Contrary to what it said last week, Tokyo Electric Power Co. (TEPCO) now believes that it is unlikely that the March 12 hydrogen explosion at the No. 1 reactor building of the Fukushima No. 1 nuclear power plant was caused by the reverse flow of hydrogen gas from the containment vessel into the reactor building.
Contrary to what it said last week, Tokyo Electric Power Co. (TEPCO) now believes that it is unlikely that the March 12 hydrogen explosion at the No. 1 reactor building of the Fukushima No. 1 nuclear power plant was caused by the reverse flow of hydrogen gas from the containment vessel into the reactor building.
The utility said that on June 4, it found records saying that one of two valves in an exhaust pipe was designed to shut down automatically when a power source was lost, and probably did close. That would have prevented the flow of hydrogen from the containment vessel into the reactor building.
TEPCO has yet to confirm whether the valve actually shut down automatically, company officials said.
When the Great East Japan Earthquake and subsequent tsunami hit the nuclear power plant on March 11, all power was cut off.
In the No. 1 reactor building, the exhaust pipes from the reactor building and the containment vessel were joined into a single pipe that then vented all exhaust gases from both to the outer atmosphere.
On March 12, authorities attempted to release hydrogen gas that had built up inside the containment vessel to prevent an explosion. The gas was supposed to be released to the outer atmosphere.
Earlier, TEPCO officials theorized that hydrogen gas from the containment vessel had instead flowed down the pipe into the reactor building through two open valves. As a result, hydrogen would have accumulated inside the reactor building, leading to the explosion.
On June 3, the utility reported that theory to the Nuclear and Industrial Safety Agency (NISA) of the Ministry of Economy, Trade and Industry (METI).
On June 4, however, TEPCO officials rechecked their records from last year's regular inspections. They discovered then that one of the two valves was designed to seal itself automatically when power was lost.
As it is contained in a simple automatic system, officials think it likely that the valve shut down properly.
TEPCO is continuing its investigation into the cause of the explosion.