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How To Line a Thermonuclear Reactor
sciencehabit writes "One of the biggest question marks hanging over the ITER fusion reactor project — a giant international collaboration currently under construction in France — is over what material to use for coating its interior wall. After all, the reactor has to withstand temperatures of 100,000C and an intense particle bombardment. Researchers have now answered that question by refitting the current world's largest fusion device, the Joint European Torus (JET) near Oxford, U.K., with a lining akin to the one planned for ITER. JET's new 'ITER-like wall,' a combination of tungsten and beryllium, is eroding more slowly (PDF) and retaining less of the fuel than the lining used on earlier fusion reactors, the team reports."
Its a little like the old puzzle "What do you use to hold an acid that can eat anything?" Difficult, but interesting, problem.
Python: 'And then suddenly you have a language which says "we're all stuck with whatever the whiniest coder wants".'
Space-age materials are pretty amazing, but Fusion-age materials are at a whole different level. I think the community hasn't expressed to the public just how daunting the challenges are. Controlling the plasma is one thing, but engineering the plasma-facing components (PFCs) is a whole 'nother kettle of fish.
The so-called "first wall" is the interior layer of the fusion reactor. It has to stand up to neutron bombardment, but it also has to avoid shedding particles into the plasma. For example high-Z materials such as tungsten, molybdenum, and vanadium are interesting for their neutron tolerance, but if atoms scrape off into the fusion plasma they will radiate like crazy (proportional to Z^2) and drain a lot of energy out of the plasma. That's why they are testing a Be coating (Z=4).
On the other hand, you have divertors, which sit in direct contact with the plasma and basically hold it in place so it doesn't randomly hit the wall. These have to withstand a high heat load. I admittedly don't know much about divertors so I will stop there.
There's also the superconducting material in the coils of the tokamak to consider. Of course there's a whole bunch of neutrons flying around. But also but it turns out that a lot of the issues with superconducting magnets are mechanical in nature. The HEP community has figured out how to build SC magnets consistently, but I think the magnets needed for a tokamak are quite different.
There is supposed to be a International Fusion Material Irradiation Facility, part of the ITER project (and basically a consolation prize to Japan), that will provide intense neutron beams for materials studies. But I am not really sure what the situation/timeline is for that given the funding problems ITER has faced.
The problem isn't the temperature alone, it's also that heavy atoms will pollute the plasma if they come loose at all. The Princeton Plasma Physics Laboratory is working on a liquid-lithium walled reactor to try and handle several of these problems. Check out LTX (Lithium Tokamak Experiment).
Twenty years ago I was a program officer at the Office of Fusion Energy, US Department of Energy. The ITER planning had started. My take -- there is no way on Earth that a tokamak can be cost competitive. Even if it works, even if the first wall problem is solved as may be indicated above, the engineering costs are so prohibitive as to price the whole concept out of consideration.
I earlier worked on Trisops, a simpler fusion concept that might be economically feasible, but I even doubt that. In the official fusion community, which is fixated on the the tokamak, it suffered from the NIH ( Not Invented Here ) syndrome and was defunded.
Thorium is very heavy. This is a bad thing.
For a tokamak first wall, you want a very tough, lightweight material. Something with very few electrons to strip off when it inevitably contaminates the plasma. If you use heavy elements, loads of energy is wasted ionizing the contaminants, and the energy is radiated away.
They're using beryllium, which is a very lightweight metal, doesn't retain expensive fuel, but toxic six ways to Sunday. It melts at a low temperature, but the operators of JET have installed elaborate safety systems to prevent as much as possible, damage to the first wall.
For the divertor (the 'exhaust pipe'), they use tungsten: heavy, but has the highest melting point of any known material, and there are few worries about contamination of the plasma, where the plasma edge ('scrape off layer') contacts a physical surface inside the reactor.
These are way better than the old material: carbon composites; which are incredibly tough and don't melt or sputter easily, but trap fuel away from the plasma.
Don't forget Dr. Bussard's Polywell concept.
It's under a publishing blackout because it's a project currently being funded by the Navy, but the fact that it's still being funded is encouraging.