An alien shape for an awesome task
6 Mar 2017
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R.A.
A component fallen from an alien star ship wouldn't look stranger than a cassette body for the ITER divertor. Depending on the angle of view, the heavy steel structure is vaguely evocative of a sled (the alien captain's seat?) or a drakkar bow. It's got ridges, dips, protrusions, curves and angles... but nothing that allows us to give it a name from our repertory of known objects.
At CNIM, an hour-and-a-half drive from the ITER site, the divertor cassette body prototype is in the last stages of completion. The alien-looking component is being produced in collaboration with the Italian firm Simic.
Unless of course we know about tokamaks.
Many things about a tokamak are extreme (vacuum, heat, cold, electromagnetic loads ...). But there is a place at the bottom of the vacuum vessel where conditions are more extreme than elsewhere ─ a place where it feels like standing at the surface of the Sun.
This explains why a cassette body for the ITER divertor does not resemble anything familiar—not only is it designed to withstand at close range the monstrous heat and radiation of a star, it must also resist the monstrous mechanical forces occasionally generated by electromagnetic loads.(1)
Arranged in a circle at the bottom of the vacuum vessel, 54 cassettes bodies—each protected by actively-cooled tungsten components—form the divertor, an essential system of the ITER machine that will extract the heat and the helium "ash" from the burning plasma and protect the surrounding structures from thermal and neutronic loads.
Three companies, the Italian Walter Tosto, the Finn Hollming Ltd and a French-Italian consortium formed by CNIM and Simic were each awarded a contract to manufacture a divertor cassette body prototype.
In the 18 months since fabrication began, twenty-five tonnes of high-grade steel have been patiently transformed into a lean, four-tonne ITER component.
Once the prototypes are finalized, the European Domestic Agency (responsible for the procurement of the divertor cassettes) will choose the company or consortium for the fabrication of all or part of 58 cassette bodies—54 to be installed in the machine plus 4 to 6 spares.
In the Mediterranean harbour of La Seyne-sur-Mer, France, an hour and a half drive from the ITER site, the CNIM-Simic prototype is in the last stages of completion.
The fabrication of the component was led by CNIM and shared between the two companies: the "primary segments" were machined and electron beam welded at CNIM in La Seyne, sent to Simic in Camerana, Italy, for additional tungsten inert gas welding and non-destructive controls, and eventually brought back to CNIM for final machining, verifications and functional tests.
Resting on sturdy steel supports in an enclosure inside CNIM's 3,000 m² hall, the prototype is now in the last stages of the 18-month process that progressively transformed twenty-five tonnes of high-grade steel into a lean four-tonne ITER component.
"We are finalizing the rough machining phase," explains Eric Mercier one of CNIM's methods engineers. "Three more millimetres to remove and we're done." Tolerances are especially tight and, despite its name, the operation is so precise and delicate that it will take three weeks to remove the remaining three millimetres ...
Following finishing works, a series of operations will be performed—dimensional inspection, a hydraulic test, a hot helium leak test, and eventually functional tests that will check the mechanism for the insertion of the component into the rail running at the bottom of the vacuum vessel.
Amidst shiny metal shavings and splashes of cutting fluid the massive and elegant metal component is acquiring its final form—an alien shape to be duplicated some 60 times before being exposed to the fire and forces of the ITER sun.
(1) The cassette bodies are designed to resist transient electromagnetic induced forces as high as 100 tonnes, which are expected to develop during a few tens of milliseconds during the most severe disruptions.
After initial forging and machining at CNIM in La Seyne-sur-Mer, France, the prototype was sent to the Simic factory in Camerana, Italy, for tungsten inert gas welding and non-destructive controls. It was back at CNIM in January for final machining, verifications and functional tests.
Resting on sturdy steel supports in an enclosure inside CNIM's 3,000 m² hall, the prototype is now in the last stages of the 18-month process that progressively transformed twenty-five tonnes of high-grade steel into a lean four-tonne ITER component.
A cassette body for the ITER divertor does not resemble anything familiar. It forms the backbone of a unique system designed to withstand heat loads comparable to those one would encounter at the surface of the Sun and to resist the monstrous mechanical forces that could be exerted by plasma disruptions.
Amidst splashes of cutting fluid, a five-axis boring machine delicately shaves the prototype's surface. "Three more millimetres to remove and we're done," says Eric Mercier, one of CNIM's methods engineers. But the operation is so precise and delicate that it will take three weeks.
It takes 54 cassettes to form the ITER divertor, each of them equipped with a "dome" and two "targets" that will directly face the fiery plasma. Cassettes connect to the vacuum vessel by way of a knuckle whose "fingers" are inserted in a rail running at the bottom of the vacuum vessel.