Critical cooling systems for four pools containing thousands of nuclear fuel assemblies at the Fukushima No. 1 power plant shut down due to a loss of power overnight Monday, highlighting the vulnerability of the ad hoc equipment set up after the meltdowns two years ago.
Tokyo Electric Power Co. said repairs to fully recover the cooling functions of the fuel pools at reactors 1, 3 and 4 were completed on Tuesday night. Meanwhile, the utility said another huge common fuel pool would be fully functional by 8 a.m. Wednesday.
Tepco was still trying to pinpoint the cause of the power loss Tuesday afternoon, raising concern about the soundness of the facilities at the badly damaged nuclear plant, where decommissioning work will take decades.
Tepco emphasized that it would take at least three more days for the water in the spent fuel pool in the damaged reactor 4 building, potentially the most dangerous one, to reach the threshold control temperature of 65 degrees, giving the operator ample time to restore power before the coolant water started to boil and evaporate.
The utility speculated that a makeshift switchboard set up after the meltdowns of reactors 1-3 probably malfunctioned Monday evening, causing two more switchboards and other equipment to automatically shut down.
It is thought that this led to the power loss to the cooling systems for the fuel pools in the reactor 1, 3 and 4 buildings as well as the large common pool, which contains 6,377 nuclear fuel assemblies, Tepco said.
The injection of coolant water into the damaged cores of reactors 1, 2 and 3 was not disrupted, Tepco said.
“We are still trying to identify the cause (of the power loss). We need to investigate further,” said Tepco executive Masayuki Ono, who served as a spokesman at the news briefing Tuesday morning.
Tepco announced the loss of cooling functions shortly after 10 p.m. Monday, about three hours after the power went out, drawing criticism from the media.
Tepco said the information was withheld because it took several hours to figure out which equipment was affected by the power disruption.
Chief Cabinet Secretary Yoshihide Suga, meanwhile, sought to ease concern by assuring the public that the plant has backup equipment on hand for worst-case scenarios.
“We are preparing (backup safety) measures so that you don’t need to worry,” Suga told a news conference Tuesday morning.
According to Tepco, fire engines are deployed at the Fukushima plant, and other water pumps are available as well to inject water into any of the pools in the event the cooling equipment goes down for an extended period.
Radiation monitoring posts in and around the Fukushima plant showed no abnormal readings, according to both Tepco and the government.
But the current problem pulls back the curtain on the unreliability of the cooling systems at the Fukushima plant.
Containing 1,533 fuel assemblies, the reactor 4 pool is the hottest of the four. Its water temperature was estimated at 30.5 degrees at 10 a.m. Tuesday, and would rise 0.368 degree per hour while the cooling system is out, Tepco said.
At 100 degrees, the water would boil and evaporate. If all the water was lost, the fuel assemblies would melt down andthe pool collapse, releasing vast amounts of radioactive material into the environment.
Tepco now plans to transfer all the fuel assemblies from the reactor 4 pool to the sturdier common pool by the end of this year.
But the cooling system for the common pool was also shut down by the recent power loss.
It is expected to take more than three decades to decommission the three troubled reactors at the Fukushima complex, which has six reactors. It has not been decided what will happen with the rest of the crippled plant. Tepco will have to keep the cooling systems running throughout the long process to prevent further meltdowns of the still-hot reactor cores.
Ono said that for now at least Tepco believes the makeshift switchboard either lost current or surged, tripping other switchboards and equipment connected to the network, and cutting power to the cooling systems.