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  • CODAC | The "invisible system" that makes all things possible

    It is easy to spot all the big equipment going into ITER; what is not so visible is the underlying software that makes the equipment come alive. Local control [...]

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  • Assembly | Zero-gravity in a cramped place

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  • Image of the week | A closer look at KSTAR

    Over its twelve years of operation, the KSTAR tokamak (for Korea Superconducting Tokamak Advanced Research) has built an extremely valuable database for the fut [...]

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  • Pre-compression rings | Six of nine completed

    The European Domestic Agency is responsible for the fabrication of nine pre-compression rings (three top, three bottom and three spare). The first five have bee [...]

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  • Industrial milestone | Japan completes the first D-shaped coil of the ITER Tokamak

    In a ceremony on 30 January, a major industrial achievement was celebrated in Japan—the completion of the first 360-tonne D-shaped toroidal field coil for the I [...]

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Of Interest

See archived entries

The fellowship of the composite rings

Juan Knaster, ITER Magnet Division

At the Italian laboratory ENEA, Juan assesses the failure mode of one of the ten mock-up rings tested to rupture. (Click to view larger version...)
At the Italian laboratory ENEA, Juan assesses the failure mode of one of the ten mock-up rings tested to rupture.
The pre-compression rings will be one of the most challenging composite structures ever manufactured. Weighing more than 3 tonnes each, they will tightly hold the ITER toroidal field coils from the top and bottom with a radial load of 7,000 tonnes per coil and withstanding hoop stress of 350 MPa per ring.

Ten years of successful R&D performed by the Italian laboratory ENEA near Frascati, under Task Agreements with the European Fusion Development Agency (EFDA), the European Domestic Agency, and direct contracts with the ITER Organization, have recently been brought to a close. The work performed at ENEA by the team of Paolo Rossi identified two suitable fabrication processes for the rings, and developed applicable non-destructive examination methods by x-ray and ultrasound. The Italian team further completed the full mechanical characterization of the glass-fibre/epoxy composite at room and operating temperatures, allowing a final optimization of the ring design, and determined the ultimate tensile stress (UTS) of six mock-up rings (in average over 1500 MPa) in a purpose-designed machine that, with 18 independent hydraulic pullers, simulates the configuration of the 18 toroidal field coils (see related article).

The last challenge was to achieve the prediction of the long-term performance of the mock-up rings: this challenge has also been met. Tests on specimens at different constant loads during long periods had allowed the definition of the creep behaviour, however the correlation with long-term tests on the mock-up rings was pending. Early last year a ring was loaded at 950 MPa (65 percent of UTS) with no measurable degradation rate after 210 days maintained at such stress. A new ring was loaded last month at 1100 MPa (77 percent of UTS) and maintained under constant load until rupturing after 140 hours, right between the breaking times of two specimens loaded equivalently.

This accomplishment occurred concurrently with the end of the European Domestic Agency call for tender for the pre-compression rings. It enables our Division to wrap up the technical understanding of the rings' performance before industry enters in the game. The expertise and the purpose-design equipment available at ENEA will continue to play an essential role for industry in the years to come.


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