Canisters
After interim storage the spent nuclear fuel will be encapsulated in copper. The copper canister is one of the barriers that prevents radioactive substances from escaping into the environment.
The canisters will be deposited in the final repository at a depth of 500 metres. They have been designed to withstand earthquakes and future
ice ages.
The copper canisters that will encapsulate the spent nuclear fuel are nearly five metres long and over a metre in diameter. They weigh between 25 and 27 tonnes when filled with spent nuclear fuel. The outer casing is a five-centimetre-thick layer of copper to protect against corrosion. Inside is a nodular cast iron insert to provide additional strength.
The canister is designed to withstand corrosion and any mechanical forces caused by movements in the rock. We have thoroughly tested the canister material for its capacity to cope with such stresses. Provided the canister is properly sealed and impermeable, no radioactive substances can escape into the surroundings.
Buffer
Before the canister is lowered into the final repository's deposition hole, the hole is lined with blocks and rings made of bentonite clay.
The bentonite clay will act as a buffer between the walls of the deposition hole and the copper canister. The buffer damps both mechanical movements and chemical changes in the rock.
The task of the buffer in the final repository is threefold:
- To prevent corrosive substances from getting to the canister.
- To protect the canister in the event of small movements in the rock.
- To delay the spread of radioactive substances that may escape from
a canister that has become permeable.
Rock
The rock surrounding the final repository will serve to isolate the waste from man and the environment. It will also give the canister and the bentonite clay buffer a stable chemical environment and protect them from whatever is happening at ground level.
The Swedish crystalline bedrock is not an impenetrable, compact material. There are fractures in the rock through which the groundwater passes in various ways. Water flow is generally very slow, though it can vary slightly from fracture to fracture and within a single fracture. The large fracture zones in the rock can provide channels for major water flows. These zones also act as shock absorbers by absorbing natural movements in the rock.
If radioactive substances are dissolved in water they follow the water's movements. Nearly all radioactive substances in the spent nuclear fuel can attach to fracture surfaces and minerals, and be caught inside the micropores of the rock. It is only certain forms of iodine and carbon that are fairly mobile.