Trial Disposal of Spent Fuel Started at Finnish ONKALO Repository
Trial Disposal of Spent Fuel Started at Finnish ONKALO Repository
Three canisters for storage of spent nuclear fuel have been encapsulated at ONKALO deep geological repository site within the trial disposal. In the near future, the canisters will be transferred to the repository for trial final disposal.
The history of the ONKALO deep geological repository (meaning “small cave”) started in 1983 with the siting for construction and the study of the rock in 1983. The rocks had to be not too hard and the layer under the repository should not contain groundwater. After geological exploration, a granite bedrock at the Olkiluoto site in the municipality of Eurajoki was selected in 2000.
Initially, the repository was an underground research facility to study bedrock of Olkiluoto for final disposal. The construction works on the repository started in 2004 and were divided into four phases:
Phase 1 (2004-2009) focused on excavation of the large access tunnel to the facility, spiraling downward to a depth of 420 meters;
Phase 2 (2009-2011) continued the excavation to a final depth of 520 meters;
Phase 3 (2015-2017) consisted of the construction of the repository and excavation of vertical tunnels;
Phase 4 for the encapsulation and burial of areas filled with spent fuel will begin in 2026.
The ONKALO repository has been excavated with the drill and blast method and the shafts by raise boring. The drill and blast method begins with the drilling of the holes which are charged. Rock mass is blasted and blasted rock is transported away. Rock is sealed where necessary with grouting and structural waterproofing. Grouted rebar bolts and shotcrete are used for rock reinforcement.
The main difficulty in constructing the repository was not in excavation of the serpentine of horizontal tunnels, but in precise work on vertical holes, where spent fuel in canisters will be stored. Machines that can ensure accuracy needed were designed specifically for drilling vertical holes at the ONKALO repository. Strict requirements were set for the holes, the most important of which are clearly vertical axes, so that all points of a deposition hole, which is 8 m deep, do not deviate by more than 25 mm from the vertical center lines.
Special canisters with spent fuel developed according to KBS-3V method will the placed into vertical tunnels after reinforcement of walls. The KBS-3V method aims to protect the environment from the ingress of radioactive substances by using different barriers: cast iron, copper canister, buffer of bentonite and gravel, and hundreds of tons of rock.
The absence of temperature fluctuations, excessive corrosion that would affect the condition of the materials and the structure resistant to seismic activity will ensure reliable and long-term spent fuel storage for centuries.
