gas-tight, corrosion-resistant canister made of copper and cast iron. Canister protects the fuel assemblies from the mechanical stress occurring deep inside the bedrock.
Bentonite Barrier: The final disposal canister is surrounded with bentonite clay that protects the canister from any potential jolts in the bedrock and slows down the movement of water in the proximity of the canister.
Bedrock: The bedrock provides the canister and bentonite with conditions where changes are slow and predictable. Deep in the bedrock, the canisters are protected from any changes occurring above ground, such as future Ice Ages, and kept away from people’s normal living environment.
Vertical or Horizontal Disposition?
Posiva’s current plan is to place the final disposal canisters inside vertical holes drilled in the final disposal tunnels. Nevertheless, Posiva in cooperation with SKB is also exploring the possibility to place the canisters in horizontal final disposal holes.
In horizontal solution, the canister and the bentonite are placed in perforated steel casing. Several canisters, separated with bentonite clay blocks, are then placed one after another in the hole. There are, however, still some uncertainties with the horizontal disposition relating to such factors as the behaviour of the bentonite barrier during the disposal.