Let's say you have a velocity requirement of 6 km/s. A chemical hydrogen-oxygen rocket might have exhaust velocity of 4.5 km/s so mass ratio is exp(6/4.5) = 3.

Let's say you have a velocity requirement of 6 km/s. A chemical hydrogen-oxygen rocket might have exhaust velocity of 4.5 km/s so mass ratio is exp(6/4.5) = 3.8. A nuclear rocket with exhaust velocity 9 km/s gets a mass ratio 1.9. Most likely the nuclear rocket uses hydrogen only, meaning the tank is likely bigger than the chemical rocket's! The thrust to weight ratio of a normalish solid nuclear engine is about ten times worse than a chemical one. You do save on the oxygen mass, but that's it. It's probably bigger, heavier, more complex and definitely more expensive and dangerous than an ordinary chemical rocket. For higher velocities, it starts making more sense...