Drilling of deposition holes starts at Onkalo

Radioactive waste management company Posiva said a kick-off meeting was held on Tuesday, where it went through the safety aspects, technical specifications and more detailed plans for the drilling operation once again. The drilling of the first deposition hole using the Deposition Hole Boring Machine (DHBM) was started in the 1.9-billion-year-old Olkiluoto bedrock at a depth of more than 400 metres on Wednesday afternoon.

"The final adjustments to the equipment were made during the morning and early afternoon, and before long the drill was already getting to work on the rock," said Kimmo Lehtola, Posiva's Construction Manager for Operations. "We have already practiced with a couple of holes and preparations for the drilling of the actual deposition holes have been ongoing for quite some time; this day has been much anticipated by our team. It is great to have the opportunity to drill for the first time a hole which will actually become part of the final disposal process."

The Deposition Hole Boring Machine rig was delivered to the Onkalo site in June 2022 from its German manufacturer, Herrenknecht AG. It is more than 9 metres long, 3 metres wide and almost 4 metres tall, and weighs 78 tonnes. The drilling rig complex also includes a 7-metre and 22-tonne energy unit and a 4.1-metre-high and 18.5-tonne drill processing plant. Four holes were drilled in late 2022 as test drilling operations of the boring machine by a crew from Herrenknecht. Posiva's own crew then drilled a total of four test holes assisted by the supplier.

The deposition hole now construction is being drilled vertically into the floor of a previously excavated final disposal tunnel. The finished hole will have a depth of 8.4 meters and a diameter of 1.75 meters. The centre point of the deposition hole must not deviate from the vertical centre line by more than 25 millimetres along the full length of the hole. The first deposition holes that are drilled will undergo even more stringent checks than normal. The first three holes, at least, will each be measured again as soon as they are completed before continuing with the next holes. If the permitted deviation is exceeded despite the monitoring, the hole will be rejected from final disposal use.

According to Lehtola, the drilling of an eight-metre hole in Onkalo takes about ten hours of effective drilling time. "The harder the rock, the slower the drilling process, but ten hours is a pretty good rule of thumb in our conditions," he said. Once the drilling operation becomes part of the normal production process, the goal is to produce about two finished deposition holes in one working week.

A deep geological repository comprises a network of highly-engineered underground vaults and tunnels built to permanently dispose of higher activity radioactive waste so that no harmful levels of radiation ever reach the surface environment. Countries such as Sweden, France, the UK and the USA are also pursuing this option.

_{A rendering of the underground used fuel repository at Olkiluoto (Image: Posiva)}

At Onkalo, 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 final disposal tunnel that is now being worked on is about 330 metres long and will house a total of about 30 drilled deposition holes.

The Onkalo repository

The site for Posiva's repository at Eurajoki, near the Olkiluoto nuclear power plant, 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 (TVO) - 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 would be 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.

In July 2022, Posiva announced that the excavation of the first five actual disposal tunnels had been completed. The total length of the tunnels, whose excavation began in May 2021, is about 1700 metres. 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 350 metres. The tunnels will be about 4.5 metres high and about 3.5 metres wide.

Posiva submitted its application, together with related information, to the Ministry of Economic Affairs and Employment on 30 December 2021 for an operating licence for the used fuel encapsulation plant and final disposal facility at Olkiluoto. Posiva is applying for an operating licence for a period from March 2024 to the end of 2070. The encapsulation plant and final disposal facility are intended for the used nuclear fuel generated by TVO's Olkiluoto and Fortum's Loviisa nuclear power plant units.

The government will make the final decision on Posiva's application, but a positive opinion by the Radiation and Nuclear Safety Authority (STUK) is required beforehand. The regulator began its review in May 2022 and earlier this month announced the safety requirements for granting the operating licence for the world's first used nuclear fuel repository have been met. The Ministry of Economic Affairs and Employment said it "considers that a proposal for the decision on the operating licence could be prepared and submitted to the government in the autumn".

Posiva President and CEO Ilkka Poikolainen said: "Our ambitious goal is to achieve operational readiness for the start of final disposal by the end of 2026."