I was curious what might happen if the containment field collapsed, and found this[1] on reddit from 6y ago. I'm not sure how accurate it is, but it sounds pretty bad -- though not nearly as bad as a nuclear meltdown at a fission plant.

> It will depend on the reactor design. Let's assume a worst case scenario, there is a breach in the reactor containment vessel and the fuel gets out. In that case, your reactor is likely destroyed, and a fuel leak will contaminate local groundwater.

> Fusion reactors very broadly have two main species that are being developed, inertial confinement and magnetic confinement reactors. Inertial confinement uses lasers to heat little pellets of hydrogen fuel on all sides to the point that it fuses, and I won't discuss them here. Magnetic confinement is more of your 'typical' reactor design. For instance, the tokamak. That's the ITER reactor in France. It has a donut shaped chamber where hydrogen plasma is heated to 150 million degrees Celsius and contained by magnetic fields in order to fuse hydrogen into helium. Let's suppose it gets out. First, the magnetic field fails. The magnetic field is supported by superconducting magnets which have to be kept at temperatures close to absolute zero in order to carry the electric current that produces the field. The heat of the escaped plasma will cause the wires to stop superconducting and all of the energy of the magnetic field will very quickly be dumped into heat in the wires, boiling off the cryogens. This is called a quench and it can seriously fuck up the magnet and cryogen system. (Come to think of it, if the reactor has an outer cryogen tank that's basically at absolute zero and a plasma at some hundred million degrees then this reactor probably has one of the steepest temperature gradients in the universe- this kind of thing just doesn't happen naturally).

> Anyway- the expanding cloud of gas. When the gas expands it cools very very quickly and ceases fusing almost immediately, stopping the fusion reaction dead in its tracks. Unfortunately, the fuel is a mixture of hydrogen isotopes- deuterium with one proton and one neutron, and tritium with one proton and two neutrons. Deuterium is stable and found in nature and is actually harvested from the hydrogen in sea water since that's a readily available source, but tritium is actually unstable and highly radioactive.

> A tritium leak into the environment is bad because hydrogen is so reactive it makes it really hard to clean up. Greenpeace has a good fact sheet about the risks of tritium leaks, and as pro-nuclear energy as I am, I have to acknowledge that tritium leaks are really fucking bad.

> When radioactive material gets out into the environment you want it have a very very short half life, or a very very long one. That way it'll either all decay away immediately, or it'll decay so slowly that it's not really irradiating anything. Tritium has a half-life of 12 years, which is neither fast nor slow, it's right in the sweet spot of being terrible. That tritium will bond with oxygen to make water molecules. Radioactive water molecules. These will make their way into the water table and are impossible to remove because they look just like any other water molecule... except sometimes it'll spit out a high energy electron. If this water is in your body, then that radioactive decay will shoot a high energy electron through you, destroying organic molecules and scrambling your DNA. At the least, this can increase your risk of cancer a marginal amount. At its worst, it could kill you.

> Best case scenario: the reactor is destroyed but the gas is contained by some secondary containment vessel so the tritium leak doesn't happen, and the gas can be collected and processed properly. Anyway, even in worst case scenario I can imagine for a fusion reactor failure is still a million times less dangerous than fission reactor meltdown.

>> Lockheed Martin said that they can have a fully functional fusion reactor in three years.

> I'll believe it when I see it.
[1] https://old.reddit.com/r/askscience/comments/2nbn11/what_wou...