The United States has no unaccounted nuclear weapons. There are two American nuclear reactors laying on the bottom of the Atlantic Ocean, but I seriously doubt any terrorist can get to them. The same goes for Soviet nuclear missiles lost in the Pacific.
You need to research any loss of nuclear weapons by either the United States and the Soviet Union and study how each nation went about recovering them. You should search through the JSTOR database on "broken arrows".
>The Palomares and Thule incidents are well known "broken arrows". The A-4 incident is new to me. Global security website does have a list of "broken arrows", including the A-4 falling off a carrier elevator.
I do question the A-4 incident, however. The B-52s, like the ones were lost over Palomares and Thule, normally carried nuclear weapons as a strategic deterrent. The A-4 was a rather small aircraft, so its nuclear payload would also be small, and the A-4 was not a strategic bomber.
Internet searches are good, but you have to be careful of what sites you use in your paper. You will want to cite reliable sources; university papers have a higher criteria than high school projects. I would suggest that you use the internet searches as a starting point, then search for academic articles and books on the subject.