Didn't the Russians drop a Burevestnik missile into the Barents sea in 2018 and then send a load of recovery vesels to fish it out or was that fake news. Anyway, how hot does the exhaust gas need to be in order to make a viable ramjet, 1000C maybe I don't know, the hotter the better. Now make a fission reactor that will run at those sort of temperatures, it's probably possible with new alloys and ceramics developed over 60 to 70 years of nuclear and rocket engine research.
Project Pluto back in the early 60s was a success but deemed to be "too provocative" and was cacelled.
Nuclear fission propulsion is feasible with modern materials and it's one step above the ICBM concept because they can fly for days, weeks if not months and therefore no part of the planet is safe.
It's a nuclear war game changer if and only if you could fly it around for a day or two.