Heavy lifting and transport company Mammoet used a customised, hydraulically powered solution – the SBL 1100, a moveable four-point hydraulic lifting gantry with a capacity of 1 100 t – as part of a safe and efficient solution to lift a 400 t nuclear reactor pressure vessel out of the reactor pool at the Germany-based Unterweser nuclear power plant (UNPP).

Unterweser, once considered the most efficient single-block nuclear power plant in the world, was shut down following Germany’s decision to phase out nuclear energy.

Mammoet was tasked by contractor Höfer & Bechtel to lift the reactor pressure vessel, with the company’s technical solution being adapted to suit the confined conditions, thereby making it also suitable for use in UNPP operator PreussenElektra’s fleet, which had a total of five pressurised-water reactor plants that needed to be decommissioned.

Since the tracks of the lifting gantry rested on the operating floor, and partly also spanned the reactor pool itself through a cantilever, there were strict limits on load-bearing capacity of the building structure.

Tackling this, 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 of UNPP. The preparation of documentation for existing equipment required considerable expertise, according to Mammoet.

Together with Höfer & Bechtel, Mammoet conducted all coordination with the authorised 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 of the UNPP, piece by piece.

This equipment was also prepared in such a way as to ensure it could be easily decontaminated and removed after completion, if necessary.

However, before the lifting system was installed on the operating floor, steel load distributors had to be installed, 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, enabling the reactor pressure vessel to be lifted and lowered vertically and moved laterally.

With the successful load test for all components, Mammoet received the final approval for execution.

Successful Execution

The execution proceeded according to plan, following a series of steps. Firstly, the reactor pressure vessel was lifted from its installation position within the reactor pool, after which it was moved horizontally to a designated parking location.

Concurrently, Höfer & Bechtel positioned a dismantling rack and the reactor building crane at the original installation site.

Subsequently, the reactor pressure vessel was returned to its original position and carefully lowered onto the dismantling rack to enable safe removal of the calotte.

Finally, the vessel was transported to the final dismantling position and cautiously lowered, requiring it 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 company comments, adding that the company’s expertise from the pilot project will be used for the contractor on another of PreussenElektra'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 Mammoet Germany engineering manager Philipp Boettcher.