As of 14:00 local time (05:00 GMT) on Friday, the temperature of water inside the cooling pool at reactor 3 was 15.1C, well below the safety limit of 65C, Tepco said.

BBC: Fukushima nuclear plant: Cooling system power restored

Restarting cooling to the spent fuel pool at reactor 4 had been of particular urgency for Tepco as it had the highest number of fuel assemblies stored there, Kyodo news agency reported.

BBC: Power at Fukushima nuclear fuel ponds restored

All of the rods from the fuel core were put into the spent fuel pool above the reactor at that time.

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The key risk of storing spent fuel at reactor sites is radiation exposure from spent fuel that has caught fire when it is stored in a pool.

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