Posiva Oy, the company responsible for the disposal of Finland’s spent nuclear fuel, announced last week that it has begun excavating the first disposal tunnels at the Onkalo deep geologic repository near the Olkiluoto nuclear power plant.
Posiva, which is owned by Finnish nuclear plant operators Fortum and Teollisuuden Voima, said that the start of construction is a significant milestone, as it comes after years of research and development activities on methodology for rock construction.
The Onkalo repository will be the first geological disposal facility in the world for spent nuclear fuel when it begins disposal operations, expected in the mid-2020s. Initial construction work on Onkalo, which will be constructed at a depth of 400 to 430 meters (about 1,300 to 1,400 feet), began in 2004. The Finnish government granted Posiva a license for constructing the final disposal facility in 2015.
First five tunnels: Posiva said that the first five tunnels, to be excavated during the next 18 months, mark the beginning of an extensive building effort. It is estimated that 100 deposition tunnels will be excavated during Onkalo’s 100-year operational period, totaling a length of about 35 kilometers. The tunnels, which will have a maximum length of 350 meters, are about 4.5 meters high and about 3.5 meters wide.
Disposal method: Posiva has opted for final disposal based on the KBS-3V method developed by the Swedish nuclear fuel and waste management company SKB. The method involves placing spent fuel canisters in deposition holes drilled into the repository’s disposal tunnels. The copper and steel canisters will be surrounded by a bentonite clay buffer within the deposition hole.
Depending on how many deposition holes there are in a tunnel, which is determined by the volume of suitable rock based on the rock fractures, about 30 canisters will be placed in one tunnel, accommodating about 65 tons of spent nuclear fuel, Posiva said.
The encapsulation of spent fuel and the emplacement of the canisters in the deposition holes will start once the Finnish government grants an operating license for the facility.