I'd absolutely agree with your concerns about the safety of launching a fission reactor into orbit. Hopefully we'll find another method of generating that amount of power that efficiently before the EM drive is ready to be put to a useful purpose but I'd hate to see the technology sitting on the shelf because we're unable to make reactor technology safe enough to launch.
You're probably right and my suggestion to use existing submarine reactors probably wouldn't be considered safe enough to fly, after all they are normally surrounded by an ocean of emergency coolant
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I don't want to divert this thread to the pros/cons of nuclear fission (we've already had a thread on that recently) but perhaps there's hope in some of the much safer, more contemporary, reactor designs.