[mywittyname]

It was from atmospheric testing of nuclear weapons in general, but the last atmospheric nuclear test was conducted in 1980, leaving Chernobyl as the last event to disperse significant nuclear material into the atmosphere. Though, I think the bulk of the radionuclides have decayed enough that there's no longer a need to salvage old WW1 battleships for their low radiation steel. Specifically, the two major "medium-lived fission concerns," caesium-137 and strontium-90 have a ~30 year half-life.