In general, nuclear plasma is dangerous

Now, this isn't really very difficult here. If you have gaseous medium heated to 150 million degrees Kelvin, Celsius or whatever, it's very volatile, by definition. It has a great deal of kinetic energy, and will spread it around very quickly. It's not just the amount of energy here, it's the form it's in. In contrast, for example, Mount Everest contains an enormous amound of gravitational potential energy, but, in general, that's not too much of a problem, the energy is contained within a stable structure. Unless you get caught in a landslide or an avalanche. So, again, by definition, Tokamak fusion reactors are dangerous. If the electromagnetic containment breaks down, that superheated nuclear plasma will dissipate in an explosive fashion. And, the more efficient the reactor, the more energy the plasma will contain, and the the more dangerous the explosion will be. Basically, a Tokamak fusion reactor is simply an electromagnetically contained H-bomb. Any commerically viable fusion reactor will have to contain at least dozens, and perhaps hundreds of times more energy in the plasma than in the conducting coils energizing it. So, if the containment breaks down, we can expect the entire region to be vaporized, just like an H-bomb would do it. Not really that "safe" after all, eh? Now currently, these reactors really don't work at all. They'll have to be vastly more efficient to be commercially viable. And, admittedly, currently, with hundreds of cubic meters of superheated plasma at 2.6 atmospheres, they're dangerous as all Hell, but, still not nearly as apocalyptically dangerous as an H-bomb at its epicenter. However, since efficiency will have to be increased by orders of magnitude for any commerical applications, it's quite likely plasma density will have to be increased correspondingly. So, actually, if electromagnetic containment breaks down after these developments, it will have effects identical to those of an H-bomb. Goodbye hundreds of square miles. Just glaze. Have a nice day. If a nuclear fusion power plant were no more efficient or powerful than a conventional power plant, then we need only concern ourselves with the equivalent of a 0.1 kiloton Hydrogen Bomb. Not a very big Hydrogen Bomb, at all. So, it would only liquidate the power plant and the immediate area, if electromagnetic containment collapsed. However, I think the idea is that fusion power plants would be much, much more efficient and powerful than conventional power plants. Hence, much closer to actual, larger H-bombs in the devastating effects of a collapse of electromagnetic containment. In the tens, hundreds or thousands of kilotons.