The NERVA engine, developed at the late 60ies, was a nuclear thermal rocket developed for vacuum use in space craft. It was supposed to use Liquid Hydrogen, a cryogenic fuel with quite a few issues such as high cost and difficulty in containment that has greater efficiency than typical hydrocarbon fuels.
So here's where the question comes in: Why did NERVA had to use cryogenic fuel to be worth its trouble (so I've heard)? Cryogenic fuels are far bigger trouble in vacuum stages than in launch stages where they have to stay inside the tank without boiling off years on end. What would the specific impulse of a NERVA using traditional RP-1 or another rocket fuel would be? Is regenerative cooling a part of the reason why LH2 was chosen?