With Germany's decision to phase out nuclear energy, the nuclear power plant Unterweser was shut down. Now the once most efficient single-block nuclear power plant in the world is being decommissioned.
For the operator PreussenElektra, this was the first of a total of five pressurized water reactor plants to be decommissioned. Thus, a pilot solution was needed that could be adapted to the other power plants within the fleet.
Based on its high standards for quality and safety in engineering, Mammoet was tasked by contractor Höfer & Bechtel to lift the reactor pressure vessel with a weight of approx. 400t out of the reactor pool as a whole component. During an intensive planning phase, the team of operator, contractor and Mammoet worked together to develop a safe and efficient solution.
The safest lifting solution for the reactor pressure vessel
Mammoet proposed a customized special solution using the SBL 1100, a moveable four-point hydraulic lifting gantry with a capacity of 1100t.
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.
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 the contractor, Mammoet conducted all coordination with the authorized experts and met the high quality requirements by providing the necessary material verifications and production manuals.
Once the documentation was completed, all equipment could be brought into the control area, piece by piece. It was foiled so 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 installed first. 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.
Smooth operation safely removes reactor pressure vessel
The execution was carried out exactly according to the planning in these steps:
First, the reactor pressure vessel was lifted from its installation position in the reactor pool.
Next, it was 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 reactor pressure vessel was then moved back to its installation position and lowered onto the dismantling rack so that the contractor could safely cut off the calotte.
The reactor pressure vessel was finally moved to the final dismantling position and lowered. In the process, it had to be lifted over a wall.
Mammoet ensured smooth, safe and on-time execution at the construction site by bringing together the right expertise from throughout the global business.
The expertise of Mammoet from this pilot project will be utilized for the contractor on another of the operator's decommissioning projects.
"The removal of the reactor pressure vessel was a very demanding project. 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.", says Philipp Boettcher, Manager Engineering at Mammoet in Germany.