A fire and explosion at the state-run Power Reactor and Nuclear Fuel Development Corporation reprocessing plant at Tokaimura contaminated at least 35 workers with minor radiation in Japan's worst atomic accident to date. They said they thought that while radioactivity was released into the atmosphere, the radioactive material itself remained contained. The Yomiuri newspaper reported that the plant was an experimental facility and the workers may have been conducting experiments without all necessary safety precautions, such as protective gear and remote control systems. At one point, as much as 15,000 times the normal level of radiation was measured in the vicinity of the plant. A team was removing water from the cooling equipment around the tank in hopes that it would suppress further nuclear fissioning.
Water can stimulate neutron activity, spurring along a nuclear reaction.
By early Friday morning, radiation levels were cut to about one-fifth the level they had been at originally, but they were still at the dangerously high level of about .7 milli Sieverts per hour. Tokyo Electric Power Co. delivered 880 pounds of sodium borate as authorities worked to get close enough to dump the powder on the radioactive holding tank and snuff the fissioning, company official Kohgo Usami said. Officials said the plant was not designed to block the escape of radiation, officials said. A total of 34 other workers were being examined for possible contamination, said Junichi Takahashi, an official with JCO Co., the private company that runs the plant. Hair samples from 11 of the workers showed traces of radioactivity, said Tatsuhito Matsuzaki, a local official.