The Yomiuri Shimbun
An emergency cooling system of the No. 1 reactor at the Fukushima No. 1 nuclear power plant malfunctioned after the March 11 earthquake and before the tsunami hit, data released by plant operator Tokyo Electric Power Co. have revealed.
According to a large cache of data concerning plant operations from March 11 to 14 released Monday by TEPCO, the No. 1 reactor's isolation condenser, which operates on direct-current power, began to malfunction shortly after the Great East Japan Earthquake struck the Tohoku and Kanto regions.
TEPCO on Sunday announced that after the tsunami completely knocked out the cooling system, fuel rods in the No. 1 reactor melted down 16 hours after the earthquake hit at 2:46 p.m. on March 11. If the cooling system had operated normally, the meltdown could have been delayed, according to some nuclear experts.
The Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency demanded TEPCO release the data, which include water levels within reactor containment vessels and radiation levels for several days after the earthquake, in addition to operator logs and operation records.
According to the data, the No. 1 reactor was put into emergency shutdown immediately after the earthquake when control rods were inserted into the reactor core. The isolation condenser was then automatically activated at 2:52 p.m., initiating cooling and pressure reduction inside the reactor.
However, around 3 p.m., only about 10 minutes after it began operating, the isolation condenser stopped functioning temporarily, and then went on and off intermittently as valves between the condenser and the pressure containment vessel were opened and closed, according to operation records.
According to TEPCO, the pressure within the reactor fluctuated violently immediately after the earthquake. The cause of these fluctuations is not known, but TEPCO suspects workers manually suspended the condenser to stabilize the pressure.
When the tsunami hit, an emergency diesel generator that started after the earthquake was disabled, totally cutting off DC power. Other cooling devices failed, and temperature and pressure data for the No. 1 reactor became unavailable for a time.
At the No. 2 reactor, which has a different design than the No. 1 unit, a reactor core isolation cooling system--another type of emergency cooling system that operates on batteries--continued to operate for nearly three days after it started at 3:02 p.m. on March 11. Operators manually kept the system going until about 1:30 p.m. on March 14, to deal with fluctuating water levels inside the reactor.
(May. 18, 2011)