Novel solution developed for removal of Unterweser reactor vessel

Mammoet was tasked by contractor Höfer & Bechtel to hoist the reactor pressure vessel, with a weight of about 400 tonnes out of the reactor pool as a whole component.

Mammoet proposed a customised special solution using the SBL 1100 - a moveable four-point hydraulic lifting gantry with a capacity of 1100 tonnes. The company said this technical solution could be well adapted to the confined space conditions and would therefore also be suitable for use in PreussenElektra's reactor 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 the 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.

The medium skidding track with strand jacks installed on the beams of the lifting system (Image: Mammoet)

Mammoet said another challenge was posed by the strict verification and documentation requirements for the material used in the control area. It noted the preparation of documentation for existing equipment required considerable expertise. Once the documentation was completed, all equipment could be brought into the control area, piece by piece.

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.

With the successful load test for all components, Mammoet received the final approval for the hoisting of the vessel.

The operation first involved lifting the reactor pressure vessel from its installation position in the reactor pool and moving it horizontally to a parking position. Meanwhile, the contractor placed a dismantling rack with the reactor building crane at the original installation position. The vessel was then moved back to its installation position and lowered onto the dismantling rack so that the contractor could safely cut off the bottom of the vessel. The vessel was finally moved to the final dismantling position and lowered. In the process, it had to be lifted over a wall.

The reactor pressure vessel is lifted vertically from its installation position (Image: Mammoet)

"The removal of the reactor pressure vessel was a very demanding project," said Philipp Boettcher, Manager Engineering at Mammoet in Germany. "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."

Unterweser - a PWR with a gross installed capacity of 1410 MWe - operated between 1978 and 2011. It was one of seven nuclear power plants shut down in Germany in March 2011 when it lost its commercial operating licence under the 13th Amendment to the Atomic Energy Act.

PreussenElektra is also decommissioning the Brokdorf, Grafenrheinfeld, Grohnde, Isar 2 and Stade PWR nuclear power plants.