The safety of all the phases of the final disposal process must be ensured before the commencement of the activities. When final disposal operations comply with the safety requirements set by the authorities as well as the internal requirements of Posiva, radioactive substances will not cause significant health risks to the residents of surrounding areas, even in potential accident situations.
Final disposal is executed according to the multi-barrier principle. This ensures that even if the isolation capability of some canisters was lost, the other barriers will limit and slow down the release of radioactive substances from the fuel contained in the canister. Even in a case such as this, the consequences for people and the environment would be insignificant.
Radioactive substances are isolated by several barriers that back each other up so that one deficient barrier, a predictable geological change, or other similar factor will not compromise effective isolation.
The fuel pellet effectively binds radioactive substances
The fuel, uranium dioxide, is compressed into small pellets which are stacked inside zirconium metal fuel rods. The rods are then bundled together as assemblies. Spent fuel assemblies appear outwardly identical to new ones, but the composition of the uranium pellets has changed. The spent fuel removed from the reactor emits high levels of radiation. In time, uranium, associated fission products, and transuranium elements gradually decay into other elements and finally into non-radioactive elements. Some elements only take a few seconds to decay, others need billions of years.
Most of the radioactive fission products generated in the fuel are short-lived. The radioactivity of the fuel decreases to approximately one hundredth in one year and to one thousandth in forty years. The nature of the radiation also changes as time passes. To begin with, the most significant form of radiation is penetrating radiation. Over the long term, radiation emitted by heavy elements, such as uranium, which is not penetrating in nature, takes over. The radioactive elements remaining at this point are only toxic to humans if ingested or inhaled.
Radioactive substances released from the spent fuel disposed of deep in the bedrock could only reach the human living environment if carried by groundwater moving through cracks in the bedrock. This is only possible if the fuel dissolves into the water. However, the fuel pellet is very poorly soluble, even into boiling water, and in the conditions occurring deep in the bedrock, its solubility into bedrock groundwater is particularly poor. Even in reprocessing, rendering the fuel into a soluble form requires very strong acid solutions.
The copper canister securely contains the waste
The canister insert is made of sturdy cast iron. Its purpose is to prevent the groundwater pressure from crushing the canister. Extreme conditions, such as earthquakes, and the possibility that a layer of ice several kilometers thick may multiply the groundwater pressure in the future, have been taken into account in the strength design of the insert.
The copper overpack protects the canister from the corrosive effect of groundwater. Studies have shown that the groundwater deep inside the bedrock is oxygen-free, which means that its ability to corrode copper is poor.
The canister will remain tight in final disposal for millions of years.
Hard-compressed bentonite clay acts as a protective wall, isolating the canister from the surrounding rock. Bentonite clay is a natural clay with good waterproofing properties that make it a well-suited material for disposal operations.
The properties of bentonite include the following:
- ability to limit the migration of substances
When coming into contact with water, the clay expands, fills any empty spaces, and prevents the movement of water around the canister. The density of the bentonite prevents harmful substances from reaching the canister's surface, while its flexibility protects the canister from any rock movements, earthquake-induced or otherwise.
Old bedrock ensures stable conditions
Bedrock isolates the canister from the organic environment. The effects of any changes in above-ground conditions are limited to the surface layer of the bedrock. Therefore, disposal at sufficient depth ensures stable and predictable conditions for the canisters far into the future. The bedrock also provides good shielding from radiation; two meters of rock is sufficient to stop the penetrating radiation emitted from a canister. Furthermore, the depth eliminates the possibility of inadvertent human entry into the disposal tunnels.
The bedrock in Finland is old and stable. The Olkiluoto bedrock is 1.8 billion years old. The processes that mould the Earth's crust, possibly resulting in volcanic activity, bedrock movement, and earthquakes, ceased to affect the area hundreds of millions of years ago.
Extensive research has shown that any small bedrock movements occur along existing fracture zones. The bedrock between these zones has remained unchanged for millions of years. The disposal tunnels will be excavated in these non-fragmented bedrock blocks which move as a whole.
The processes occurring deep within the bedrock are slow and predictable. Practically the only process that could cause changes in the fuel properties is related to the bedrock groundwater flowing through cracks in the rock. Detailed research has shown that the groundwater deep inside the bedrock is oxygen-free and that its movement is particularly slow. As the groundwater moves slowly through cracks in the bedrock, it reacts with the minerals contained in the rock. In the process, a chemical balance of a predictable nature is created.
Non-fragmented bedrock with few cracks conducts water poorly. Research has shown that the number of cracks in the bedrock allowing groundwater movement decreases when the depth increases. Detailed surveys are carried out to find bedrock areas that feature little cracking and where the groundwater movement is insignificantly small. The aim is to bore the canister deposition holes in locations where no cracking exists or the water conductivity through the cracks is insignificant.