Fusion, not fission (well, fission too obviously, but we already do that). But yeah, starting an uncontrollable chain reaction that consumes the earth in a ball of fire would be bad. I have to imagine that it wouldn't be quite so "free" as the safety measures that would need to be in place along with cost of delivery would still exist.
The implications for countries in the Middle East, South America, Russia, etc who survive financially largely based on huge oil reserves would be huge. It could either take away all their power and force them to compromise, or cause them to start WWIII out of desperation.