Answer(s) to How many nuclear fission reactors have been launched into space? How many are still there? suggest that most nuclear reactors still in space are pretty darn old.
Question: What was the most recent launch of a nuclear reactor, and what are the current barriers to launching the next one?
note: I'm not asking about RTGs which produce energy from simple radioactive decay, I'm asking about reactors that use chain reactions to produce a high rate of power production.