A development and testing program lasting several years was necessary to create a spherical bearing design that met the performance requirements for the experimental nuclear-fusion generator under construction in France.

Cover-ITER-Reactor-Cryostat_Bearings-Credit-ITER-organisation

  • Name of the owner
    International Thermonuclear Experimental
  • Name of the client
    F4E
  • Delivery date of the project
    December 2017
  • Partners of the project
    Nuvia / VINCI Construction Grands Projets
Key figures

Specific spherical bearings for ITER reactor cryostat

The main focus of this globally significant project, which aims to generate power from nuclear fusion, is the assembly of a Tokamak machine within which the fusion process will take place. The machine itself is encapsulated within a cryostat structure designed to maintain it at the exceptionally low temperatures that are necessary.

The cryostat is a cylindrical vessel made of stainless steel; it is supported on a concrete ‘crown’ with an internal diameter of approximately 20 m and is itself surrounded by 2 m-thick bio-shield wall. The vessel sits on top of 18 spherical bearings which are located on the top of the crown; these high-tech elements are specially designed to allow the cryostat to expand and contract under thermal actions, but to filter any horizontal seismic forces in the event of an earthquake.

The bearings are essentially the key interface between the machine and the civil structure that protects and contains it.

  • 0.25
    Maximum friction coefficient
  • 18
    Stainless steel bearings for cryostat
  • 2,100
    Tons of vertical load on each bearing
ITER-Crysostat-Exacting requirements-Credit-ITER-organisation1

Exacting requirements

Freyssinet’s industrial department worked with Nuvia (Nuclear specialist of Soletanche Freyssinet) for several years to develop and test these spherical stainless-steel bearings. Each must accommodate a load of up to 1,200 t and thermal expansion of ±40 mm, resulting from the predicted temperature range of between -100 °C and +35 °C in a highly radioactive environment.
As well as the necessary loads and displacements, Freyssinet-Nuvia also had to guarantee a maximum coefficient of friction of 0.25. This required a great deal of work to investigate and validate the relationships between the different materials and sliding surfaces of the components that make up the bearings.
It was not possible to use existing sliding materials that were available on the market, as they did not meet the criteria in terms of creep, radiation resistance, and brittleness under cold temperatures.

Credit: ITER organisation

New qualification

But qualification of a new material for such use also presented a challenge, given that no existing facility was capable of testing the necessary range of movements and conditions on full scale prototypes.
Hence the development of the bearings had to be done with this in mind, using a step-by-step qualification process. A sample of proposed material was first tested for its sliding qualities, considering its resistance to radiation, ageing, wear and so on. Different scale model tests were then carried out using prototype bearings at a quarter size, half size and full size, testing combined loads on each different model as far as it was physically possible.

Credit: Jean-Marie Huron

ITER-Crysostat-New-qualification-Credit- Jean-marie HURON
ITER-Crysostat-Test-Track-Credit- Jean-marie HURON1

Test track

The strategy of design and testing of the bearings was led by Nuvia, with  being responsible for the industrial aspects of the project, to ensure that production of the bearings would be feasible.
A purpose-designed friction testing bed was designed and built by Nuvia at the Freyssinet facility in France, to enable the prototypes to be tested under different loading sequences, varying the vertical load, the temperature and so on, with the friction coefficient being monitored throughout.

Credit: Jean-Marie Huron

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