In 1999, there was a bad nuclear accident in Japan, exposing dozens to what? The accident exposed dozens of people to radiation. The event was the Tokaimura nuclear accident, a criticality accident at a JCO uranium-processing facility in Ibaraki Prefecture on September 30, 1999.
Workers were preparing a uranium solution when too much material was poured into a precipitation tank. The mixture reached criticality, beginning an uncontrolled nuclear chain reaction that emitted intense gamma and neutron radiation. Three workers received severe exposures, and two later died from radiation injuries. Other plant workers, emergency responders, and nearby residents were also exposed.
The accident caused evacuations and shelter-in-place instructions, as well as restrictions on local crops and water use. It was rated Level 4 on the International Nuclear and Radiological Event Scale. A common mix-up is calling it a reactor explosion: Tokaimura was principally a fuel-processing criticality accident, not a commercial power-reactor disaster like Fukushima in 2011.