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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Engineers and workers so far have managed to stave off a complete meltdown in Fukushima Daiichi's reactors 1-3 and in the spent fuel pool of unit 4.

CNN: STORY HIGHLIGHTS

According to a report by the Congressional Research Service, there are more fuel rods in the Unit 4 fuel pool than at any of the five other reactors.

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The IAEA statement was released hours after Tepco temporarily shut down the cooling system in the second unit's spent fuel pool following the discovery of dead rodents near a transformer.

BBC: IAEA calls for improvements at Japan's Fukushima plant

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

This is a very difficult situation, and there will be a lot of work continuing as we go forward to deal with continuing to cool the reactors and to provide cooling to the spent fuel pool.

WHITEHOUSE: Press Briefing

Used fuel that has been cooled in a spent fuel pool for at least five years can be put into a container called a cask and surrounded with inert gas, providing leak-tight containment, according to the U.S. Nuclear Regulatory Commission.

CNN: Nuclear disasters: Is the fear realistic?

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 .

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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.

FORBES: Nuclear Power's Long and Toxic Tail

Perhaps a more important question is: Could fuel be cooled in a way that doesn't require the pool design?

CNN: Nuclear disasters: Is the fear realistic?