Truman was, for the most part, against the idea of using nuclear weapons during the Korean War, though after the Chinese joined he certainly seemed more open to the idea.

Sometime in 1951 (I can't remember when), multiple nuclear weapons were transferred to the control of the Strategic Air Command so that they'd be ready for immediate use if necessary. This was occurring at around the same time MacArthur was beginning to be openly opposing Truman's orders, so he was overlooked as it was feared (particularly by Truman) that he would be too eager to use them.

Ultimately, it seems unlikely that they would have been used. Sure, preparations were made for nuclear weapons to be used, but that is understandable considering there was also the worry by the US that the USSR might join the war, but largely because of the fact that Truman did not seem open to the idea means that nukes probably wouldn't have been used.