Posted on: 23rd January 2012
If you are building a tokamak, you need to make it out of a quite special material. It must resist extraordinarily high temperatures, be structurally very strong and corrosion resistant, yet must not be very conductive or have high magnetic properties. For JET, the material chosen for the vessel is Inconel 600.
Most steel is over 90% iron, which is well-known for its excellent magnetic properties; exactly what you don’t want in a tokamak, with its huge magnetic fields and mega-amp currents. Hence Inconel 600 was chosen, a so-called super-alloy containing 72% nickel and about 15% chromium. Inconel 600 has only 8% iron in it, along with small percentages of other materials, such as niobium, silicon and sulphur which give it excellent temperature resistance and structural properties. “It will easily go up to 700 degrees Celsius before it begins to soften.” says Dr Chris Lowry, of the JET Department, “but it’s hell to machine!”
Although the resistance of Inconel 600 is quite high – its composition is similar to nichrome wire, commonly used for heating elements – it still does conduct some electricity. So, further measures have been taken in the structural design of the vessel to minimise currents in the vessel that might sap energy from the experiment. The rigid parts of the vessel are separated by corrugated sections known as bellows. There are two walls of bellows made of alloy merely 2 mm thick; 550 mm of alloy concertined into the 130 mm or so between the rigid sections. The extra length of thin alloy increases the resistance, as required, but to maintain the structural strength with such thin walls, a higher grade of alloy is required, Inconel 625, which has double the strength of Inconel 600. The strongest of all is Inconel 718, but because of its cost, it is only used in JET in a few vital spots, for example, as a tie material for some of the tiles.
This same Inconel vessel has been in place for the duration of JET’s 29 year operating life, while different internal walls and structures such as limiters and divertors have been installed and replaced. These days the duller lustres of the beryllium and tungsten based ITER-Like Wall completely obscure the bright sheen of the Inconel.
While beryllium and tungsten do have a future in the ITER vessel, Inconel does not, because of the neutron bombardment in that environment. Although Inconel has so many good properties, its downfall is that some of its components can be activated easily – nickel, cobalt, tantalum and niobium could all be problematic. Instead, for the ITER vessel, a careful selection of steels has been made, which will perform even better than a super-alloy!