The neutrons released by the fusion reaction can be captured by the atomic nuclei of other materials it encounters, in a sense fusion. This induces radioactivity in those materials, called neutron activation, but won't create a run-away reaction. Nuclear fission reactors also produce neutron radiation that behaves in the same way, except in nuclear fission fuel it does create a chain reaction.

> Sounds a bit like a nuclear reactor.

They are nuclear reactors. Nuclear fusion reactors, rather than nuclear fission reactors.

Right so it never goes out of control basically once you stop the input!

Short answer: No, that can't happen.

Fusion reactors and conventional nuclear (fission) reactors are very different. Only poorly designed fission reactors can meltdown and release large amounts of highly radioactive material into the environment. And no nuclear power reactor of any kind can explode into a giant fireball like a nuclear bomb; that only happens on TV shows.

The main way they seem to go out of control is in the schedule and budget.

The thousands of tons of molten lithium needed for useful operation would, if ever exposed to air, prove extremely difficult to put out.

That would be what they might call an "expensive setback".

Molten lithium (or Pb-Li) probably won't be used in magnetic fusion reactors, because the magnetic forces from induced currents in the flowing metal would cause unacceptable pressures to develop. There was hope that insulating coatings for metal structures could be developed to deal with this, but apparently even small cracks are too much.

So, fugure lithium hydride, instead?

They need a great deal of 6Li/7Li in the path of the neutrons to breed fuel. How they would extract small PPB concentration of volatiles is unclear.

A lithium hydride breach would be no picnic, either.

Lithium-containing ceramics, perhaps.

What happens if its exposed to air?

It burns very hot. Also water won't help you.

On other hand amounts used are relatively minor so it isn't massively bad issue.

There are some great videos on YouTube about how alkali metals behave in contact with air or, for extra amusement, water. Those don't generally present superheated, molten alkali metals.

Fusion reactor can, in theory, go out of control, but it won't "fuse any atom it comes in contact with". Somewhat simplified:

The failure mode for a regular (fission) reactor can be twofold. The better scenario is that by some kind of mechanical failure the radioactive materials escape the confinement, and instead of putting their energy into the electricity generation mechanisms, just start shooting it around, irradiating things, thus breaking them (including living organism's cells and DNA) and causing them to become secondary sources of radiation. The worse scenario is that that before that, radioactive materials become too close together, starting self-sustaining chain reaction, which outputs immense amounts of energy (essentially, like a nuclear bomb), inevitably leading to destruction of whatever container it is in (no container can survive it for long, too much energy) and spreading around, by which time we're back to the scenario above (since once the materials have spread around, the chain reaction would stop) only with much more material which is much more energetic and thus will spread around wider and do more mess.

The failure mode of fusion reactor, if it happens, would be different, since it does not contain fissile material. Instead, it contains some light elements (usually the mix of deuterium and tritium, both of which are just hydrogen with some extra neutrons) which are heated and compressed a lot to start forming helium. If something breaks, the elements would not have anything to contain them (since, unlike what happens in the Sun, they don't have nearly enough gravity in themselves to be able to counter the thermal forces taking them apart) so what you'd get is a lot of very hot gases (mostly hydrogen) flying around. It's no fun, especially given hydrogen likes to explosively combine with oxygen in the air under the right conditions, but there would be no radiation involved, and it won't be able to "fuse" with anything else because it won't have enough energy to initiate the fusion process (that why we needed to compress and heat it up in the first place). So if everything goes very wrong - which is not very likely, but we're assuming the absolutely worst case scenario - we will have an explosion but noting like fission reactor. The containment is absolutely necessary - at least in current fission reactors - to achieve more energy out than in - and if it fails, the energy output will stop. This is one of the reasons fusion reactors are supposed to be safer.

There still could be some radioactive contamination involved due to fusion causing neutrons to fly around, hit the surrounding materials and turn them radioactive, and these could be spread around by the explosion, but less than in the fission case.

Now you may ask how hydrogen bombs are so destructive then? The big difference they use a regular nuke to ignite the reaction. Unless somebody builds a fusion reactor inside an exploding nuke, that's not the scenario we'll be dealing with in the fusion reactor case.

Thanks for explaining it so well, that gives a lot more perspective. Some have replied fusion is still to expensive to run, is that true?

Right now nobody has a functioning fusion reactor, so nobody knows how expensive it would be to run one. Hopefully, there would be some way to make it cost reasonable money - since it has many advantages over existing solutions - but I have no idea if it's feasible with current technology.

Nope, failure of containment simply means they fizzle out. Some massively hot plasma might go to areas immediately next to reactor, but it won't blow up. There isn't just enough temperature or pressure for fusion to continue.

where does that even appear in sci-fi? you're the first person i've ever seen even type out such a thing.

The Dark Knight has Bane trucking around an explosive fusion reactor.

In Spiderman 2 Doc Oct is blowing stuff up with fusion.

Those are the two that pop into my head.

There is lots of sci-fi where the reactors go in full overload. Startrek, Starwars, Stargate. Don’t quite recall where I got the idea exactly from to be honest.

Start Trek uses matter-antimatter reaction as power source. Provided we ever find out how to do that, if this reactor stores any substantial amount of anti-matter - which appears to be the case in Star Trek, with the confinement being achieved by usage of dilithium crystals - the failure mode would be loss of confinement, with the result of antimatter coming into contact with regular matter. This will lead to all anti-matter instantly converted to energy (taking the equivalent mass of matter with it) resulting in enormous explosion probably converting any matter in the vicinity into a superheated plasma cloud and enormous burst of high-energy radiation. Star Trek reactors are not very safe, as it looks from the descriptions.

Iron Man's "arc reactor" is explicitly supposed to be a fusion reactor and it blows up, taking a building with it, during the events of the first Iron Man movie.