Mammoet, on behalf of contractor Höfer & Bechtel, has supported the decommissioning of the Unterweser nuclear power plant in Germany.

Mammoet reactor vessel

Photo credit: Mammoet.

Mammoet was tasked with lifting a reactor pressure vessel, weighing approximately 400 tonnes, out of the reactor pool as a whole component. For operator PreussenElektra, this was the first of five pressurised water reactor plants to be decommissioned and served as a pilot solution that could be adapted to other power plants in its mix.
During an intensive planning phase, the team of operator, contractor and Mammoet worked together to develop a safe and efficient solution.
Mammoet proposed a customised solution using the SBL 1100 four-point hydraulic lifting gantry, which has a capacity of 1,100 tonnes. According to Mammoet, this technical solution could be well adapted to the confined space conditions and would therefore also be suitable for the use in PreussenElektra’s fleet.
Since the tracks of the lifting gantry rested upon the operating floor, and partly also spanned the reactor pool itself via a cantilever, there were strict limits on load bearing capacity of the building structure. Mammoet’s engineering team carried out calculations and successfully proved that load distribution ramps could be used to ensure safe execution.
Before the lifting system was installed on the operating floor, steel load distributors had to be installed. 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. This allowed the reactor pressure vessel to be lifted and lowered vertically and also moved laterally.
With the successful load test for all components, Mammoet received the final approval for execution: lifting the reactor pressure vessel from its installation position, moving it horizontally to a parking position while the contractor placed a dismantling rack at the original installation position. The vessel was then moved back to its installation position and lowered onto the dismantling rack. 

The contractor could then safely cut off the calotte, after which the reactor pressure vessel was moved to the final dismantling position and lowered.