Management of long-lived radioactive waste

Long-lived radioactive waste is defined as the type of waste containing radionuclides half-life of which exceeds (²³⁵U, ²³⁸U, ²²⁶Ra, ²³⁹Pu, ⁶³Ni, ¹⁴C, ³⁶Cl, ¹²⁹I...).

The following types of long-lived radioactive waste are distinguished:

• Spent nuclear fuel;
• Metal parts and control rods of reactors, fuel collectors;
• Black lead used inside reactor;
• Used sources of ionizing radiation.

It has been universally acknowledged that in terms of environment protection the only cohesive and safe way of final disposal of spent nuclear fuel and other long-lived high-level radioactive waste is in deep geological repositories. The final disposal of this type of waste in deep geological repositories may be justified by the usage of stable geological environment. Radioactive waste is isolated by several passive barriers that reinforce and complement one another. Safety of the repository will remain sufficient even if deficiencies occur in one of the barriers or it fails to perform its functions. Safety of humans and the environment will be ensured by using natural barriers, that is old, stable rocks laying at large depths, together with engineered barriers. The engineered barriers are adjusted to the environment of the repository and designed in such a way as to isolate the radioactive waste stored in the repository and prevent it from outspread, while there is no need for post-closure surveillance of the repository.

Figure 1: Design of a deep geological repository.

Currently there is only one deep geological repository in the world – in the United States long-lived transuranic waste has been disposed of in the experimental repository constructed in layers of rock-salt. Spent nuclear fuel has not been disposed of anywhere yet. All spent nuclear fuel has been stored using interim depots which are either of “wet” type (fuel is stored in water basins) or “dry” type (fuel is stored in special containers). Both methods of storage are safe and for some time do not pose danger to the environment. The USA, Finland, Sweden, Japan, France, Belgium, Switzerland are those countries that work intensively in the sphere of managing long-lived radioactive waste. These countries are involved in designing containers used for final disposal of waste and in constructing underground laboratories in which mechanical and chemical properties of rocks are being explored.

Figure 2: A tunnel of Grimsel (Switzerland) laboratory where explorations are carried out for constructing a deep geological repository.

Crystalline basement used for final disposal of spent nuclear fuel and long-lived radioactive waste is the most examined geological medium in the world. An additional advantage of this medium is that this geological formation may be found at lower or higher depths everywhere. In Lithuania there are quite large blocs of crystalline basement not much affected by tectonic processes lying at a depth suitable for a deep geological repository to construct. Such blocs are a promising location for constructing a deep geological repository. In Lithuania the crystalline basement is covered by sediments the thickness of which varies from 200-300 metres in south-east Lithuania to 2000 metres in the Baltic seacoast. With the use of the state-of-the-art technologies, it is possible to construct shafts in rocks of crystalline basement and to bore 150-500 metres long tunnels at a depth of some 500 metres in which containers with spent fuel may be placed. All gaps between the copper containers with fuel and the rocks would be loaded by special impervious clay (bentonite). Clayey rocks are also a promising medium since clay is quite impervious, distinguished by its properties of sorption. Clay, however, is not so steady and less stable than rocks of a crystal base. Therefore, to construct a repository in the medium of clay would be a much harder task than to do so in the rocks of crystalline basement. In Lithuania both the crystalline basement and the promising layers of clay are covered by the thickening of sediments of various origin and properties.

Handling of long-lived radioactive waste in Lithuania

In Lithuania spent nuclear fuel is stored in special containers within depots of “dry” type, in the territory of the Ignalina Nuclear Power Plant (NPP). The storage of waste in these containers will be safe for the period of at least 50 years. During that period it has to be decided how to ensure final disposal of long-lived radioactive waste.

Figure 3: Nuclear fuel used in Ignalina NPP is stored in special containers.

In Lithuania several methods of final disposal of spent nuclear fuel and long-lived radioactive waste are being examined:

1. Final disposal of spent nuclear fuel in Lithuania’s deep geological repository.
2. Final disposal of spent nuclear fuel in regional deep geological repository. To construct such a repository, cooperation among several countries is required.
3. Making use of technical possibilities provided by other countries.

Strategic decisions on further management of spent nuclear fuel and other long-lived waste will be made after considering natural, social and economic conditions as well as taking into account political factors.