Mammoet Extracts 365t Steam Generators at Grafenrheinfeld

Mammoet re-engineered its 500-tonne DHS-500 handling system to remove four 365-tonne steam generators in one piece from the Grafenrheinfeld nuclear power plant in Germany.

The work formed part of the plant’s ongoing decommissioning programme, led by PreussenElektra, with Framatome responsible for dismantling. Grafenrheinfeld entered service in 1981 and was shut down in 2015.

Mammoet previously used the DHS-500 at the Unterweser nuclear power plant, where four steam generators weighing less than 300 tonnes were extracted. At Grafenrheinfeld, however, the heavier components had to remain within the 400-tonne lifting capacity of the reactor building’s existing polar crane.

To make this possible, Mammoet redesigned the DHS-500 and reduced the weight of its steam generator handling ring by around 20%.

“These were around 365 tonnes. So we had to think about ways we could reduce the weight of our DHS-500 system to stay within 400 tonnes – the lift capacity of the polar crane,” said Philipp Boettcher, Manager Engineering at Mammoet Germany.

The redesigned ring was fabricated from higher-grade steel, allowing the same lifting capacity with less material. Its connection method was also changed to suit the geometry of the Grafenrheinfeld generators, with the ring secured to steel brackets on the casing rather than extending to the base of the generator head.

The DHS-500 uses an equaliser beam connected to the polar crane hook, with strand jacks suspended from the beam and attached to the handling ring surrounding the steam generator.

Each generator was initially lifted approximately six metres inside the reactor building. The component then had to be lifted and tilted simultaneously using the strand jacks and crane trolley to negotiate internal walls and avoid contact with the polar crane.

Additional restrictions came from the large beams supporting the crane trolley. Because of the load, operators had to ensure the trolley did not travel across the joints between two beam sections. Framatome also installed two A-frames beneath the polar crane to reinforce it during the operation.

Once horizontal, each steam generator was lowered onto saddles and moved approximately 25 metres out of the reactor building using a hydraulic skidding system.

Outside, a semi-gantry crane lifted each unit at a height of around 21 metres before lowering it onto two 12-axle-line SPMT combinations for transport to the site laydown area.

Each complete extraction and transfer took approximately one week.

By removing the steam generators intact rather than dismantling them inside the reactor building, the project reduced the amount of work required within the controlled area while demonstrating how the DHS-500 can be adapted for heavier-generation nuclear components.