Reactor operators typically discharge about one-third of the fuel assemblies every 18 months to 2 years and place this spent fuel in a pool to cool .

FORBES: Nuclear Power's Long and Toxic Tail

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