Once the MOX fuel assemblies have been irradiated in commercial power reactors, the plutonium can no longer be readily used for nuclear weapons.

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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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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.

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If the U.S. finds a place for its nuclear waste to go, some fuel assemblies will have to be removed from casks and transferred to transportation packs, a dangerous enough task without the prospect of fuel rods ruptured or crumbling.

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