The problem with a fission reactor is that it's all built around a couple of tons of very angry plutonium or uranium. If things shut down that radioactive metal doesn't get any less angry.

Fusion, from what I understand, is about using lasers and magnets to force hydrogen atoms to fuse in to helium. Even if something goes wrong, the worst case scenario is a small amount of super-heated plasma that has basically no mass and will cool down pretty quickly.

I could be completely wrong, though. I am but a humble liberal arts major.