Well I would imagine it would be with current rockets, but what if something goes wrong and the fissile material just goes?
We’ve been launching fissile material into space for decades. It powers long range craft like Voyagers 1 and 2.
And with the exception of the satellite that crashed after orbiting Venus (with radiation leaking that caused the zombie outbreaks of the ‘60s and ‘70s), no accidents have yet to occur.