South Korean negotiators had been seeking a new nuclear-cooperation agreement with the U.S. that would allow it to begin enriching uranium and reprocessing spent reactor fuel, arguing these technologies are crucial for Seoul to expand and secure its civilian nuclear-power program.
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.
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.
That could be a problem if the time comes to transport spent fuel from reactor sites to a central storage facility.
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Where most attention has centred on reactors 1, 2 and 3, there is also concern about the spent fuel pond in reactor building 4.
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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In early April, the cooling system at one of the pools containing spent - but highly radioactive - fuel rods in reactor number three broke down for about three hours.
As many as 15 percent of the 5, 000 previously approved welds in the reactor's auxiliary plumbing, containment dome and spent-fuel cooling systems were X-rayed and found to be defective due to air pockets, poor soldiering and heat damage.
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Electricity is needed to keep the reactor cores covered with coolant and to keep safe temperatures in spent fuel pools.
Each fuel assembly is typically used in the reactor for 4 to 6 years, after which most of the fuel it contains is spent, and the uranium dioxide is no longer cost-efficient at producing energy.
An alarm at reactor number three earlier revealed the system had stopped at one of the pools containing spent, but highly radioactive, fuel rods.
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