It's the only thing it possibly can be, to have limitless range. The Americans mothballed it as it was considered "too provocative".

The technology is totally feasible, and not even that complex - it's just that it's pretty much the most evil thing man could devise.

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

From the Wikipedia article:

"After delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing tremendous ground damage with its shock wave and radiation from its unshielded reactor. When it finally lost enough power to fly, and crash-landed, the engine would have a good chance of spewing deadly radiation for months to come."

"Too provocative" was a pretty mild way of putting it.

They also couldn’t figure out how to test it. Once you turn it on, you can’t stop it, and if you lose control it could go anywhere on the planet.

Can you not stop it by sending it commands to turn off the reactor?

Then it’ll crash and you’ll have a nice Chernobyl to deal with.

Several orders of magnitude less than Chernobyl. The reactor that lost containment was a gigawatt production plant. A gigawatt nuclear ramjet would be so huge there would be no trouble detecting it and shooting it down near the launch site.

Most of Chernobyl’s material stayed in the reactor, though. A hypersonic crash would disperse a much bigger proportion of the material. I don’t know how the two factors balance out exactly.

Just crash it into the ocean, sea water is an excellent neutron moderator.