Thermonuclear composites

Pultrusion company Exel Composites is collaborating with French industrial contractor CNIM to make glass fiber components for the magnet support structure of the International Thermonuclear Experimental Reactor (ITER), the world’s largest experimental fusion facility.

To fulfil the quality demands for the pre-compression rings, Exel Composites had to produce defect-free pultruded profiles of around 3 km in length.

The €18 billion ITER under construction in Saint-Paul-lez-Durance, France, has been designed to demonstrate that fusion power can be produced on a commercial scale, providing a safe, environmentally sustainable energy source. The ITER will use hydrogen fusion, controlled by superconducting magnets, to produce massive heat energy. In the commercial machines that will follow, this heat will drive turbines to produce electricity.

The composite pre-compression rings ensure the operation of the toroidal field coils that are employed to create a magnetic 'cage' to confine the 150 million°C plasma. To reduce fatigue and deformation of the coils resulting from the powerful magnetic fields, three pre-compression rings will be placed on top of them and three below, while an extra set of three will be manufactured in case replacement becomes necessary in future. The pre-compression rings are required to withstand maximum hoop stresses of up to 500 MPa at room temperature. Glass fiber epoxy composite with a high fiber content was selected as the most suitable material to withstand such extreme loads, avoid circulation of electromagnetic currents and deliver a long service life. The composite rings will have a diameter of approximately 5 m, a cross-section of nearly 30 cm x 30 cm and will weigh slightly more than 3 tons.

The process uses on pultruded composite profiles produced by Exel Composites whereby each ring will be produced by winding a 2 mm thick, 2.8 km long, flat pultruded profile around a metal tool. A 0.12 mm thick epoxy adhesive tape is wound over each layer. The completed ring lay-up is cured and then machined to the required final dimensions.

Length challenge

Exel formulated an epoxy resin system to meet the mechanical specifications for the pre-compression rings and ensured that the 3 km profiles supplied were defect-free along their entire length by means of online non-destructive testing (NDT). The handling of this length of profile presented a further challenge, which Exel solved by winding the product on a customised bobbin for supply to CNIM.

CNIM has already manufactured a series of prototypes, which are currently undergoing NDT and qualification tests. Production of the full-scale pre-compression rings will begin later this year.