Spent nuclear fuel is radioactive. It must be isolated from
organic nature until its radioactivity has decreased to a harmless
level. This isolation is called final disposal.
When disposed of, the spent fuel will be sealed in disposal canisters made of copper and cast iron. The canisters will be placed into deposition holes drilled in a deposition tunnel, more than 400 meters below the ground surface. Finally, the tunnel system will be filled and sealed.
The final disposal will be realized according to the multibarrier principle. The canister, filling materials within the tunnel, and the bedrock will limit and slow down the release of radioactive materials from the fuel contained in the canisters. Even if the isolation capability of some canisters was lost, the impact on people and the environment would remain insignificant.
In Finland, each producer of nuclear power is fully responsible for its own nuclear waste management. Teollisuuden Voima Oyj and Fortum Power and Heat Oy have established Posiva Oy to manage the research and technical implementation of the final disposal of nuclear waste produced in their power plants in Finland, deep in the Olkiluoto bedrock.
From the reactor to final disposal
Spent fuel assemblies are cooled down for some years in the water pools of the nuclear power plant unit.
After four years, the fuel assemblies will be transferred to the water pools of an interim storage facility located at the power plant site. The spent fuel will stay in the interim storage facility for several decades to cool down before final disposal.
After interim storage, the spent fuel assemblies will be sealed inside copper and cast iron canisters in an encapsulation plant to be built in Olkiluoto.
The canisters will be placed at a depth of 400–450 meters inside the Olkiluoto bedrock.