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.

FORBES: New Video From Fukushima Shows Containment Vessel Dome Amid Debris

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.

FORBES: Nuclear Power's Long and Toxic Tail