A magnitude 7.4 earthquake that struck northeastern Japan has not affected the country's nuclear power plants, according to utility TEPCO. No main measured parameters have changed, nitrogen gas injection into unit 1 continues, no significant changes in water levels in trenches have been noticed, no workers have been injured, it said.
Data from unit 3 of the Fukushima Daiini nuclear power plant, which so far is in stable condition (unlike its neighbour Fukushima Daiichi) show that the highest acceleration due to the earthquake (58 Gal) was only about 15% its maximum, and about a third of what was experienced on 11 March. No data was available for Fukushima Daiichi; the Kashiwazaki Kariwa plant was even less affected, and none of the four operating units was shut down.
In contrast, TEPCO estimates that the acceleration from the March earthquake (507 Gal in east-west) exceeded the plant's designed resistance in unit 3 (441 Gal), although no data was available for units 1, 2 and 5 at the time of that 22 March report.
[Editor's note: this story has been modified from the original, to reflect subsequent media reports that the Onagawa plant lost external power, but successfully started up its emergency generators for a time].
FilesFukushima-Daiichi parameters 11 April Reactor-by-reactor, system-by-system summary from JAIF as of 11 April