Seach
Final disposal of the spent nuclear fuel from the nuclear power plants owned by Teollisuuden Voima and Fortum will be in copper canisters placed at a depth of some 400 metres in the bedrock at Olkiluoto. Posiva Oy is responsible for research into the final disposal of spent nuclear fuel and for the practical implementation. The tests relating to final disposal are carried out at ONKALO, the underground rock characterisation facility at the nuclear waste final disposal plant. Comatec Group has designed a prototype for installing the bentonite buffer blocks. It is planned to start testing the machine this year. Final disposal should start at the beginning of the 2020s and will continue for almost 100 years.
‘Teollisuuden Voima Oyj’ and ‘Fortum Power and Heat Oy’ are Finnish energy companies that own Posiva Oy, which is responsible for research into and the actual implementation of the final disposal of the spent nuclear fuel of its owners. The disposal of spent nuclear fuel must be organized in such a way that it is not harmful to organic nature. Final disposal of the spent nuclear fuel from the nuclear power plants owned by Teollisuuden Voima and Fortum is in copper canisters at a depth of four hundred metres in the bedrock at Olkiluoto. The tests relating to final disposal are carried out at ONKALO, the underground rock characterisation facility at the nuclear waste final disposal plant.
The key issue in the final disposal of spent nuclear fuel is the long-term safety of the solution, meaning the safety of the repository after the final disposal plant has ceased its operations and the facilities in the bedrock have been closed. Long-term research is being conducted to examine the suitability of the conditions in the bedrock for final disposal and assess how they affect safety. R & D aims to ensure that the canisters, the bentonite and the back-fill material will last for the required period of time.
Long-term safety is based on the multi-barrier approach. The radioactive matter is placed within several barriers to release that together make each other more effective but are as far as possible independent of each other, so that if one barrier fails it will not put the effectiveness of isolation at risk. These barriers are the ceramic state of the fuel, the copper/cast iron canister, the bentonite buffer, the tunnel back-fill and the surrounding bedrock. The spent nuclear fuel is packed into the final disposal canisters at an encapsulation plant. After packaging the canisters are transported by lift to the underground repository. The canisters are placed into holes in which bentonite blocks have been installed.
The bentonite buffer blocks are therefore one part of the barriers. Bentonite clay is sensitive and reacts to moisture. This sets its own challenges for handling the material. Altogether 10 blocks will be placed in one final deposit hole. To achieve the target speed set in the project, the blocks should be installed in one hole in two hours. Extremely high precision is required for installing the blocks, to the millimetre, to make it possible to place the canister in the hole. The betonite blocks have a diameter of 1.65 metres, the highest block has a height of 96 cm, and the heaviest weighs four tonnes.
Posiva plans to test the installation of the betonite blocks in the holes in the repository during 2013. Comatec Group is designing the prototype of a special installation machine for the tests that will be made in Finland. This will be the first of its kind in the world.
The prototype machine for installing buffer blocks will be 7.7 metres long, 3.7 metres high and 2.6 metres wide, and it will weigh some 20 tonnes.
“Comatec’s product development project began in spring 2011 with the concept design. Closely connected with the concept design for the machine was logistics planning, where we developed various methods for bringing the bentonite blocks to the installation machine,” says Ari Haanperä, who was in charge of the project at Comatec.
“When we started designing, we knew what the installation machine had to achieve and the technical requirements for this. These formed the basis for the design.
“The first step was logistics planning, in other words deciding on the most effective concept for bringing the bentonite blocks to where they would be installed. Bentonite’s sensitivity to moisture was a major factor here,” says Haanperä.
“Detailed design of the prototype began once we had decided on one concept out of the several options available, and we then started
to implement this. Simplicity of structure, reliability and installation safety have been of primary importance in the design of the prototype. The machine is remote controlled and contains no hydraulics. We’re using electrical equipment instead of hydraulics,
and during the project we also examined other ways to perform the required movements,” states Haanperä.
Designing the prototype has been one of Comatec’s largest product development projects, and it is currently being built at the workshop of Konepaja Laaksonen in Turku.
“Altogether some 30 experts at Comatec have taken part in the design project. We have needed all areas of Comatec’s expertise in the design, such as mechanics, hydraulics, electrics and automation, as well as expert services including strength calculation and product safety design. Just to mention a few of our areas of expertise,” says Haanperä.
“We’re finishing off the documentation and reports at the moment and we still need to test the machine and the concept,” he adds.
Designing the prototype for installing the buffer blocks is part of the LUCOEX project financed by the EU, in which Posiva is working with other international organisations responsible for final disposal. The other organisations are SKB in Sweden, Andra in France and Nagra in Switzerland. The LUCOEX project is a four year project that began in 2011 and should finish in 2014. Posiva’s goal in the project is to develop equipment for final disposal.
“Each country is developing its own final disposal concept, but we exchange information and insights. At the same time we cooperate in studying and developing installation solutions,” says Keijo Haapala, development engineer at Posiva.
“In the EU project the installation of buffer blocks is divided into three parts: installing the buffer blocks, quality assurance for this, and handling any problems.
“Working underground it is easy to forget that there are very high precision requirements for installation,” stresses Haapala.
“Developing a completely new machine is extremely challenging. It is hard to foresee everything that needs to be taken into account,” says Haapala.
“Cooperation with Comatec has gone very well. For example we decided on a completely new solution, using a special transport container for transporting the bentonite buffer blocks to where they will be installed,” explains Haapala.
The LUCOEX project is a public project and information about progress in the project is made freely available. The LUCOEX project has its own website: www.lucoex.eu. Posiva also communicates openly about its activities on its website at www.posiva.fi.
TEXT: TAINA SYRJÄNEN