Nope, humans were trying to figure out how to manage the consequences of nuclear weapons, which were judged to be a necessary strategy to end World War II to attempt to defeat fascism and save democracy.

Not "just to see what would happen" but to know how vulnerable our Navy would be to the very real threat of atomic attack from the Russians, and thus to responsibly inform strategy and tactics should that nightmare ever come to pass.

Testing decontamination also necessary to inform e.g. necessary fleet sizes in case of nuclear war -- could ships ever be reused or not? An existing shipyard is the logical place to do that, and unfortunately widespread environmental consciousness just didn't happen until the landmark publication of Silent Spring in 1962, more than a decade later.

Storing the contaminated sand on the ship and sinking it was a pretty reasonable thing to do at the time, not "just to use it as target practice" but to kill two birds with one stone.

Nobody's perfect, but you seem to really be judging people from several generations ago according to current societal standards, and completely misportraying their motives.