The cooling system for the reactor 4 spent fuel pool at the Fukushima No. 1 power plant temporarily halted Tuesday morning after an alert over electrical equipment was issued, Tokyo Electric Power Co. said.
The operation to remove fuel from the spent fuel pool was also halted. Tepco said workers damaged a cable during drilling work near the electrical equipment.
The alert over the electrical equipment was issued at 9:40 a.m. and the cooling system stopped about five minutes later.
While the temperature of the water inside the pool rose to 13.1 degrees from 13 while the cooling was suspended, Tepco said it was able to restart the system by 2:16 p.m.
The damaged cable also caused a fire that was immediately extinguished, Tepco said.
Tepco started the delicate work of removing more than 1,000 fuel assemblies from the spent fuel pool inside the damaged reactor 4 building in November.
Reactors 1, 2 and 3, all in operation at the time of the earthquake in March 2011, experienced core meltdowns.
Reactor 4, offline for periodic maintenance work, had all of its fuel stored in the spent fuel pool and avoided a meltdown, but concerns have remained over the continued storage of the large volume of fuel in the building, which was weakened by a hydrogen explosion.
Water management hit
Nuclear regulators say Tokyo Electric Power Co. might have been able to minimize the leak of 100 tons of toxic water last week from a tank at the crippled Fukushima No. 1 nuclear complex if it had implemented sound water management.
The direct cause of the incident was an erroneous valve setting, which led toxic water to be injected into a tank that was already nearly full. But Nuclear Regulation Authority Commissioner Toyoshi Fuketa said the plant should have been managed so that leaks would be prevented even if valves are mishandled.
Tepco admitted during a meeting with regulators and experts Monday that workers in a control room last Wednesday had not been paying close attention to the water levels of the tanks that should have been receiving the toxic water.
“The water transfer operation to a tank area called E started Feb. 17 and was going smoothly. So the monitoring activities became lax (during the time the wrong tank in the H6 area was receiving water),” one of the Tepco officials said.
The water levels in the E-area tanks were basically flat, even though a pump for radioactive water was operating, Tepco said.
Meanwhile, an alarm went off, warning that the water level in the H6-area tank was rising, but workers judged that a gauge had malfunctioned because of the irregular readings it started to show afterward.
It wasn’t until more than nine hours after the alarm went off that workers noticed the tank in the H6 area was overflowing.