[IdeaHamstir]

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.

[mikeash]

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.