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Currently a megaproject called ITER (International Thermonuclear Experimental Reactor) is being built in France. The device would use magnetic containment of plasma reacting at temperatures of over 100 million Kelvin. What would happen if the magnetic field containing this reaction were to fail? Realistically?

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    $\begingroup$ Calling ITER a megaproject is stretching it a bit. The total cost of building ITER is projected to be about 13 billion euros. (Initially it was expected to cost 5 billion euros, but this kinds of projects have a tendency to overshoot their initial budgets.) For comparison, an American Zumwalt-class destroyer costs about 4 billion euros, and the Americans have built three of them for no discernible purpose. $\endgroup$ – AlexP Sep 21 at 10:26
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    $\begingroup$ This question seems asked and answered on the ITER website FAQ $\endgroup$ – user535733 Sep 21 at 14:29
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    $\begingroup$ I miss the world building content here. Apart from the lack of research effort, it's a plain what if question. $\endgroup$ – L.Dutch Sep 21 at 14:44
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Realistically, what you have is a very small amount of superheated hydrogen gas escaping in the environment.

Other than ruining a very expensive piece of machinery, and a loud noise, just about nothing will happen.

Fusion reactions cannot take place under natural conditions on Earth, so the reaction will stop as soon as containment is breached. The contents of the reaction vessel is a few grams of superheated heavy hydrogen, which will do what hydrogen does; a part of it will burn producing a tiny amount of heavy water, the rest will raise up in the atmosphere and eventually be lost in outer space.

(For comparison, a cubic meter of ordinary sea water contains about 300 grams of heavy water.)

Then they will have to find the funds to repair their expensive machine.

ITER themselves have the following to say about this topic:

Although 100 million degrees Celsius is an extremely high temperature, the density of the plasma (atoms per cubic metre) is very low—about one million times less than air—and the total energy in the plasma is not very great. The very rapid release of the energy could cause superficial damage to some plasma-facing components (i.e., surface melting) but would not be sufficient to produce structural damage.

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    $\begingroup$ Yes it is very hot. No, there is not a gigantic amount of thermal energy in there, because we are speaking of 10 grams of hot gas -- its heat capacity is minuscule. It will melt a hole in the equipment, no doubt. It will make a loud noise -- that's the shock wave. It will not trigger any kind of nuclear reaction, if only because there is nothing around it which could conceivably undergo such reactions. The stuff around it is mostly steel and copper. No uranium is present on site -- why would there be any? $\endgroup$ – AlexP Sep 21 at 10:35
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    $\begingroup$ "100M Kelvin seems like a lot": There's a difference between heat and temperature. You have 10 grams heated to 100M; you spread that out over a swimming pool, and it would raise the temperature less than a degree. (I calculated the weight of the water at 2.5 billion grams, 250M times as large as the sample; divided out that's less than a degree of change.) $\endgroup$ – user3757614 Sep 21 at 18:09
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    $\begingroup$ @Chameleon If you have got a way to make a few grammes of gas at 100 million kelvin “trigger nuclear chain reactions” without any kind of confinement, then you can build a working fusion reactor and win a Nobel prize. $\endgroup$ – Mike Scott Sep 21 at 18:48
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    $\begingroup$ Just lool at sparkler fireworks. The individual sparks are incredibly hot, much hotter than most fires you normally encounter like when burning gas. But because each indivindual spark is so small it can heat up anything it touches like your skin less than a degree. Not enough to warm you or set you on fire. The same applies to this reactor. The contents might be hot and have a lot of individual energy, but the total amount of energy if you released it would be relatively small. $\endgroup$ – Demigan Sep 21 at 20:06
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    $\begingroup$ If you could trigger a nuclear reaction that easily you wouldn't need ITER in the first place :) $\endgroup$ – David Hambling Sep 22 at 19:15
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Expanding my comment out into an answer: Temperature is not heat.

Imagine you have a boiling pot of water on the stove. You can reach through the steam to turn off the element. Steam has a high temperature (100C), but it doesn't have much heat, because there's not much of it. You could also stick your hand in the boiling water, but I wouldn't recommend it; boiling water is also 100C, but there's a lot more mass, so the amount of heat is much greater. (Steam gets dangerous under pressure; the amount of steam in a given place increases and so the heat increases as well.)

Here we have 10g of matter at 100M degrees. A calorie is the amount of energy needed to raise 1g of water 1 degree, so there are 1 billion calories of energy in this sample. (EDIT: as noted, the actual number is higher; Hydrogen at room temperature carries at least 3.5 times more heat than water of the same mass, and things get screwy at 100M degrees.)

A bathtub has about a 300L capacity. Conveniently, that's also 300kg of water. That 1 billion calories of energy is enough to bring 33 bathtubs from freezing to boiling. Which is impressive, but 33 bathtubs isn't that many.

An olympic-sized swimming pool has 2,500,000L, or 2,500,000kg of water. That 1 billion calories of energy is enough to raise the temperature by 0.4 degrees.

So, how much danger is there? I wouldn't want to stand next to it if it broke, but it's unlikely to hurt anyone outside the actual building. A heated swimming pool is going to use more energy.

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    $\begingroup$ IIRC Hydrogen has around 3 times higher specific heat than water. Not that it changes much. $\endgroup$ – Revolver_Ocelot Sep 21 at 19:19
  • $\begingroup$ Wasn't able to quickly find reliable looking numbers to run the cals myself, but would adding examples to show the temperature difference in that 10g of matter compressed to its 'reaction volume' of its magnetic containment, vs what the temp would be if allowed to expand to the total volume of the vessel help reinforce how little thermal energy is involved? $\endgroup$ – TheLuckless Sep 23 at 18:22
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There would be no ecological impact; nothing would get out of the building.

Even in terms of thermal pollution your car has more effect.

The ecological impact of having excavated such a huge site and hauling in the 30,000 tons of concrete that went into it in the first place, were, however, massive.

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  • $\begingroup$ That "huge site" is not huge at all. Look at the parking lots in the picture to get a sense of scale. It is a reasonably sized campus for a research institution. And 30,000 tons is not really a huge amount of concrete; an ordinary ten-story building weighs about 30,000 tons. The Empire State building weighs 360,000 tons. The Hoover Dam contains 8,000,000 tons of concrete. $\endgroup$ – AlexP Sep 22 at 20:10
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    $\begingroup$ Sorry, I was misremembering. It's 400,000 tons of steel and concrete -- iter.org/newsline/-/2237. And any of the things you mention has a pretty significant ecological impact compared to a few grams of hot plasma inside a building. $\endgroup$ – David Hambling Sep 24 at 7:27

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