Tohoku late Thursday suffered its largest aftershock since the March 11 killer temblor, but no abnormalities were found in radiation levels around nuclear reactors along the region’s Pacific coast, including the crippled Fukushima No. 1 power plant, government officials said Friday.
At the Onagawa nuclear plant at Ishinomaki, Miyagi Prefecture, and the Higashidori plant in Higashidori, Aomori Prefecture, cooling systems for spent fuel pools stopped for 20 to 80 minutes Thursday night after the quake.
However, the cooling systems were all restored after that, with little change in water temperatures of the pools during that time, according to the Nuclear and Industrial Safety Agency.
The aftershock also cut off external power supplies to the Higashidori plant and the nuclear fuel reprocessing plant in Rokkasho, Aomori Prefecture, but their backup generators automatically kicked in to keep supplying electricity as they are designed to do during emergencies, the government said.
The external supplies were restored by Friday morning.
“Everything has been restored and the cooling systems are working steadily,” Chief Cabinet Secretary Yukio Edano said.
“There has also been no abnormality in data at the monitoring spots” around the plants measuring radiation levels.
Edano warned Friday morning, however, that more aftershocks with a seismic intensity of lower 5 and upper 6 on the Japanese scale to 7 may occur.
“We call on everyone to be prepared for such aftershocks,” he said.
Meanwhile, up to 3.8 liters of radioactive water was reported to have leaked or spilled over from eight locations within the Onagawa atomic plant compound, including from three spent fuel pools.
But no leakage outside the compound was reported.
At the Fukushima No. 1 plant, the aftershock did not affect ongoing work to keep pumping fresh water into the Nos. 1, 2 and 3 reactors to avoid overheating and the infusion of nitrogen gas into the reactor 1.
But the temperature inside reactor 1′s core surged to 260 degrees at midnight Thursday from 223 as of 7 p.m. before the aftershock. The cause of the rise was not immediately known, Tokyo Electric Power Co. said.
The temperature went down to 246.6 as of 1 p.m. Friday, according to NISA.
NISA said the cooling systems for spent fuel pools at the three reactors at Tohoku Electric Power Co.’s Onagawa nuclear plant and the Higashidori plant stopped at around 11:32 p.m. Thursday soon after the aftershock occurred.
An important preparation is to make sure the plants can secure electricity when the external power supply is knocked out, as happened at Fukushima No. 1.
That situation became grave after the external power supply was lost and backup emergency generators were knocked out by tsunami, halting the plant’s cooling systems.
If the backup system fails, generators are the next emergency measure to provide electricity to keep vital reactor cooling systems working.
Power supply vehicles have now been provided for Fukushima No. 1, and arrangements are being made for other nuclear plants nationwide to get them.
Tepco said none of the workers at the Fukushima plant was injured in the aftershock.
NISA official Hidehiko Nishiyama also noted that the level of contaminated water in an underground trench connected to the No. 2 reactor building at the Fukushima plant rose 2 cm as of 7 a.m. Friday.
Although the reason is still being looked into, Nishiyama said the increase may stem from the sealing of a crack to prevent further contaminated water from pouring into the sea.
Earlier this month, high-level radioactive water was found to be leaking from the cracked cable storage pit near the No. 2 reactor. A few days later, Tepco succeeded in stopping the leak by injecting sodium silicate, also known as “water glass.”
“We must keep an eye on the rise in the water level and if the increase were to continue everyday, we would need to transfer (the water) elsewhere,” Nishiyama said.
Information from Kyodo added