Tokyo Electric Power Co. said Wednesday that some of the melted fuel in reactor 2 at the Fukushima No. 1 nuclear plant may have triggered a brief criticality event.
Although the Nuclear and Industrial Safety Agency said there have been no drastic changes in the reactor's temperature and pressure level, and the reactor itself is stable overall, the discovery may mean the goal of Tepco and the government to achieve cold shutdown of all three crippled reactors by the end of the year may not be possible.
Suggesting that criticality, or a sustained nuclear chain reaction, may have occurred temporarily, or partially, Tepco said one hundred thousandth of a becquerel per cubic centimeter of xenon-133 and xenon-135 was detected in gas samples.