Project Pluto

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In 1957 the United States Air Force and the Atomic Energy Commission began a joint project to study the feasibility of high-speed unmanned nuclear-powered aircraft. By 1961 a working prototype of the propulsion system was completed and earmarked for a proposed production cruise missile known as SLAM, a beautifully evocative acronym for Supersonic Low Altitude Missile. The research program for SLAM, known as Project Pluto, is one of the most terrifying defense ideas ever seriously considered, and you can read more about it here:
Project Pluto

I don't think there's much to add to Project Pluto beyond what the Wikipedia article has to say, other than that this is a weapon that would satisfy the defense department of Satan himself. SLAM was to carry a dozen thermonuclear warheads for months at a time, never slowing below the speed of sound, ready to drop them on Soviet targets on short notice. Once these bombs were depleted, the reactor would still have fuel enough to fly on almost indefinitely, presumably to blast enemy troops with the shock waves riding on the missile's nose and to irradiate them with gamma rays from the unshielded reactor. It was a baroque idea that never quite made sense due to the advent of reliable guidance and reentry systems for ballistic missiles. Unlike its distant technical cousin Project Orion, I have no mixed feelings about Pluto's permanent assignment to the dustbin of history.