(In case you haven't read about it during my absence...)
Nikkei Shinbun reports that NISA admitted the hydrogen explosions that took place in Reactor 1 and Reactor 3 at Fukushima I Nuclear Power Plant in March may have been caused by hydrogen flowing back from the exhaust stack. In other words, vent may have caused the explosions.
From Nikkei Shinbun (12/27/2011):
The Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry disclosed during the experts hearing on December 27 on the cause of the Fukushima I Nuclear Power Plant accident that a possible cause of the hydrogen explosions in Reactors 1 and 3 may have been that the hydrogen that was vented from the Containment Vessel [to the exhaust stack] flowed back into the reactor building through a different pipe. As the power was lost due to tsunami, the valve of this different pipe remained open, and unable to prevent the reverse flow of hydrogen, according to the NISA.
In Reactors 1 and 3, hydrogen accumulated in the Containment Vessels after the core meltdowns, and TEPCO carried out the vent in order to remove hydrogen. The exhaust pipe for the vent connects to the exhaust pipe for the "standby gas treatment system" for the air ventilation of the reactor building, and then to the exhaust stack.
The valve of the exhaust pipe for the standby gas treatment system opened when the power was lost, so that the air ventilation of the reactor building would continue. In fact, the investigation of Reactor 3 after the accident showed the valve was open. When TEPCO did the vent, hydrogen may have flowed back to the reactor building through the open valve, and with the hydrogen leaked from the top lid of the Containment Vessel caused the hydrogen explosion.
All the other nuclear reactors in Japan has the same system whereby the valve opens when the power is lost. As a countermeasure, the NISA suggests two separate exhaust pipes and installing a valve to prevent backflow. Professor Tadashi Narabayashi of Hokkaido University points out that the vent process needs to be improved fundamentally.
Narabayashi, one of the "Three Plutonium Brothers" who said the toxicity of plutonium was the same as salt, used to work for Toshiba.
So after more than 9 months since the accident NISA feels like telling the truth for some reason, now that the accident is officially "over".
The very act of venting probably caused the explosions, says NISA. How about that, GE?
Product liability lawsuits anyone?