Soviet nuclear rocket test

I read somewhere that the Soviets tested a nuclear rocket engine in space in
the 80's. What were the results of these tests and why hasn't anyone
followed up on this?

If a nuclear rocket engine is 20 times more efficient than a conventional
cryogenic engine, does this mean that the rocket only needs to carry 1/20th
the fuel? This could mean that a Saturn V would only be 30m tall instead of
110m.

The only problem with nuclear thermal engines is that they expulge
radioactivity but this could be solved by better casings around the
radioactive core.