ITER seismic isolation bearings

The Tokamak machine in which the fusion process will take place is being assembled with all the associated facilities inside a mammoth building measuring 180 m long by 60 m wide and 60 m high. The building sits within a huge concrete-lined excavation, and rests on pedestals on a total of 493 seismic isolation bearings.

These bearings had to be designed to isolate the reactor building from the impact of ground motions generated during an earthquake. As well as meeting the structural performance criteria, the materials that are used in the bearings have to be durable and guaranteed to retain a certain level of performance over the 60 years of service that is required.

The bearings consist of a sandwich of steel plates of varying thicknesses in between layers of polychloroprene that provide the isolation. The sandwich is injected with mortar between two steel bearing plates that are attached to the structures above and below them by steel studs, and each assembly measures 1.1 m square and 285 mm thick.

Compliance with new seismic protection devices standards

Additional testing of the bearings had to be carried out to ensure that they would comply with the new standard for seismic protection devices that was published while the design was being finalised. EN 15129 specifies the functional requirements and general design rules of the devices, the material characteristics, manufacturing, testing, installation and maintenance requirements.

The testing was largely to demonstrate that the ageing characteristics of the polychloroprene material met the requirements of the new standards, and also that the bearings were able to withstand the static and dynamic loads in all directions, in particular the significant vertical loads which are an average of 770 t per bearing.

Nuvia-Freyssinet had previously designed and tested an identical product in 2010 for the experimental Jules Horowitz Reactor which was built on the same site; almost 200 isolation bearings were manufactured and installed to prevent any earthquake damage.

These protective elements, which measure 900 mm square and 181 mm deep, were also specified for use under the Tokamak building, to isolate it from the same seismic motions.

Polychloroprene material resistance

Meeting the ageing criteria for the polychloroprene material was the most difficult, with no more than 40% hardening of the material – measured by increase in horizontal stiffness – permitted over the 60-year service period for the bearings. Additives are used in the chloroprene material to improve its resistance to ozone and to the oxidation process that promotes ageing.

The compound that was selected not only meets the ageing criteria, but it is also resistant to fire, reducing the likelihood of damage in the event of hostile attack. An extensive programme of testing was used so that the dynamic behaviour of the bearings could be fully characterised.

Further qualification of the production and installation procedures was also necessary to prove that they met the quality standards demanded by the nuclear industry – this work was carried out by sister company Nuvia.