[akiselev]

NASA uses plutonium for radioisotope thermoelectric generators whereas the OP is talking about nuclear pulse propulsion, which uses the shockwave from nuclear warhead detonations to propel a spacecraft. These are two fundamentally different things. The latter is illegal in most countries because of the aforementioned Partial Nuclear Test Ban Treaty, which bans any atmospheric or outer space detonations.

[XorNot]

Also unnecessary because thermal lightbulb rockets deliver better ISP.

[akiselev]

Yes but unfortunately after the UTC research was canceled all development on the nuclear lightbulb design was stopped while other methods of nuclear propulsion received at least some mindshare from theoreticians and engineers. The nuclear lightbulb now rots in the NASA technical archives. Most of the engineers who worked on it are dead and we haven't made much progress on the material science needed to pull it off (I.e. we still don't have a solution to neutron damage and runaway heating in the single crystal berrillium oxide sheath as well as a viable design for the fuel injector).

[XorNot]

https://en.wikipedia.org/wiki/Project_Pluto

But we did actually build prototype engines of a similar principle. There were actual mechanical artifacts test-fired. This is a much more viable looking option then Orion which only proved the pusher-plate design seems viable on a small scale, and which is unlikely to ever clear the problem of "build a lot of nuclear weapons...then deliberately start setting them off".