The Associated Press has finally received heavily redacted freedom of information copies of documents from the Armed Forces Special Weapons Project, in which planners considered uses for radiological weapons. That is, the U.S. investigated potential 'dirty bomb' uses for radioactive materials, including large-scale area denial and assassinations.
The AP article mentions that an "unknown assailant used a tiny amount of radioactive polonium-210 to kill Kremlin critic Alexander Litvinenko in London", and that Polonium is precisely the type of attack the Special Weapons Project foresaw.
The whole premise and the project itself is a rather Strangelovian exercise to be sure, but it came about in the depths of the Cold War, at a time when people were not at all creeped out by anything and everything radioactive. Calling something 'atomic' meant is was new! and modern! and not poisonous and evil. It is not at all surprising that the military looked into this realm of warfare - it would only have been surprising it they hadn't.