Until March 2011, the worst nuclear accident in the history of Japan took place at a uranium facility in Tokaimura on September 30, 1999. Three workers were attempting to mix nitric acid and uranium to form the fuel uranyl nitrate. However, the workers unknowingly used seven times the allowable limit of uranium, and the reactor couldn’t stop the solution from reaching critical mass.
The three technicians were exposed to massive gamma and neutron radiation poisoning, which killed two of them. More than 70 other workers received high doses of radiation as well. After an investigation, the International Atomic Energy Agency saidthat the accident had been caused by "human error and serious breaches of safety principles."