High-level waste is in the form of spent fuel assemblies which are kept in water pool storages at nuclear power plants. Later on, the fuel assemblies will be transferred to the disposal site into a facility where they are inserted into specific disposal containers, called waste canisters. The construction of the encapsulation plant will be started in the first half of 2010s.
At an above ground encapsulation plant, spent nuclear fuel is transferred into disposal canisters by remote control handling.
Unlike at nuclear power plants, there are no high temperatures or pressures at the encapsulation plant. Spent nuclear fuel cannot explode by itself. There are no easily flammable substances in the encapsulation chambers, either. Prevention of sabotage will be taken into account in the planning and operation of the facility.
The waste canister is comprised of an insert of nodular cast iron, surrounded by the external canister made of copper which is about 5 cm thick. The canisters are lowered into the final disposal repository. The repository comprises a network of tunnels excavated in the bedrock at a depth of 400-700 metres.
Spent nuclear fuel is isolated from the environment by multiple safety barriers. These independent barriers ensure that no significant amounts of radioactive substances are released to biosphere. For instance, the corrosion-resistant disposal canister surrounded by the bentonite buffer isolate spent nuclear fuel from bedrock and groundwater. The bedrock of the disposal site must be favourable with regard to isolation of radioactive substance, for instance the groundwater flow rate around waste canisters must be small. If radioactive substances would come into contact with groundwater, they would be retarded in the surrounding bedrock. Therefore they would move in bedrock slower than the rate of the groundwater flow.
Final disposal must be planned and carried out so that there will be no need to oversee the safety of the final disposal facility after its closure. However, the Government decision of 1999 requires that retrieval of the disposed waste canisters must be feasible even after the closure of the repository. Furthermore, the conditions in the repository may be monitored for some time after the completion of the disposal operations, if so decided in due course.
Even one meter of bedrock layer would be enough to dampen the radiation level to a level that would be safe for man. Spent nuclear fuel is disposed of at a depth of several hundreds of metres so that no actions of man or natural catastrophes could damage the final disposal repositories in the future, either.
