The Final Disposal Canister
Disposal canisters are massive metal casks. Their interior is made of nodular graphite cast iron, and the exterior is made of copper. With the spent fuel from the four nuclear power plants in operation and the fifth one under construction, some 2800 canisters will be needed for the final disposal.
Construction materials, structure, production methods and sealing technology of the final disposal canister have all been thoroughly examined so as to achieve a mechanically and chemically resistant container that can remain leak-tight deep inside the bedrock for at least 100,000 years. Leak-tightness is ensured with setting high quality requirements to the production and controlling the observance of these demands with an extensive inspection system.
Copper Canister
The outer part of the final disposal canister is 5 cm thick and made of copper. It will protect the insert and the fuel assemblies from the corrosive effect of groundwater. According to extensive theoretical and experimental research, the corrosion, or erosion to the extent that leak-tightness would be lost, of the canister would take hundreds of thousands of years even in very unfavourable conditions.
Reserves of natural copper show that copper can last hundreds of thousands of years inside the bedrock without corroding. This is why copper is the material of choice for the exterior of the final disposal canisters. A further piece of evidence is a chisel, made of natural copper and found in Suomussalmi in Finland, that had resisted the aboveground conditions and remained in good shape for thousands of years.
Cast Iron Insert
The interior of the final disposal canister is made of cast iron and is strong enough to resist the mechanical stress that the canister inside the bedrock is subjected to. The canister has been perfected to endure even extreme conditions, such as earthquakes, or the pressure inflicted by a continental glacier.
Cast iron has been selected because it is tenacious and strong and thus an ideal material for the interior of the canister. Cast iron is also easier to cast than many other materials, such as steel.
Three Different Canisters
There are three different fuel assembly types in the Finnish nuclear power plants: one in plants Loviisa 1 and 2, another in plants Olkiluoto 1 and 2, and yet another type in Olkiluoto 3, the plant currently under construction. Each fuel assembly size has its own canister design. All canister designs are 1.05 m wide, but their length varies according to the particular fuel assembly type in question.
