Excavation of the first final disposal tunnel has started at the Onkalo underground used nuclear fuel repository near Olkiluoto, Finnish radioactive waste management company Posiva Oy announced today. The repository - the first in the world for used fuel - is expected to begin operations in the mid-2020s.

Excavation of the first deposition tunnel, which will eventually be up to 350 metres in length, begins at Onkalo (Image: Posiva)

Used nuclear fuel will be placed in the bedrock, at a depth of about 450 metres. The disposal system consists of a tightly sealed iron-copper canister, a bentonite buffer enclosing the canister, a tunnel backfilling material made of swellable clay, the seal structures of the tunnels and premises, and the enclosing rock.

The first five tunnels to be excavated during the next 18 months mark the beginning of an extensive building effort, Posiva said. It is estimated that 100 deposition tunnels will be excavated during the 100-year operational period of the final disposal facility, and will have a total length of about 35 kilometres. The maximum length of each tunnel will be 350m. The tunnels will be about 4.5m high and about 3.5m wide.

The excavation of the first five tunnels is part of Posiva's approximately EUR500 million (USD607 million) EKA project that covers all the final disposal facilities needed, including both their construction and equipment, as well as the start of the final disposal operation in the first deposition tunnel.

The company said the start of the excavation is a "significant milestone" for Posiva, as it comes after years of development activities on research and methodology for rock construction. It noted that Finland's Radiation and Nuclear Safety Authority (STUK) verified before the excavation began that the pre-conditions specified for starting the work had been fulfilled.

"The years of research and development of rock construction that have produced procedures for the construction of a nuclear facility suited to the Finnish bedrock have culminated in this moment," said Posiva Construction Manager Juha Riihimäki, adding that the development of the methodology started with the construction of the Onkalo facility in 2004.

Encapsulated fuel

Posiva's plan is for used fuel to be packed inside copper-steel canisters at an above-ground encapsulation plant, construction of which began in September 2019 and is scheduled to be completed in mid-2022.

About 30 canisters will be placed in each tunnel, Riihimäki said. This number depends on how many deposition holes there are in the tunnel, which is determined by the volume of suitable rock based on the rock fractures. The 30 canisters placed in each tunnel can accommodate a total of about 65 tonnes of used nuclear fuel.

Posiva noted the encapsulation of used nuclear fuel and the emplacement of the canisters in the deposition holes will start once the Finnish government grants the operating licence for the final disposal facility.

"The current estimate is that the final disposal operation will start in the mid-2020s," said Kimmo Kemppainen, Posiva's programme manager. "The excavation and reinforcement of the tunnels, which have now been started according to tight regulatory requirements, signify for Posiva a transfer to the execution of the most complex and exacting rock structure system in Onkalo."

In March this year, Posiva announced that work to excavate a tunnel had started at the Onkalo for the 'joint functional test', which involves small-scale final disposal under actual conditions, and is part of commissioning the geological repository. That tunnel will be about 80m long and four deposition holes will be drilled into it during the joint functional test, which will take place in 2023. This test will demonstrate that the processes and procedures related to final disposal are in order and, only then, can the disposal facility be granted an operating licence.

The site for Posiva's repository was selected in 2000. The Finnish parliament approved the decision-in-principle on the repository project the following year. Posiva - jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj - submitted its construction licence application to the Ministry of Employment and the Economy in December 2013. Posiva studied the rock at Olkiluoto and prepared its licence application using results from the Onkalo underground laboratory, which is being expanded to form the basis of the repository.

The government granted a construction licence for the project in November 2015 and construction work on the repository started a year later.

The Onkalo geological repository will be the first in the world for used nuclear fuel. A similar repository is planned at Forsmark in Sweden.

Researched and written by World Nuclear News