Re: "super-heated plasma that turns the Earth into another star"
Um, no. I'm not a physician, but I'm certain that no fusion reactor Man can make will turn Earth into a star. We might be able to return it to the molten slag state it started in, but we have nowhere near the mass, nor the hydrogen required to make Earth sustain fusion reactions in its core.
Personally, I admit that there will be mistakes - to think otherwise wold be foolish, given Man's history. I also think that a broken fusion reactor will simply make one spot rather hot for a bit, and that should be it.
Maybe we should always build them underground ?