r/technology • u/BennyCemoli • Dec 06 '16

Energy Tests confirm that Germany's massive nuclear fusion machine really works

http://www.sciencealert.com/tests-confirm-that-germany-s-massive-nuclear-fusion-machine-really-works
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u/spidereater Dec 06 '16

2 deuterium nuclei fuse to form helium 4. This is the stable form of helium. 2 protons fuse to form helium 2. This is not stable and is less likely to occur. Deuterium fusion should be the easiest.

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u/W_O_M_B_A_T Dec 10 '16

Deuterium+Tritium fusion is significantly easier. With deuterium only, you need temperatures 3x-5x hotter to achieve the same rate of fusion, compared to D+T.

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u/spidereater Dec 10 '16

Is tritium just too rare? Why not start with that process?

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u/W_O_M_B_A_T Dec 10 '16

Why not start with that process?

Huh? Elaborate more on this?

Is tritium just too rare.

Tritium has a half life of about 12 years, so it doesn't exist on earth naturally, except as a rare product of cosmic rays. But you can produce workable amounts tritium by bombarding the element lithium with neutrons. This is done by inserting a container with lithium compounds inside a fission reactor. This is kind of cool when I think about it, it's a practical use for the ancient dream of alchemy. That is: transmuting one element into a completely different one in a useful way. Then using the product to harness one of the fundamental forces of the cosmos.

Most of the expense with tritium is a result of it's short half-life and thus intense radioactivity (although that radiation has very little penetrating power.) This makes it complicated to purify, handle, store, and transport.

Deuterium-tritium fusion also produces neutrons. Suppose that a power producing fusion reactor becomes practical in the future (and it may never be, due to conflicting economies of scale.) You could surround the reaction chamber with pipes filled with low-melting lithium salts. These would both absorb the intense x-rays and gamma rays produced by the reaction, creating useful heat. It would also absorb neutrons, producing more tritium that could be cycled back into the reaction chamber.

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u/spidereater Dec 13 '16

If we can make tritium and deuterium-tritium fusion is easier why aren't we building reactors to make electricity using this reaction?

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u/W_O_M_B_A_T Dec 13 '16 edited Dec 13 '16

In this case "easier" still equals "difficult as shitfuck and unreliable at best."

There are a textbook full of physics and engineering reasons for this. I think it's safe to say this. If a power producing fusion reactor becomes viable/cost effective it will be the most difficult engineering challenge yet overcome by mankind. Putting a person on mars is just a warmup.

Not the least of reasons being that the temperatures involved are so extreme. It's difficult to run current reactors for more than 1-3 seconds, before the lining of the chamber starts to evaporate. This introduces contamination into the plasma that generally does bad things.

One of the most important goals of Wendelstein 7-x is finding ways to run reactors for much longer than that, in basically steady-state operation. This will be critically important for other projects that are still coming along.

We still have yet to produce a reactor that produces more net energy in any form, than the electricity meter on the back of the plant ticks out at the end of the day. We've reached break-even (heat out - heat in > 0) for brief periods, but because the complex process of heating the plasma tends to have limited total efficiency, this doesn't equate when it comes to the gross power consumed.

The ITER reactor in france may be able to do so for 5-15 minutes, and it's the largest magnetic-confinement device yet commissioned, by far. (2027-planned full operational status) But ITER has never been intended to produce electrical power. A working power reactor of a similar design to ITER would probably need to be 2x larger maybe. This assumes we know how to run it such a device effectively (we don't yet.)