NASA first developed nuclear propulsion from 1965-1972 under the NERVA Project.
Scientists were so ecstatic with the results they boasted of sending astronauts to Mars in 1980 and Saturn in 1990.
WIKI
https://en.wikipedia.org/wiki/NERVA
Remember about those as a teenager. Radioactive as hell, had remote tugs to pull them in and out for testing. No one could get close.