ITER Divertor cassette bodies pass crucial leak tests

At the bottom of the ITER Vacuum Vessel, the divertor will form a massive “ashtray” where all plasma impurities will fall, while enduring the blazing heat. The divertor stretches roughly 142 m² and is made up of 54 cassettes assembled in the shape of a doughnut. They each weight eight tonnes and consist of three plasma-facing components mounted on a stainless-steel body.

Europe is responsible for the cassette bodies and the inner vertical targets. Fusion for Energy (F4E) is working with several companies to produce in total 58 units of each. The four extra ones will be kept as spares, since the cassettes will need to be replaced during operations using special robotic tools.

The divertor components are extremely challenging to manufacture, given their geometry, size and location in the device. The stringent tolerances of ITER are a true exam to the skilled suppliers: they must pull off highly precise machining and welding, and, in the meantime, carry out detailed inspections.

Currently, the first cassette bodies are nearing the end of this journey. The four units entrusted by F4E to the SIMIC–CNIM consortium are undergoing final acceptance tests. Following the hydraulic pressure tests, the teams recently completed Hot Helium Leak Tests, a key checkpoint to certify the components are tight and are ready for ITER.

During summer, the tests were carried out in a specialised facility owned by the Criotec,  an Italian SME. Laurent Guerrini, F4E Programme Manager, travelled there a few times to follow the tests firsthand with his team: “We worked with SIMIC and the subcontractor to adapt and qualify the facility for the tests, which require high cleanliness and isolation conditions,” he explains.

How do the tests work? The first step is heating the components in a vacuum chamber to temperatures of up to 250 degrees Celsius to mimic the thermal stress in ITER. Then, pressurised helium is injected into the component and, outside, highly sensitive spectrometers look for traces of the gas escaping. The tests took roughly two weeks per component, as they were done at three different temperature ranges each.

The results brought very good news. No helium leaked through the structure, confirming the compliance of the cassette bodies. The four units returned to SIMIC for the final steps, including additional machining and dimensional checks. Afterwards, the technical teams will need to fill in the paperwork to transfer the components to ITER. “We expect the first deliveries in early 2026. It will be a big and well-deserved achievement for F4E, our suppliers and ITER Organization. For more than five years, we have put our expertise and efforts into making the fabrication a success,” says Vassilis Stamos, F4E Project Manager. 

“The successful completion of the tests is a big milestone, that confirms the robustness of our manufacturing processes and the dedication of our teams to meet the most demanding standards. It is the fruit of the excellent collaboration with F4E, built on mutual trust and a shared commitment to excellence,” states Ermano Franchello, SIMIC-CNIM Project Manager.