There were some genuine nuclear reactors in space, the soviets used them for a few high-power radar satellites. However, three of them have already re-entered (two had a failure in the system that boosted the core to a higher orbit, and one didn\'t reach orbit at all); two landed in the ocean, and were barely noticed. The other one burnt up over canada, dispersing a few highly radioactive fragments, but still clearly not something you could compare to a \'full-fledged nuclear bomb\'.