https://en.wikipedia.org/wiki/NERVA
We actually had the NERVA nuclear rocket engine that was killed by Nixon in the early 70’s.
Even had it up and running for tests. It would have been used for manned Mars missions and building a moon base.
Although NERVA was a great idea and it clearly worked, “it would be bad” if those engines disintegrated in the atmosphere especially above land. Assembling and using them it space might be OK.