Sweco involved in the design of the world’s first encapsulation and final disposal plant for spent nuclear fuel

Posiva Oy is the world’s leading final disposal company, which will start the final disposal of the spent nuclear fuel of its owners, TVO and Fortum, to ONKALO® deep in the bedrock. Posiva is currently building an encapsulation and final disposal plant, which was granted a construction permit by the Government in 2015. The final disposal of spent nuclear fuel in Olkiluoto will utilise a process never before seen anywhere in the world.

Sweco was selected to carry out the structural engineering of this advanced encapsulation plant and the coordination of different information models into a single combination model. ‘The solution presented by Sweco was of the highest technical level and their competence was the most convincing,’ Unit Manager of Structural Engineering Juha Matikainen from Posiva Oy describes the starting points of the partnership.The site has been under construction for almost a year, but design work to implement the project has been carried out for years. In the design, special attention has been paid to the improvement of safety, the reduction of waste on site and the long-term maintenance of the site.Safety as a starting point for structural engineering work

Sweco’s Anssi Mäntynen, Structural Engineer and Project Manager in charge of the project, points out that the Posiva project is subject to nuclear safety supervision and includes safety-classified structures. The structures and loads are similar to those on other heavy industry sites, but here the safety aspects are emphasised and reflected in the level of quality required from the designs. “For example, the design and calculation work for the acid-resistant steel cladding in the processing chamber of the encapsulation plant was exceptionally demanding,” Mäntynen says, praising his team for their strong expertise.

Key elements have been that any problems are openly brought up and that all steps are carefully documented so that the information can be traced afterwards. The importance of information security has also been highlighted. All design information is stored in Posiva’s own information systems, and the company has instructed Sweco on data storage.

Virtual tools were utilised in BIM coordination

BIM coordination is in charge of information model integration, drawing up and inspecting the combination model and monitoring its corrections.

The exceptional performance of the VirtualSite™ tool developed by Sweco enabled all information models of the final disposal plant to be combined into a single virtual model. The VirtualSite model includes, for example, dozens of kilometres of tunnels, mechanical models, all HVAC and electrical models, structural engineering, architectural design and the realisation data of tunnels gained through laser scanning. Furthermore, the model provides the 1:1 scale and allows for the design situation to be monitored in a group virtually and with remote connections, regardless of place.

The virtual model of the encapsulation plant reflects reality and includes an animation of the entire encapsulation process from start to finish. The virtual model can be used to ensure the operation and safety of the entire process.

“At Sweco, we have unique expertise in providing tools that traditional software companies have been unable to produce. I would never have believed that the entire enormous plant could be included in a single virtual model without cutting it into pieces,” Sweco’s Specialist Antti Hämäläinen, responsible for the project’s BIM coordination, happily describes the opportunities provided by the new tool.

“BIM coordination has gone well despite us being unable to progress exactly as planned with foreign operators,” Matikainen assesses. He praises Sweco’s courage in promoting new tools: “Personally, I would not have thought to request these new opportunities. These are not available straight from the shop!”

There are many benefits to using virtual tools

It is easier to make observations in the virtual model and any deficiencies can be noticed early on. For example, occupational safety risks can be dealt with already during the design stage and possible hazards identified. Correcting these issues before construction both increases safety and enables cost savings. Making changes later is significantly more expensive.

In structural engineering, reinforcements for cast-in-place and precast structures, as well as supporting steels, have been modelled in great detail. “Although the detailed modelling of reinforcements takes time and hard work, the accurate BIM is a great help in the implementation and coordination of the entire project,” Mäntynen states.

Accurate modelling also cuts down waste in material acquisitions. ”For example, if the site orders reinforcements according to the BIM lists, the site will always receive the correct amount of correct bars without material waste,” Mäntynen describes.

In addition to use during construction, Posiva’s Matikainen thinks that virtual tools will be used especially for maintenance purposes. Maintenance is of great importance in a nuclear facility since the encapsulation plant has a service life of a hundred years, for example.