The reactor pressure vessel (RPV) at Germany’s single unit Unterweser NPP in the Wesermarsch district of Lower Saxony has been removed as part of its decommissioning using novel technology. The 1410 MWe pressurised water reactor was commissioned in 1978 but was shut down in March 2011, along with seven other NPPs, as part of Germany’s post-Fukushima decision to phase out nuclear power. The permit to decommission and dismantle the plant was granted in February 2018 and decommissioning began two weeks later.

Until it was shut down, PreussenElektra’s Unterweser plant was the world’s most efficient single-unit NPP. PreussenElektra is also decommissioning the Brokdorf, Grafenrheinfeld, Grohnde, Isar 2 and Stade PWR NPPs. A pilot solution was therefore needed that could be adapted to the plants within the fleet.

Dutch heavy lifting and transport specialist Mammoet was tasked by contractor Höfer & Bechtel to lift the 400-tonne RPV out of the reactor pool as an intact component. Working together with PreussenElektra and Höfer & Bechtel, a safe and efficient solution was developed. Mammoet proposed a customised special solution using the SBL 1100, a moveable four-point hydraulic lifting gantry with a capacity of 1100 tonnes. This solution was well adapted to use in a confined space and would therefore also be suitable for the rest of PreussenElektra’s fleet.

The tracks of the lifting gantry rested upon the operating floor and partly also spanned the reactor pool itself via a cantilever. This imposed strict limits on load bearing capacity of the building structure. Mammoet's engineering team carried out calculations, which proved that load distribution ramps could be used to ensure safe execution.

Mammoet said another challenge was posed by the strict verification and documentation requirements for the material used in the control area. The preparation of documentation for existing equipment required considerable expertise. Together with Höfer & Bechtel, Mammoet co-ordinated with the authorised experts and was able to meet the requirements by providing the necessary material verifications and production manuals.

With the documentation completed, the equipment could be brought into the control area, piece by piece. This was done in such a way that it could be decontaminated and removed after completion if necessary. Before the lifting system was installed on the operating floor, steel load distributors had to be put in place. These were followed by tracks, four hydraulic lifting stamps and two strand jacks to increase the lifting height of the system. The strand jacks were installed on a hydraulic skidding track on the beams of the lifting system, allowing the RPV to be lifted and lowered vertically and also moved laterally.

First, the RPV was lifted from its installation position in the reactor pool. It was then moved horizontally to a parking position. In the meantime, the contractor placed a dismantling rack with the reactor building crane at the original installation position. The RPV was then moved back to its installation position and lowered onto the dismantling rack enabling the contractor to safely cut off the bottom of the vessel. Finally, the RPV was moved to the dismantling position and lowered. This involved lifting it over a wall.

The expertise learned from this pilot project will be utilised by the contractor on another of PreussenElektra’s decommissioning projects. Philipp Boettcher, Manager Engineering at Mammoet in Germany said removal of the RPV was very demanding. “Through the intensive cooperation, all parties involved have grown together and become a team. We are delighted that we were able to contribute to a safe project with our custom lifting solution and pave the way for another decommissioning project within the power plant fleet,” he noted.

Image: Removal of the reactor pressure vessel at Unterweser (courtesy of Mammoet)