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 (f

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.