The powerful earthquake that rocked the Tohoku region Tuesday morning temporarily stopped the critical cooling system for a spent fuel pool at the Fukushima No. 2 nuclear plant for about 100 minutes, Tokyo Electric Power Company Holdings Inc. said.

The cooling system was automatically shut down at 6:10 a.m. at the pool for the No. 3 reactor after sensors detected changes in the water level of a tank that pumps cooling water into the pool, which is home to 2,360 still-hot spent fuel assemblies and 184 unused ones.

Tepco spokesman Kenichi Nakakuki said the quake likely shook up the water inside the tank, triggering the automatic shutoff.

After confirming that no water had leaked from the tank, Tepco reactivated the cooling system at 7:47 a.m., Nakakuki said by telephone.

No radioactive materials have been leaked from Fukushima No. 2, Tepco said. The station is located 12 km south of the heavily damaged Fukushima No. 1 facility.

At 6:38 a.m., a 1-meter tsunami reached the coast facing the No. 1 plant, but there were no reports of new damage, Tepco said. The plant was wrecked in the triple meltdown crisis after the March 2011 tsunami-quake disaster.

The Fukushima No. 2 power plant was also damaged by the 2011 tsunami, but workers there managed to safely shut down its reactors, barely avoiding a meltdown crisis.

The four reactors at the No. 2 plant have been suspended since the 2011 disaster due to strong opposition from local residents.

During Tuesday’s automated suspension of the cooling system, the water’s surface temperature at the No. 2 plant rose by 0.2 degrees to 29.5 degrees.

If the water in the spent fuel pools had boiled off and exposed the rods, thousands of nuclear fuel assemblies would have been severely damaged, releasing massive amounts of radioactive materials into the environment.

Tepco guidelines state that the pools’ surface temperature must not exceed 65 degrees. If cooling functionality was completely lost, the surface temperature would likely rise to the limit in about seven days, according to Tepco. In the wake of the 2011 meltdown crisis at Fukushima No. 1, all 48 commercial reactors in Japan were eventually taken offline. Since then, four have been reactivated after passing stringent new safety rules set by the Nuclear Regulation Authority.

All 48 reactor buildings have spent fuel pools that house thousands of fuel assemblies because Japan has yet to build a final disposal site due to opposition from residents near candidate sites.

Prime Minister Shinzo Abe’s government has worked to reboot more reactors once they pass NRA’s safety standards test.

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