High radiation readings taken in the No. 1 reactor building the night of March 11 suggest it was the quake rather than the loss of cooling that critically damaged the Fukushima No. 1 power plant, a utility source said Saturday.
The belated disclosure could trigger a review of quake-preparedness at nuclear facilities across the country. Many have been focusing on increasing defenses against tsunami, which knocked out the plant’s poorly placed emergency power generators.
On March 11, the nuclear plant shut down automatically just after 2:46 p.m., when the magnitude 9 quake occurred. Within an hour, it was hit by at least two tsunami. The external power supply then shut down, stopping the emergency cooling system from injecting water into the reactor core at 4:36 p.m.
That evening, Prime Minister Naoto Kan declared the country’s first state of nuclear emergency and residents near the plant were asked to evacuate.
Workers entered the No. 1 reactor building during the night to assess the damage only to hear their dosimeter alarms go off a few seconds later, sources at Tokyo Electric Power Co. said. Since they thought the building was filled with highly radioactive steam, the workers decided to evacuate.
Based on the dosimeter readings, the radiation level was about 300 millisieverts per hour, the source said, suggesting that a large amount of radioactive material had already been released from the core.
The source of the steam was believed to be the No. 1 reactor’s overheated pressure vessel.
But for that scenario to hold, the pressure in the reactor would have to have reached enormous levels ~~~- damaging the piping and other connected facilities. It should have taken much more time to fill the entire building with steam.
A source at Tepco admitted it was possible that key facilities were compromised before the tsunami.
“The quake’s tremors may have caused damage to the pressure vessel or pipes,” the official said.
The Nuclear and Industrial Safety Agency has so far said the reactor withstood the shaking and that the unexpectedly large tsunami caused a station blackout, which led to explosions.
On the night of March 11, Tepco did not open the containment vessel vents to relieve pressure that was supposed to be rising. The move was finally taken the following morning, releasing radioactive steam from the vessel.
In the No. 1 reactor, the water level began falling from the night of March 11. Though Tepco sprayed in large amounts of water, the fuel was exposed and the reactor core melted down.
Subsequently, the fuel pellets’ zirconium casings began reacting with the hot steam, generating the hydrogen that blew the reactor building’s roof off at 3:36 p.m. on March 12.