Heating up substances using a nuclear reactor in order to produce propulsion has been demonstrated before successfully, usually with liquid hydrogen, as it offers the best specific impulse, see for example the US NERVA project (https://en.wikipedia.org/wiki/NERVA), which was a great success, but sadly abandoned anyway, and the Soviet RD-0410 (https://en.wikipedia.org/wiki/RD-0410). But from what I can find this new Russian weapon is a subsonic cruise missile, which is most likely closer to something like the old US Project Pluto engine (https://en.wikipedia.org/wiki/Project_Pluto) which just pumps air through a nuclear reactor to heat it, which then cools the reactor and provides trust. Honestly, we don’t know nearly enough about this new Russian weapon to be able to say how exactly it works, but it is probably not as groundbreaking as people might think, since all the projects I have mentioned above date back quite a while back.
Heating up substances using a nuclear reactor in order to produce propulsion has been demonstrated before successfully, usually with liquid hydrogen, as it offers the best specific impulse, see for example the US NERVA project (https://en.wikipedia.org/wiki/NERVA), which was a great success, but sadly abandoned anyway, and the Soviet RD-0410 (https://en.wikipedia.org/wiki/RD-0410). But from what I can find this new Russian weapon is a subsonic cruise missile, which is most likely closer to something like the old US Project Pluto engine (https://en.wikipedia.org/wiki/Project_Pluto) which just pumps air through a nuclear reactor to heat it, which then cools the reactor and provides trust. Honestly, we don’t know nearly enough about this new Russian weapon to be able to say how exactly it works, but it is probably not as groundbreaking as people might think, since all the projects I have mentioned above date back quite a while back.