Finland has adopted disposal techniques developed by Sweden, with Finnish input. They rely on a series of barriers to prevent escape of radionuclides.
Spent fuel is thought to be reasonably resistant to the escape of radioactivity since the fuel rods are made up from uranium embedded in insoluble ceramic pellets.
After they have cooled, the rods will be stored for 40 years in interim storage water tanks before they are transported to the repository where a surface ecapsulation building will prepare them for long-term disposal.
There, the main defence is copper, chosen for its resistance to corrosion. Researchers cite deposits of native copper, including one in the UK, that have resisted corrosion over several hundred thousand years.
'We could have used gold too, ' laughs Niemitalo, 'but people might dig it up again.' The rods are bundled inside copper cylinders which are lled with neutral argon gas and then welded shut. The fuel inside the 50mm thick copper casing is held in an iron framework which supports and separates them. But its primary function is to give the cylinders resistance to crushing and bending forces. This means greater reliability and reduces the amount of copper used.
The cylinders sit in 7.5m deep vertical holes drilled at intervals in the base of the tunnels. They are packed in with blocks of bentonite which will gradually absorb groundwater and expand blocking off groundwater flow around the cylinders, explains Niemitalo. More bentonite mixed with rock chip spoil will fill each tunnel once full.
An alternative of horizontal storage in bentonite along the tunnels is also being researched.
The rock itself is the final barrier of course, as long as it is sufciently sound, restraining groundwater movement to a very slow pace.