Mistakes were made. Nuclear bomb testing in the South Pacific didn’t always produce a successful explosion. That was why they had to test them. The US cleaned up the areas by creating an enormous pile of radioactive debris, and then covering it with a concrete dome. The islands are barely above sea level, which means the domes are barely above sea level. As the oceans rise, the domes will effectively sink, and a long-term nuclear waste storage scheme may have only lasted less than a hundred years. If the domes don’t leak, then there’s no change. If the domes do leak, highly radioactive debris will swirl out into the ocean. The domes probably weren’t designed to be submerged forever, or to be opened and redistributed.
“Climate Change Threatens Radioactive Storage Dome In South Pacific” – The Guardian