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  • A world in itself

    From a height of some 50 metres, you have the entire ITER worksite at your feet. The long rectangle of the Diagnostics Building stands out in the centre, with [...]

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  • US completes toroidal field deliveries for ITER

    The US Domestic Agency achieved a major milestone in February by completing the delivery of all US-supplied toroidal field conductor to the European toroidal fi [...]

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  • Thin diagnostic coils to be fitted into giant magnets

    Last week was marked by the first delivery of diagnostic components—Continuous External Rogowski (CER) coils—from the European Domestic Agency to the ITER Organ [...]

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  • Addressing the challenge of plasma disruptions

    Plasma disruptions are fast events in tokamak plasmas that lead to the complete loss of the thermal and magnetic energy stored in the plasma. The plasma control [...]

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  • Blending (almost) seamlessly into the landscape

    Located in the foothills of the French Pre-Alps, the ITER installation blends almost seamlessly into the landscape. The architects' choice ofmirror-like steel c [...]

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Of Interest

See archived articles

Bearing extreme forces

-R.A.

Tremendous forces will be exerted on the ITER machine when plasmas begin pulsating inside the vacuum vessel. As a consequence, the machine's support system needs to be both extremely robust, with a strong connection to the Tokamak Complex basemat, and yet also accommodating to the wobbling, expansion and occasional displacement of 23,000 tonnes of mass (tokamak plus cryostat).

In order to simulate the vertical compression forces that the bearings may be subject to, a powerful hydraulic press—capable of delivering up to 7,000 tonnes of vertical pressure—was purpose-built in the Nuvia facility near Milan, Italy. (Click to view larger version...)
In order to simulate the vertical compression forces that the bearings may be subject to, a powerful hydraulic press—capable of delivering up to 7,000 tonnes of vertical pressure—was purpose-built in the Nuvia facility near Milan, Italy.
A massive pedestal—connected to walls that are radially anchored into the three-metre-thick bioshield—will allow for the even distribution of loads and efforts; a set of 18 spherical bearings acting like ball-and-socket joints will allow for the smooth transfer of horizontal, vertical and rotational forces, whether stemming from normal operations, a vertical displacement event or an earthquake.

Spherical bearings (semi-spherical actually) are commonly used when large structures, such as bridges, need a small allowance of movement.

But while mechanical constraints can be compared, a tokamak's environment is quite different from that of a bridge: radiations and extreme temperature gradients (from 35° C down to minus 100) require specific materials and techniques.

At a facility near Milan, Italy, two full-size bearing prototypes have entered the qualification phase at Nuvia—the ITER contractor that also designed and installed the anti-seismic bearings of the Tokamak Complex.

Made of high-resistance steel, the 18 cryostat bearings play an essential part in accommodating the wobbling, expansion and occasional displacement of the 23,000 tonnes that represent the combined mass of the Tokamak and cryostat. (Click to view larger version...)
Made of high-resistance steel, the 18 cryostat bearings play an essential part in accommodating the wobbling, expansion and occasional displacement of the 23,000 tonnes that represent the combined mass of the Tokamak and cryostat.
In order to simulate the vertical compression forces that the bearings may be subject to a powerful hydraulic press, capable of delivering up to 7,000 tonnes (1) of vertical pressure, was purpose-built. Equipped with additional jacks, the press also simulates the cryostat's lateral and rotational loads.

An important factor in the behaviour of the bearings during both normal and incidental conditions is temperature. Because it can get as cold as minus 100 °C close to the bottom of the cryostat a small cryoplant has been added to the hydraulic press in order to inject liquid nitrogen into the test apparatus.

The latest qualification tests mark the last step before launching the fabrication in late November. Further tests will be carried out during the manufacturing stage on the actual production bearings.

(1) 7,000 tonnes represents about two times the force that could actually be exerted on each of the 18 individual bearings including safety provisions.


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