When old meets new8 October 2020
Sweden’s central storage facility for spent fuel is unique in many ways. Preparations are now under way to add an encapsulation plant. Kristina Gillin explains
SWEDEN’S NUCLEAR POWER FLEET INCLUDES nine boiling water reactors (BWRs) and three pressurised water reactors (PWRs). The reactors are distributed over four sites along the coast in the southernmost third of the country. (See NEI, February 2020 p32).
The original plan was to reprocess the spent fuel from the Swedish reactors. This was to be carried out overseas, although the option to build a national reprocessing plant was also explored. Eventually, Sweden steered toward direct disposal and developed a multi-barrier concept referred to as KBS-3. It involved encapsulating the fuel bundles in reinforced copper canisters and emplacing them in a repository located 500 metres down in the bedrock, surrounded by bentonite clay.
With this change in strategy, a new problem arose. As the reactors were built with the assumption that spent fuel would be shipped off site for reprocessing, the storage pools in the reactor buildings were too small for the planned operational lifetimes.
Instead of expanding at each reactor, Sweden opted to build a central interim storage facility (Clab) in Oskarshamn — one of the nuclear sites identified earlier as suitable, in the event a national reprocessing plant would be pursued. Oskarshamn also hosts three BWRs.
Clab received its first shipment of spent fuel in 1985. The fuel bundles arrive by sea using a purpose-built ship. For on-site transport (from the harbour or the Oskarshamn units), the spent fuel casks are moved by truck. Up to now, 7200t of spent fuel has been accepted. In addition, Clab is currently used to store BWR control rods and other core components.
Clab is owned and operated by the Swedish Nuclear Fuel and Waste Management Co (SKB), which is responsible for managing all radioactive waste from the nation’s nuclear power plants. Before leaving the site, the fuel bundles will be transferred into copper canisters in an encapsulation plant, which SKB is planning to build directly adjacent to Clab.
After a period of decay in Clab, the plan is to transport the spent fuel to a KBS-3-type repository in Forsmark. Other radioactive waste from operation and eventual decommissioning is also shipped to Forsmark, for disposal in SKB’s existing shallow geological repository for low- and intermediate-level waste (SFR).
The original facility
Clab was built between 1980 and 1985, in parallel with construction of the third Oskarshamn reactor. Construction of the two facilities was organised as a joint project, to gain efficiencies and reduce the risk of competition for resources. Per H Grahn worked at Clab during construction and he says: “There was strong pressure from the utilities to have the facility ready by mid-1985. Luckily, construction and commissioning went well, and we were able to accept fuel before any of the reactors ran out of storage space and had to shut down.”
The original Clab facility had capacity for 3000t of spent fuel and comprised a receiving building at ground level and a storage building inside a rock cavern more than 30m below ground.
In the receiving building, each transport cask with spent fuel is cooled, using a cooling water system that connects to the cask interior. Next, the cask is lowered into a receiving pool, opened and emptied of fuel assemblies. The fuel bundles are placed in storage canisters, which originally could hold either 16 BWR assemblies or five PWR assemblies. Once filled, the canisters are transferred to the storage building using a water-filled lift.
The storage building has a series of five water-filled pools, built as a block inside the rock cavern. The underground location was selected for physical protection. The bottom of the pools rest on sliding bearings, which enable the pools and the surrounding rock to move independently (to accommodate heat expansion or a seismic event).
To prevent criticality, the pools are equipped with racks that ensure that the storage canisters are sufficiently separated. Other options for preventing criticality that were considered when designing Clab included using borated water.
The deionized water in Clab’s pools is filtered and cooled using separated cooling circuits, which, in turn, are cooled by water taken from the adjacent Baltic sea. The decay heat removed by this system is used to heat the Clab buildings.
Adding storage capacity
In the 1990s, Clab switched to a storage canister that could hold 25 BWR assemblies or nine PWR assemblies. This increased the site’s storage capacity to 5000t of spent fuel, without new construction. To maintain criticality safety, the new ‘compact’ canisters are made of borated steel plates.
A decade later a second storage building was added to increase the capacity further. This building consists of a similar block of pools in a rock cavern built in parallel with the original building. The buildings are connected via a tunnel that includes a water-filled channel for fuel transfer.
The original facility was built with future expansions in mind, so the first part of the tunnel had already been excavated. Similarly, the first part of a tunnel was blasted during construction of the second storage building, in case a third building is needed in the future.
Clab is now licensed for up to 8000t of spent fuel, but additional capacity will be needed while awaiting implementation of the new repository in Forsmark and a capacity increase to 11,000t is in the licensing process. In parallel, the cooling system has been upgraded to accommodate a higher heat load.
The increase to 11,000t is planned to occur without any new construction. Instead, fuel stored in the original canisters will be transferred to the new compact canister. Core components can also be compacted or removed for storage elsewhere.
In 2011, SKB applied for a licence to build an encapsulation plant along the receiving building. Other sites were considered, including co-locating the plant with the spent fuel repository in Forsmark. An important reason for selecting the site next to Clab is that the repository can be kept free from contamination, since the spent fuel will arrive at Forsmark as sealed sources.
Hot cells at the encapsulation plant will house a series of process steps. This includes drying the fuel bundles, filling the canisters with inert gas and bolting an inner lid onto a cast-iron insert inside the copper vessel. A copper lid will be friction-stir welded to the copper vessel. Lastly, machining and non-destructive testing of the weld area will be performed.
Once completed, the copper canisters will be transported to Forsmark using SKB’s existing ship, M/S Sigrid. The plan is to produce and ship around 150 canisters per year.
Preparing for the future
SKB estimates that it will receive the initial licence for the encapsulation plant within a few years. Following that, an application for a construction licence will be submitted to the Swedish Radiation Safety Authority (SSM). Construction is due to commence in the middle of this decade and be completed during the first half of the 2030s. In preparation for construction, the design is being refined further. This includes specifying which existing systems will be used in the encapsulation plant and how they need to be modified and upgraded. Further work is also required to ensure that the encapsulation plant can be built and commissioned without risking safety in existing buildings.
The original parts of Clab have a design life of 60 years. With the current timeline for implementing the encapsulation plant and spent fuel repository, this will be exceeded by a couple of decades. Ageing management is therefore a key area when preparing the future operations.
Tomas Rosengren, who is responsible for both Clab and the encapsulation plant at SKB, says: “The wet storage design chosen for Clab has proven to be very fortunate, since the open concept makes inspection, maintenance and replacement relatively easy.”
SSM is monitoring ageing management closely. It recently requested further assessments to address building structures that cannot be replaced, such as the pools.
SSM has also raised questions about ageing of the spent fuel. Elisabet Ho¨ge, analyst at SSM, explains: “Most international research on long-term storage has been focused on dry storage, where the temperature is higher and other degradation mechanisms prevail. So further work is needed to understand how fuel bundles degrade when stored long term in the wet storage conditions in Clab.”
“Clab has played an important role for the whole fleet of Swedish power plants,” says Johan Dasht, SKB’s managing director. “It has several unique features, like technology and the whole mind-set of using a central interim solution rather than a local solution for each power plant. From a system point of view, this is a better solution considering safety, efficiency and competence, while we are preparing to build the final repository for spent fuel.”
Author information: Kristina Gillin, Principal Consultant at Lloyd’s Register