The PLUTO project had the goal to create a nuclear-powered ramjet Supersonic Low-Altitude Missile (SLAM). The Pluto would be launched most likely from a hardened underground site via three solid rocket boosters. These would accelerate the missile to ramjet speed, then be jettisoned.
Now, today it should be classified as a unmanned combat aerial vehicle (UCAV) due to its capacity to fly by days and do high and low maneuvers.
It would climb to 35,000 feet at Mach 3, until arriving near enemy terrain, where it would descend to tree-top altitude (still going supersonic) and launch between 14 and 26 nuclear bombs until depleting, and then crash itself.
If that were not enough the engine was a nuclear reactor, having the “friendly” effect of contaminate all its travel route from launch to crash site.
The project continued until the testing phase. But were do you test an engine that will contaminate by centuries any place were it could crash (and by 1964 crash test were more common than success)
So by 1964 the project was canceled, and the last reactor was dissembled on 1976.
This model shows how the test units would had look if it had been developed as originally planed.