Japanese authorities say the atomic reaction at the heart of the nation's worst ever nuclear accident is under control. Officials say radiation levels in all but two spots near the accident have returned to normal. Workers accidentally pumped too much liquid uranium into a tank of nitric acid while making nuclear fuel. This led to Japan's first ever self-sustaining nuclear fission chain reaction.
Nuclear fission is the principal behind the atomic bomb.
When nuetrons hit uranium, atoms split and huge amounts of energy and radiation are released. The Tokaimura plant was not designed to contain this sort of accident. News of the disaster was slow to emerge. The owners of the plant waited one hour before informing local authorities about the mishap. And it took most of Thursday for the government to fully assess the situation.