Last week officials from the Tokyo Electric Power Company announced that extremely radioactive water had leaked from a storage tank at the Fukushima Daiichi nuclear power plant.
According to the utility over 100 tons of water containing 240,000,000 becquerels per liter of beta-ray emitting substances leaked out of the storage tanks over a period of nine hours before the leak was discovered. The water which leaked was largely partially treated water from early on in the disaster in March 2011.
Alarms indicating a rise in water levels in the tank had gone off nine hours before the leak was discovered, but was ignored and shut off by workers who assumed that it had malfunctioned.
TEPCO spokesman Masayuki Ono told reporters at a press conference that the radioactive water had leaked because three valves which should have been closed were open. At the time, Tokyo Electric speculated that one of the valves may have malfunctioned or have broken.
This week, the utility admitted that the valves may have been wrongly operated by workers, instead of an equipment failure. Around the time of the leaks workers were attaching numbered plates to valves in the tank farm, and it is possible that the valves may have been opened during these operations.
Tokyo Electric is interviewing workers to understand how and why the valves were opened, debating whether or not to clarify valve operations, and improving the design of the water storage tanks rainwater collection system.
TEPCO also admitted that workers were not sufficiently monitoring the levels of water inside of the tanks and ignored alarms which indicated the water levels in the tank were increasing.
Chief Cabinet Secretary Yoshihide Suga told reporters that the utility needed to review its measures to prevent leaks of radioactive water to ensure that similar accidents are prevented, and should also collect the water which escaped and soil it subsequently contaminated.
Last August, over 300 tons of highly radioactive water leaked from a different tank, some of which flowed into the Pacific Ocean. Ever since that leak, Tokyo Electric began increasing their efforts to monitor the tank farm and detect potential leaks through additional physical monitoring and the installation of water-level indicators to all of the steel tanks.
Officials from the Nuclear Regulation Authority, Japans nuclear safety regulation body criticized TEPCO for failing to prevent the leaks. Toyoshi Fuketa, an official from the NRA, told reporters on Monday that water gauges, alarms, and other devices were installed in water tanks in order to prevent leaks, but have not been made operable yet. Fuketa demanded that TEPCO officials ensure that these devices are functional.