Project Pluto was an attempt during the 60's to create a cruise missile powered by a nuclear ramjet. In fact, this was the first attempt to actually create a nuclear ramjet, which has limited use outside of military applications. This is for a number of reasons. First, this ramjet used an unshielded half-gigawatt reactor, which would put out massive amounts of radiation in a somewhat large area around it. This would also make it totally unsuitable for launching in the US, and test centers for prototype engines had to be unmanned. Second, this missile operates using a ramjet, which is largely unsuitable for nonmilitary applications. Ramjets either have to be boosted to operating speed (often half of Mach 1) by rocket boosters, or by aircraft to which they're attached (as in the case of the MBDA Meteor, for example). Ramjets operate best at speeds that are difficult for humans to stand. As far as I'm aware, there are no commercial or hobby applications for aircraft moving at mach 3 or 4. Because of the speeds involved, noise is often a large factor with ramjets as well. Sonic booms are also considered a nuisance, and in many countries are banned outside the military.

The fact that this missile is nuclear-powered also presents another interesting use: loitering. Because the missile has a nearly inexhaustible fuel source, it could fly around, for example, the pacific ocean for a very long time before making its final strike at the enemy.

Of course, a missile this large and with this kind of power wouldn't be useful with just one nucler warhead. Designers decided that the Pluto would fire a number of sub-munitions, almost certainly nuclear warheads. In this way, the Pluto could fly over a pre-determined map (of Russia, most likely) firing warhead after warhead until it ran out. At a low altitude, it would be much more difficult than a bomber to intercept. Once out of submunitions, it could fly randomly over enemy territory, irradiating anything it passed, or it could pick a place to melt down, making that area uninhabitable for some years.

Project Pluto only got as far as a few (successful) engine tests. Work was made difficult because the engine could only be tested by remote. The Coors porcelain company, later made popular by its brewery branch, made important elements for the engine's reactor. Tolerances were supposedly very tight in the Pluto's reactor, with only slight imperfections resulting in serious disaster. In the end, it was decided that 60's ICBM technology was looking good enough that Pluto was unnecessary, and that making a doomsday missile that could not be fired without killing anybody near the silo was a bad idea.