Safety

Deep repository safety, both operational and in the long-term, will be ensured by satisfying the legal limits and conditions for safe operation defined in current legislation, i.e. prior to repository commissioning These conditions can be met only if the long-term isolation of high-level waste and spent nuclear fuel from the biosphere is ensured.

The operational safety of a deep repository must meet the same standards as those required of nuclear and mining safety; safety requirements in each of these individual categories may be adopted to improve the standards of the other. Experience gained through the operation of mining and nuclear facilities is of great importance when establishing the operational safety conditions for a deep repository.

Long-term safety is based on the so-called multi-barrier principle: individual barriers in the disposal system such as the long-lifetime container, the sealing materials used in the underground chambers and, primarily, the surrounding rock environment will ensure the long-term isolation of SNF and RW from the environment with which human beings may come into contact in the future.

The safety function of each individual barrier within the multi-barrier system is fully backed-up: if the isolation function of the container fails, its isolation function is replaced by that of the backfill, if the retardation properties of the backfill begin to break down, radionuclides will be retarded by the surrounding rock, etc. This provides a stable system with easily predictable properties.

The aim of long-term safety assessment is to prove that the system meets the requirements for both human and environmental protection for tens of thousands of years after repository closure. Such an assessment is required to prove complete compliance with the radiation protection requirements set out by relevant legislation.

Since processes in a deep repository are very long-term and the only way to verify their development is to employ model calculations the results of which are compared to so-called analogues (both natural and artificial) in order to improve the reliability of the results. Alternative calculations are required to take into account highly improbable scenarios (e.g. the impact of unsuitable or faulty technology, adverse natural events or human intervention in the disposal system). The safety assessment should provide solutions to a whole range of issues the most important of which is proof of both operational and long-term safety.

As mentioned above, the most significant source of information for the long-term study of disposal system behaviour is natural and artificial analogues, i.e. events or processes similar to those expected in the future. The behaviour of the disposal system can be assessed to a relatively high degree of accuracy based on such analogues.

A well known natural analogue can be found at Oklo, an underground uranium deposit in Gabon, Africa, in which a nuclear fission reaction took place 2 billion years ago for approximately half a million years. The long-term products of the natural fission reaction, including neptunium and plutonium, moved through the surrounding environment very slowly, i.e. at a rate of approximately 10 metres per one million years. It can be reasonably assumed that in the same or similar natural conditions, plutonium and other spent nuclear fuel nuclides will never enter the biosphere from a repository located half a kilometre below ground level.

A similar natural analogue was also subject to study in the Czech Republic (which is known for its numerous uranium deposits) involving the movement of uranium deposited in clay layers at Ruprechtov in West Bohemia. The uranium ore is surrounded by a clay layer which slows down uranium transport to the surface to such an extent that uranium cannot be detected in the local environment.

In addition to natural analogues, artificial analogues have also been studied. For example, it was found that copper items from sunken ancient Greek and Egyptian ships which had lain on the sea bed for more than two and a half thousand years were practically undamaged, i.e. they bore no signs of corrosion despite such long-term exposure. It can be assumed therefore that if waste containers are made of copper, the lifetime of the disposal system will be extended (see Fig.). In Finland and Sweden experts suppose that such containers will be able to preserve their integrity for tens of thousands to hundreds of thousands of years.

Finnish disposal container made of copper

In addition to natural analogues, artificial analogues have also been studied. For example, it was found that copper items from sunken ancient Greek and Egyptian ships which had lain on the sea bed for more than two and a half thousand years were practically undamaged, i.e. they bore no signs of corrosion despite such long-term exposure. It can be assumed therefore that if waste containers are made of copper, the lifetime of the disposal system will be extended (see Fig.). In Finland and Sweden experts suppose that such containers will be able to preserve their integrity for tens of thousands to hundreds of thousands of years.