Much as I like nuclear power for deep space stuff, even put them in the novel I've never finished writing, they're politically unacceptable* where they're most useful: take-off.
Basically all the people going "What if 9/11 was done with a flying Chernobyl?"**, some of whom are concerned voters, some of whom are the engineering team, some of whom are the foreign politicians who threaten to put sanctions on you.
Once you get to interplanetary, ion drives take away most of the advantage, because of how many people are willing to put a few extra years on a mission in exchange for not having to care about the risks.
Still, incredibly useful if you can get past all that.
* outside of warfare
** Which is essentially also something that happens in my novel, as the intersection of accident with Newton's first law
