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The latest tremor, whose magnitude was revised to 7.1 from 7.4, rocked the northern and central parts of Miyagi Prefecture, which was the worst affected by last month's mega quake and tsunami, at 1132 hours (local time) on Thursday night. Radioactive water spilled from pools holding spent nuclear fuel rods at the Onagawa power plant in Miyagi following the aftershock, the nuclear safety agency was quoted as saying by Kyodo news agency on Friday.
However, the powerful quake did not hamper the ongoing work to restore reactor cooling systems at the crippled Fukushima plant, though workers were briefly evacuated after a tsunami advisory issued, the operator Tokyo Electric Power Company said. It said no new irregularities were detected in radiation readings or other indicators, except for the surface temperature of the No 1 reactor at the plant, where engineers have been battling for nearly a month to bring the situation under control.
Before the quake, the reading stood at 223 degrees Celsius. Just after the tremor, it rose to about 260 degrees Celsius at midnight -- but fell back to 246 degrees Celsius this afternoon, according to national broadcaster NHK. After the tsunami warning issued for the area was lifted, the workers inspected the site and resumed their critical task to cool the overheating reactors at the facility. Thursday night's quake triggered panic among residents already being sheltered at local facilities following the last month catastrophe that has left nearly 30,000 people dead or unaccounted for in Japan's northeast.
Two men, aged 85 and 79 respectively, died in Miyagi while a 63-year-old woman was killed in the neighbouring prefecture of Yamagata after the tremor, according to the Fire and Disaster Management Agency. An 83-year-old woman in Miyagi was reportedly taken to hospital immediately after the earthquake and later confirmed dead. About 140 people were also injured, Jiji Press reported.
The National Police Agency said five buildings were totally or partially destroyed in Miyagi and three each burnt down in Miyagi and Iwate. Tohoku Electric Power Company said as many as four million homes lost power at one point and, despite its restoration effort, the outage continued across Aomori and Iwate and in some areas in Tohoku prefectures till Friday morning, affecting 3.04 million homes.
Some external power supply was disrupted at suspended nuclear plants and a spent fuel reprocessing facility in Miyagi and Aomori prefectures, causing them to use backup generators. A spent nuclear fuel disposal facility in the village of Rokkasho, Aomori Prefecture, lost external power supply and switched to an emergency generator but power was restored on Friday morning, according to the government's nuclear safety agency.
The Higashidori nuclear power plant in another village in Aomori also got power from an emergency generator after Thursday's quake, but its external power supply was restored early this morning. There is also no information that radioactive materials had leaked due to the aftershock.
Higashidori's only reactor was undergoing regular maintenance at the time of the temblor and its fuel rods were not inside the core but stored in a spent fuel pool, the Nuclear and Industrial Safety Agency said. The spent fuel pools at the Onagawa plant and Higashidori nuclear power station in Aomori Prefecture lost their cooling functions for 20 to 80 minutes after the quake, but their temperature hardly rose, it said.
A small amount of contaminated water spilled on the floor inside the buildings at all three reactors at the Onagawa plant, Kyodo said. In all, water spilled or leaked at eight sections of the plant as a result of last night's quake, according to Tohoku Electric. As much as 3.8 liters of water leaked at one of them, with the highest level of radioactive isotope -- 5,410 bequerels per kg --- found in the spilled water on the floor beside a spent fuel pool in the building housing the No 1 reactor.
TEPCO, whose workers have been scrambling hard to stabilise the Fukushima nuclear facility since the March 11 quake, said no new abnormalities have developed in any of the six reactors at the radiation-leaking power station and none of is workers there were hurt following last night's tremor. Its remarks came as engineers continued to pump fresh water into the No 1 to No 3 reactors to prevent them from overheating and inject nitrogen into the No 1 unit to avert a possible hydrogen blast.
TEPCO also continued to release relatively low-level radioactive water into the Pacific Ocean to make room for far more contaminated water that has flooded the basement of the No.2 reactor's turbine building. The utility would discharge 8,000 tonnes of contaminated water.