> (nobody wants a dirty bomb going off in the sky)

Before you start up a reactor for the first time, the core is low radiation. Reactors produce short-life fission products, which is what makes nuclear waste dangerous.

Rocket mass is in kg, not moles. Exhaust velocity is ~9 km/s for hydrogen, vs ~4.5 km for H2-O2 engines.

I'm a space systems engineer, who has worked on nuclear rocket designs. My opinion is the time for nuclear-thermal propulsion is past. Solar-thermal can get the same performance - both heat H2 to the limits of the materials. But solar doesn't have all the nuclear baggage to deal with.

Nuclear-electric has much higher performance (3-20 times), though like all electric systems it has longer burn times. The reactor can be much smaller (1 MW rather than 1 GW), making radiators and such easier to do.