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Deep geological repositories are considered globally
as the safest way to dispose of high-level waste
Deep geological repositories are considered globally as the safest way to dispose of high-level waste and spent nuclear fuel. The Czech Republic aims to construct such a facility.
The selection of a suitable site is crucial to the safety of the DGR. Since suitability primarily depends on the rock environment, the initial development stages are mainly related to research. The first phase involves the assessment of archived geological data, followed by the conducting of surface investigation work at the selected sites, based on the results of which the number of candidate sites can be reduced. The next stage involves the gathering of detailed information on the rock mass employing more detailed exploration research, including the drilling of deep boreholes. Priority will be accorded to sites that meet all the safety criteria including a reserve. This will ensure that even the occurrence of unpredictable events will not significantly affect the safety of the DGR. While the determination of a long-term stable rock massif is important, it is not the only element of the barrier system in the DGR, which is made up of both a natural (the rock massif) and so-called engineered barriers (the disposal container, bentonite buffer and backfill, etc.).
It is necessary to consider and analyse in detail all the risks that might occur during the period over which the disposed of waste remains active. The first step is, therefore, to determine so-called potential repository development scenarios which are usually divided into normal and alternative scenarios. Normal development scenarios include all the processes and events that might occur within the repository system with a certain level of probability, whereas alternative processes consider processes and events with a very low probability of occurrence, e.g. earthquake-induced events, major climatic change, or the unpredicted failure of the engineered barriers due, for example, by external intrusion into the repository. By taking all the potential risks into consideration, we are able to provide for safety in virtually all potential situations.
While finding a suitable site and analysing all the potential and less probable scenarios is essential, the highest level of physical protection is provided by the natural barrier, i.e. the rock massif in which the repository will be constructed and the containers in which the waste will be disposed of. Currently, the design of the container envisages a double-layer vessel made of two types of steel – carbon and stainless steel. Steel-based containers will be capable of isolating spent nuclear fuel for thousands to tens of thousands of years. The repository will also be protected against human intrusion, either deliberate or unintentional. In the case of deep repositories in which waste containing very long half-lives will be disposed of, human intrusion will be prevented mainly by the depth of the repository, i.e. several hundred metres below the surface of the earth.
We are conducting detailed research in cooperation with a wide range of research organisations and universities. At the same time, we are actively involved in a number of European EURATOM programmes and enjoy close cooperation with similar organisations to SÚRAO in countries such as Finland, Sweden and Switzerland where the development of deep geological repositories has been underway for nearly 40 years. For more on international cooperation, see HERE (prolink).
How do we demonstrate safety for hundreds of thousands of years into the future?
It is necessary to understand the physico-chemical processes that will take place in the repository and its surroundings.
The description of these processes must be based on generally valid physical laws and the comparison of laboratory data with data obtained in a real environment. This is enhanced by considering data from so-called natural analogues.
One of the world’s most famous natural analogues consists of the so-called Oklo phenomenon. Two billion years ago, a chain fission reaction took place in a uranium deposit in Gabon, Africa, that lasted for around half a million years. The long-term products of the reaction such as neptunium and plutonium move extremely slowly in the nearby environment, i.e. at a rate of a mere 10 metres per 1 million years. It can reasonably be assumed, therefore, that under the same or similar natural conditions, radionuclides from spent nuclear fuel disposed of at a depth of at least 500 m will never reach the biosphere.
A similar natural analogue has also been studied in the Czech Republic, which is known for its numerous uranium ore deposits. The study involved the investigation of the movement of uranium deposited in clay layers at Ruprechtov in Western Bohemia. It was found that the clay with which the uranium ore is surrounded retards the transport of uranium to the surface so that it does not occur in the environment at all.
The selection of an appropriate site together with the so-called engineered barriers will ensure the isolation of the disposed of waste for hundreds of thousands of years. In addition, all radioactive waste repositories are monitored on a continuous basis to ensure that the strict limits imposed are not exceeded. The annual dose limit is only a fraction of what we receive for example from medical examinations involving the taking of an x-ray.
The surface area of the repository will resemble any other light industrial complex and will exert a minimal effect on the appearance of the landscape. Moreover, the future deep geological repository will provide a number of benefits, the most important of which will consist of financial contributions and increased employment opportunities accompanied by investment in the local infrastructure and transport services and an increase in inward tourism.
The lifetime of the first barrier, the waste container, will be tens of thousands of years and a further barrier will consist of half a kilometre of rock massif. The combination of these barriers will provide for the isolation of the radionuclides contained in the disposed of waste from the surface of the Earth for hundreds of thousands of years, i.e. for the time required for the activity of the waste to drop to the level of the environment. Thus, the waste will exert no adverse impact on the environment.