The long-term safety of a final disposal facility
The long-term safety of a final disposal facility
The basic principle in the use of nuclear energy is that it must be utilised safely and in a way that does not cause harm to people, the environment or property. This principle applies to the entire nuclear fuel cycle, including waste management and final disposal.
An important ethical principle in the final disposal of nuclear waste is that the radiation safety criteria of individuals and populations of the future are the same as those of current individuals and populations. However, the determination of an acceptable level of safety is very complicated and depends on people’s perceptions of risks and the acceptability of different risks.
The average radiation dose received by a Finn from various radiation sources is approximately 5.9 millisieverts per year. The final disposal of nuclear waste is subject to a radiation dose constraint, according to which the radiation dose from disposed radioactive substances may not exceed 0.1 millisieverts per year, which is a fraction of the average annual radiation dose of a Finn.
The main principles of the geological final disposal of nuclear waste are the isolation of waste from the biosphere for as long as possible and the prevention of people from accessing a final disposal facility. The final disposal depth and technical release barriers play a role in both of these principles. They make it possible to achieve isolation from disturbances in the ground surface environment and radioactive substances have a longer or slower route of migration to the surface of the ground if insulation fails. The possibility of unauthorized access is also smaller in deep final disposal compared to near-surface final disposal or storage.
The purpose of isolation is to ensure that the hazardousness (radioactivity) of radioactive nuclear waste decreases during the final disposal to a level that is insignificant for the safety of the biosphere. The length of this period depends on the characteristics of the nuclear waste in question. In practice, periods range by waste type from several hundreds of years to hundreds of thousands of years. For example, the activity of spent nuclear fuel decreases rapidly at first but reaches an activity level comparable to uranium ore only after a very long period of time. On the other hand, in the surface final disposal of very low-level nuclear waste, the activity level that is very low to begin with reaches an insignificant level in a few hundreds of years.
The safety of final disposal is based on several release barriers
The safety of the final disposal of nuclear waste in the long term (long-term safety) is based on release barriers that complement one another. In final disposal according to the so-called multibarrier principle, the non-functionality of one or more release barriers must not compromise the safety of final disposal. Release barriers consist of the natural release barrier (bedrock) and technical release barriers.
Radioactive nuclear waste is processed, packed and disposed of meticulously. For example, in the final disposal of spent nuclear fuel, the nuclear fuel is enclosed in a hermetic copper canister that is placed in a hole lined with bentonitic clay inside bedrock, at a depth of more than 400 metres.
The purpose of the final disposal canister is to keep radioactive substances inside the canister for as long as possible. The task of the bentonite buffer surrounding the canister is to slow down the process of groundwater getting into contact with the copper canister and, on the other hand, prevent radioactive substances from getting into the rock if the canister leaks. The bentonite buffer also protects the canister from rock displacements. The purpose of tunnel backfilling is to keep the bentonite buffer surrounding the final disposal canisters in place. In addition, tunnel backfilling helps preserve the mechanical stability of the premises and prevent excavated premises from becoming groundwater flow routes. Bedrock slows down the migration of radioactive substances to the biosphere as the flow of groundwater in the cracks of rock is scant and radioactive substances adhere to the crack surfaces and to the bedrock itself.
The quality of release barriers is ensured by defining high quality requirements and extensive inspections for their production. In the construction of a final disposal facility, the goal of maintaining bedrock conditions that are favourable for safety is taken into account. Rock engineering is also subject to high quality requirements.
Demonstrating the long-term safety
Long-term safety refers to the safety in the period after the closure of the final disposal facility. In connection with the assessment of long-term safety, the factors reviewed include, among other things, the adequacy of release barrier planning, the significance of the uncertainties that the final disposal solution entails and various future developments (scenarios).
The reliability of assessments of the long-term safety of a final disposal facility are ensured by analysing technical and scientific materials, observations, experiments, tests and other evidence. The potential radiation effects of final disposal are investigated with safety analysis calculations. The results of the safety analysis are compared with the safety requirements in order to assess long-term safety.
The future radiation risks caused by final disposal are assessed with scenario-based safety analyses. Scenarios describe potential future developments, such as ice ages or displacements that might cause radioactive substance releases. Analyses involve pessimistic assumptions to compensate for uncertainties related to future developments.
When talking about very long periods of time (millions of years), the technical release barriers of a final disposal facility finally lose their functionality. The final disposal solution aims at ensuring that in this case, the remaining radioactive substances would no longer cause hazards for people living in the area.
When assessing the safety of the final disposal of spent nuclear fuel in the long term, safety must be assessed as a whole that includes the long-term functionality of the release barriers in the final disposal system as well as the migration, delay and dose calculation of released radioactive substances in the light of different future developments. However, the review of the future inevitably entails uncertainties as the future cannot be predicted. In connection with safety assessment, an understanding must be formed of the uncertainties associated with the whole and, on the basis of it, a view of risks must formed. When decisions are made about the safety and acceptability of final disposal, risks must be at an acceptable level.
The safety requirements imposed by STUK require that, in the context of the final disposal of spent nuclear fuel, the technical release barriers isolate the radioactive substances and prevent them from being released to the bedrock for at least 10,000 years, after which the radioactivity of the spent fuel has decreased considerably. The corresponding requirement applicable to short-lived low and intermediate-level waste is 500 years.
STUK evaluates the materials related to final disposal and the safety case concerning long-term safety in different permit application phases of the final disposal project. With regard to long-term safety, STUK especially supervises work carried out to demonstrate the functionality of the final disposal system, scenario and safety analysis work and their compliance as well as, ultimately, the demonstration of long-term safety and the reliability of the safety case.
The safety of the final disposal facility is subject to STUK’s regulation (STUK Y/4/2018). STUK’s YVL instructions include more detailed instructions pertaining to nuclear waste management.