A nuclear accident at a uranium-processing plant 125 km northeast of Tokyo on Thursday reached criticality, injuring three and pushing radiation levels up to 20,000 times beyond normal in Tokai, Ibaraki Prefecture.
The accident, Japan's worst, may expose thousands to radiation and remained out of control as of 1:30 a.m.
Criticality is the point at which neutrons produced in the fission process are sufficient to sustain an independent chain reaction, and early Friday morning it was feared that reaction was still simmering.