Finland: designing for disposal

Image: Posiva’s final disposal facility for spent nuclear fuel and its underground tunnels where KSAA will operate )Photo credit: Posiva)

FINLAND HAS FOUR NUCLEAR POWER units in operation. A fifth, Olkiluoto 3, will soon begin energy production. This form of electricity production generates radioactive waste, which must be handled in a manner that will not harm the environment now or in the future. This is an issue of assuring long-term safety of final disposal.

“Nuclear waste has not been disposed of in any country yet; instead, it is being kept in interim storage facilities or containers. Finland is the first country in the world to ensure safe final disposal of nuclear waste,” says Jarkko Stenfors, manager of Posiva’s Final Disposal Equipment Unit.

Final disposal is an underground process that requires equipment and logistical operations to be developed in parallel.

The process starts by boring disposal holes in tunnels that have already been excavated, using a purpose-built boring machine. The machine is currently under manufacturing and will be tested in 2022 before the actual production starts.

Underground disposal cannot begin until bentonite clay blocks are installed in the bottom of the disposal hole. Bentonite prevents water from moving around the canister and protects the canister from small rock movements.

The canister is transferred from the loading station to the disposal tunnel using a dedicated canister transfer and installation vehicle (KSAA, see below), and the same machine is used to install it in the disposal hole, which it does with an accuracy of a few millimetres.

Image: 3d-model of the canister transfer and installation vehicle (KSAA) equipment (Photo credit: Posiva)

Once the canister is lowered into the hole, the upper part of the hole is also filled with bentonite blocks and the tunnel is filled with granular bentonite clay. This tunnel backfill prevents water from flowing in the tunnels and holds the buffer material in place. When the entire disposal tunnel is filled with granular bentonite clay material, a thick concrete plug is casted at its end.

Disposal will continue until all the spent nuclear fuel in Posiva’s possession has been safely disposed of in the Olkiluoto bedrock. After this, all the tunnels and shafts leading to it will be closed, and the buildings on the ground will be demolished.

The equipment

KSAA is the world’s first spent nuclear fuel canister transfer and installation vehicle and it was designed by consultant company Comatec. Cooperation between Posiva and Comatec started more than a decade ago. Stenfors describes Comatec as a ‘royal supplier,’ which has already designed several prototype devices to test the disposal concept. Now, the final production devices are being designed.

“We do not have the required expertise to design vehicles in-house. We are a project organisation and the plant supplier of the disposal facility, as well as its end user once it is complete,” Stenfors explains.

The canisters are massive containers with a nodular cast iron insert and a copper shell. Around 2800 canisters will be required to dispose of the spent nuclear fuel from the four operational power plant units and the fifth unit currently being commissioned. The final disposal operations will continue for around a hundred years.

Strict safety requirements and limited space

Very strict safety requirements had to be considered in the KSAA’s design. Posiva had to meet thousands of rows of safety requirements, and dozens more listed in the Regulatory Guides: Nuclear Safety (YVL) publication of the Finnish Radiation and Nuclear Safety Authority. This is because the vehicle is a nuclear safety classified system (SC3).

The starting point for handling the canisters is single failure tolerance. This means that the safety of the canister must not be compromised if any single component fails, and it must be possible to restore the canister to a safe state.

During transport, the canister will be kept inside a radiation shield, which isolates it and protects the people who work close to the device.

According to the plans, there will be more than a hundred deposition tunnels with a total length of approximately 42km. These will cover an area of around 3 km2. Another major challenge has been adapting the device and assuring its functions in the limited space available underground.

Safety is further improved by KSAA’s electric transmission system. The battery pack does not emit exhaust fumes, so people working in the tunnel will not be exposed to them. Fuel tanks are also unnecessary, which reduces the risk of fire.

According to Stenfors, the battery pack is ground- breaking, because it has been approved for use in the underground deposition process. The battery chemistry has been selected with fire safety as a key attribute; as a result the risk of severe battery fire is practically non-existent.

The battery pack has also been designed so that the vehicle can complete the entire canister deposition cycle with one charge.

The vehicle, which weighs more than a hundred tonnes, will initially be remotely controlled underground. The goal is to make it move autonomously along optimal routes without touching any walls, and for it to stop when it encounters an unexpected obstacle. However, a person will still monitor the device from the control room.

“We want to increase the facility’s degree of automation in other ways as well. If we can reduce the required person-years throughout the hundred years of operation, we will achieve major savings for Finnish society at large, because increased production costs will directly influence the price of electricity,” Stenfors says.

According to Stenfors, the device as a whole represents a major innovation because it had to meet so many demands and would work in such a limited space.

“We are making industrial history, because we are the first to create a device for such a small underground space with such strict requirements,” he says.

Innovation

Another innovative aspect of the KSAA device as a whole is that it has an ‘ innovation chain’ that includes all the main functions. It is like a Transformer, in that it can move its own components and modify itself for different functions while on the move.

The detailed design phase for the equipment and the project will be completed in summer 2021.

COVID-19 made the design project even more challenging. However, engineers were able to work remotely to complete examinations of the 3D model and attend other project meetings, according to Stenfors.

He adds, “The cooperation with Finnish consultant companies has been very fluent. Indeed, I’m amazed by the pace and energy of all the companies during this project. I will tip my hat to them if they can keep up the same pace until the end. On the other hand, this project is a showcase for the global nuclear industry.”

Posiva’s equipment programme has now proceeded to the detailed design phase, which is estimated to be completed in summer 2021.

Meanwhile the company’s commercial subsidiary Posiva Solutions provides tailored expert services using all the company’s know-how. The expertise offered is based on Posiva’s decades-long experience in design, research and development of final disposal of spent nuclear fuel in Finland and we believe Posiva Solutions can add value to national nuclear waste management programmes by saving time and cost.