Tokyo Electric Power Co. officials mistakenly believed that an emergency core cooling system at the Fukushima No. 1 nuclear power plant was still functioning after the March 11 tsunami, which possibly led to an early meltdown at the No. 1 reactor.
Tokyo Electric Power Co. officials mistakenly believed that an emergency core cooling system at the Fukushima No. 1 nuclear power plant was still functioning after the March 11 tsunami, which possibly led to an early meltdown at the No. 1 reactor.Senior officials at the plant did not realize that the system, an isolation condenser, loses its functions if the electricity is cut off, according to a government investigation committee.The committee, chaired by Yotaro Hatamura, a professor emeritus of the University of Tokyo, is expected to include its finding in an interim report to be released Dec. 26.The isolation condenser cools the reactor when ordinary pumps, powered by alternating-current sources, such as outside power supplies and emergency generators, cannot operate.The isolation condenser cools steam from the pressure vessel, condenses it into water and returns water into the reactor. Steam is cooled as it passes through piping in a water-filled tank.When it loses its storage battery power, a direct-current source, valves inside the containment vessel automatically close, blocking the flow of steam through the piping.The mechanism is designed to prevent radioactive materials from leaking to the outside along with steam in case the cooling system goes offline after an interruption in the direct-current supply.But senior TEPCO officials, including those at the Fukushima No. 1 plant, were not aware of the sequence and believed that the isolation condenser at the No. 1 reactor was functioning, according to the investigation committee.The No. 1 reactor lost both alternating- and direct-current sources around 3:30 p.m. on March 11, after the Fukushima No. 1 plant was rocked by the Great East Japan Earthquake.The committee suspects that the officials failed to respond promptly to the loss of cooling functions, contributing to an early meltdown of the reactor.TEPCO said there was not a mistake in operation of the isolated condenser before or after the tsunami.TEPCO's interim report on the accident says that the officials never thought that the isolation condenser was suspended at the time of the accident.It also says the reactor was damaged early because the condenser was not functioning when the volume of heat released from nuclear fuel was at its highest immediately after the reactor stopped.According to TEPCO, a worker at the No. 1 reactor opened valves outside the containment vessel when the electricity supply was restored temporarily after 6 p.m., but closed them after a short period. TEPCO opened the valves again around 9:30 p.m.The committee also said that TEPCO suspended one of the emergency core cooling systems for the No. 3 reactor on March 13, possibly contributing to a meltdown and other damages.According to TEPCO, a reactor core isolation cooling system started pumping water into the reactor after it lost an alternating-current source, but was suspended on March 12.After that, a high pressure core flooder system automatically started operations, but it was stopped early on March 13.