Right now two institutes are working on nuclear rocket design in Russia – KB Arsenal and Keldysh Center. Literally the entire country is designing or testing parts for this device, almost 100% of the spacecraft is new. . . . There has been a lot of lab work in the US on some components of nuclear rockets (VASIMR engine, radiators, less on reactors), but only this year the defense department realized the potential of nuclear reactors in space, so I’d say the US is currently 15-20 years behind, at best 10 years behind, at worst 30 (no major experience operating nuclear reactors in space, overall degradation of nuclear competence).
[unz.com]