TECHNICAL | SHAFTS & CAVERNS


rock and each one awaiting a single sealed canister that will then be backfilled and buried in its solitary void. Implenia is also to design and build three deep


shafts down to the repository level. The shafts are for ventilation and a lift, also to serve and support the activities of the underground complex over the coming decades of its operational life. In Q3, the contractor began mobilising on site. Upon


final safety approvals, from the national radiation safety authority, and after further preparatory works on the surface, the first steps in main excavations are to begin. Excavation of the shaft, access tunnel and the


Right:


Early site investigation drilling at Forsmark site


Below:


SKB’s report on site investigation at Forsmark undertaken over 2002-2007


initial repository-level tunnels in the package are anticipated to take about eight years to complete. The full scale of the underground repository, though, is far more extensive. The network of tunnels and storage zones will only be extended in stages, matched to requirements. Some areas will have been filled with canisters and themselves sealed off, and new operational areas opened up. Current expectations are than the repository will


have been fully extended in some 60 years’ time, by the 2080s. By then, it is anticipated that 66km of tunnels will have been excavated, with in a grid network covering an area of 4km2 Expectations are that 2.7 million m3


TUNNELLING


The government granted approval for the deep repository to be located at Forsmark in early 2022, SKB told WTC 2025. A year ago, SKB was given the greenlight with an environment permit to start construction. By mid-2025, it had signed a collaboration agreement with contractor Implenia for the major initial tunnelling works required, to begin major excavations of what will be, eventually, a huge underground network.


Under the agreement with SKB, services being


provided by Implenia include planning, design and construction of the tunnels and shafts needed to reach down to the repository level, the a ‘central’ base area at that depth, and also the first main excavations to launch the underground storage grid of tunnels. More particularly, Implenia is to perform


detailed design and then excavations for the 5km-long spiralling access tunnel to reach some 470m below the surface (it is expected, WTC 2025 was told); the first of the local grid of large tunnels for various operational activities at the base of the shafts and access tunnel; and, also the initial transport tunnels that will open up the first repository storage zones. Branching off at regular intervals from the transport tunnels will be arrays of long ‘deposition’ tunnels. Along each of these slightly narrower tunnels will be storage silos, carved into the


16 | October 2025


, SKB told WTC 2025. of rock will have


been excavated, slowly and carefully, to form the tunnels at the repository level, and also the access tunnel and shafts from the surface. The scale of excavation is greater than that discussed before (1.85 million m3


) when Forsmark was picked over Oskarshamn as favoured candidate to be the host site.


DISPOSAL SKB says that when the repository has finished excavating its first access, base, transport and storage tunnels then disposal of spent fuel can begin. The present timeline anticipates the disposal activities in the deep geological vault should be able to commence in the 2030s. The plan is for Forsmark to host approximately 12,000


tonnes of spent fuel. This quantity of fuel is to be held in 6000 canisters, specially designed and sealed, and each to be individually placed in its own tightly carved slot. Each copper canister will be packed into the space with a surrounding of bentonite clay, the silo then plugged, and sealed off. This is the ‘KBS-3’ storage concept. The burial slots are to be spaced out at regular


intervals along the deposition storage tunnels, which will terminate in dead-ends. A few of these branch tunnels are to be in use at any one time, the storage activities working between them, as each of these will also be sealed up to where the most recent silo has been buried. Operational storage activities, therefore, will gradually


back out of each deposition tunnel, eventually to be filled completely, back to the junction with the main transport tunnel. The pattern of working across a few branch tunnels and gradually filling each will be ongoing.


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