These things were conceived of back in the Cold War; and were discarded as soon as ballistic missiles were invented. The rationale behind not introducing them into modern day aircraft (and damn fine rationale it is) is that if it had an accident, we'd have more radioactive messes to clean up. Don't even get me started on applying it to civilian aircraft. Plus, nowadays you can't even JOKE about sending military planes towards Russia nowadays, let alone nuclear powered ones. I highly doubt the rest of the planet would be more reciprocating to nuclear ramjets shooting around.

"That radioactive crater you have? It was caused by a plane, NOT a missile, so, y'now, that was TOTALLY not intentional. Please don't nuke us."