The commissioning of a system for drying used nuclear fuel assemblies before they are sealed within copper canisters for final disposal has taken place at Finnish waste management company Posiva's encapsulation plant under construction at Olkiluoto.

Assembly of the fuel rack for the drying chamber (Image: Markus Forsberg / Posiva)

The drying system was supplied by Platom Oy. The project started in April 2020 and the Site Acceptance Test (SAT) was successfully completed in October this year in close cooperation between the project teams of both Posiva and Platom.

"The tests proceeded without any issues and the drying system operated as planned," said Posiva Project Manager Markus Forsberg. "The licensing process of the system is quite complex, as YVL Guides [Finnish regulatory guides on nuclear safety] pertaining to several fields of technology need to be complied with and the system comprises components of different safety classes. The fuel rack, for example, is assigned to safety class 2.

Posiva said the drying of used fuel is an important phase of the encapsulation process, designed to ensure the long-term safety of the nuclear fuel in the canister. It noted the fuel handling cell is the place where the used nuclear fuel is lifted out of water for the first time since it was loaded in the reactor pressure vessel and then placed in a water storage pool to cool more than 40 years ago.

It said the used fuel drying system has "became the first commissioning process in the equipment installation subproject to be completed with regulatory approval" at the encapsulation plant.

The encapsulation plant is part of Posiva's final disposal facility complex. Once the final disposal operation starts, used nuclear fuel will be transported from interim storage to the encapsulation plant where it will be packed into final disposal canisters made of copper and spheroidal graphite cast iron. From the encapsulation plant, the cannisters will be transferred into the underground tunnels of the repository, located at a depth of 400-450 metres, and further into deposition holes lined with a bentonite buffer.

Skanska Talonrakennus Oy, the contractor responsible for construction of the used fuel encapsulation plant, handed over the building - some 72 metres in length and about 40 metres wide - to Posiva at the end of May for installation of the nuclear systems and commissioning of the process systems of the encapsulation plant.

The site for Posiva's repository at Eurajoki, near the Olkiluoto nuclear power plant, was selected in 2000. The Finnish parliament approved the decision-in-principle on the repository project the following year. Posiva - jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj - submitted its construction licence application to the Ministry of Employment and the Economy in December 2013. Posiva studied the rock at Olkiluoto and prepared its licence application using results from the Onkalo underground laboratory, which would be expanded to form the basis of the repository. The government granted a construction licence for the project in November 2015 and construction work on the repository started a year later. Operation of the repository is expected to begin in 2023. Posiva is still required to obtain a separate operating licence for the facility.

Researched and written by World Nuclear News