Hours earlier, the plant operator switched off a reactor cooling system after discovering rats near critical equipment.
The failure of diesel generators, which power water pumps that are critical to reactor cooling systems, was a leading cause of the current crisis.
For example, the Westinghouse AP 1000 reactor places cooling water at a level above the reactor unlike the Fukushima design so that it would naturally flow down.
So far, the three Japanese reactors experiencing cooling problems haven't released significant amounts of radiation, but the situation is dangerous, as an explosion Monday morning in Japan damaged a pump bringing in cooling seawater at one reactor and a third reactor began experiencing cooling problems Monday afternoon.
That came alongside news that the volume of the reactor's cooling water had declined sharply.
As of 14:00 local time (05:00 GMT) on Friday, the temperature of water inside the cooling pool at reactor 3 was 15.1C, well below the safety limit of 65C, Tepco said.
That thinking changed on March 28, 1979, when a small valve stuck open, cooling water escaped and the reactor core of TMI's Unit 2 began to melt.
The top government spokesman said TEPCO has begun new cooling operations to fill the reactor with sea water and pour in boric acid to prevent an occurrence of criticality, noting it may take several hours to inject water into the reactor.
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Even before the reported explosion, officials were becoming increasingly worried about No. 2 reactor, due to a problem in the cooling system that hasn't occurred in the others.
Workers at the Fukushima Daiichi plant are attempting to remove highly radioactive water from a reactor building to allow repair work to the cooling systems knocked out on 11 March.
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.
In December 1995, the Monju fast-breeder reactor at Tsuruga sprang a leak in its sodium-cooling circuitry.
In addition, the company chose to build its cooling facility at a lower elevation than its reactor, a decision that has played a significant role in the current disaster.
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On site, the workers in its number one reactor appear not to have been familiar with an emergency-cooling system called an isolation condenser, which they wrongly thought was still working after the tsunami.
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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The explosion at the Daiichi No. 2 reactor is the latest setback in a long battle to keep reactor cores from overheating in the wake of Friday's massive earthquake, which knocked out cooling systems and backup generators.
If the pumps are knocked out in a Generation II reactor as they were at Fukushima Daiichi by the tsunami the water in the cooling system can overheat and evaporate.
The six-reactor Fukushima Daiichi nuclear plant was badly damaged after the 11 March 2011 earthquake and tsunami knocked out cooling systems to reactors, leading to meltdowns and the release of radioactivity.
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.
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