A 7.4 aftershock last night at around midnight knocked out power in parts of the Miyagi, Yamagata, Aomori, Iwate, Akita and other Japanese prefectures. While a tsunami warning was issued, but subsequently lifted.
2 of the 3 normal power transmission lines were knocked out at the Onagawa nuclear power plant. However, backup diesel generators are being used, there is no threat to the diesel generators from a large tsunami, and Japanese agencies are saying that there have been no change in radiation levels around the plants.
Therefore, unless the aftershock directly damaged the core or storage tanks, this should not turn into any real problem.
And there are no reports of additional damage to the Fukushima Daiichi or Daini nuclear power complex.
Update: Reuters is reporting:
Water leaked out of spent fuel pools at the Onagawa nuclear plant in northeast Japan after a strong aftershock rocked the region late on Thursday, but there was no change in the radiation levels outside the plant, operator Tohoku Electric Power said on Friday.
It said water sloshed out of spent fuel pools in the plant’s No.1, No.2 and No.3 reactors which had been shut down after the 9.0 magnitude quake on March 11, and had also leaked in three other locations in the No.3 reactor complex.
“We detected a small rise in radiation levels inside the reactor buildings, and are trying to find the locations of the leaks,” a Tohoku Electric official said. “We see no change in radiation levels outside the reactor buildings.”
The International Atomic Energy Agency (IAEA), citing information from Japanese authorities, said off-site power was lost at some other nuclear facilities in the country after Thursday evening’s quake, and that emergency power supply was operating.
But NHK notes that the amount of water spilled is very small:
The power company reported on Friday that water had spilled onto the floor at all 3 reactors at the Onagawa nuclear power plant in Miyagi Prefecture. The amount of water spilled was 3.8 liters at the most.
NHK also points out:
At the Higashidori nuclear power plant in Aomori Prefecture, the quake shut down all outside power lines, prompting the plant to switch on emergency diesel power generators.
The plant had shut down its reactors and was undergoing an inspection at the time of the quake.
No serious effects were reported with the cooling systems of storage pools for spent fuel rods.
A nuclear fuel reprocessing plant in Rokkasho, Aomori Prefecture, also lost all outside power and is operating on emergency diesel generators at the moment.