TEPCO: No.4 blast due to hydrogen from No.3

The operator of the crippled Fukushima Daiichi nuclear plant says the March 15th explosion at the No.4 reactor building may have been caused by hydrogen from the No. 3 reactor.

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It was initially thought that the March 15th explosion was triggered by hydrogen produced by damaged spent fuel rods in a pool inside the No.4 reactor building. But photographs of the pool taken in April show no damage to the rods.

TEPCO focused on ducts from the No.4 and neighboring No.3 reactor buildings that join into a single duct before an exhaust pipe.

The company says that when it vented gas from the No.3 reactor through the duct, hydrogen may have seeped into the No.4 reactor building. Hydrogen that accumulated in the upper part of the No.4 reactor building may have caused the explosion. Some experts had suggested that oil in the reactor building could have been one of the reasons for the blast. By assessing damage to the building, TEPCO says the blast was likely due to a hydrogen explosion, and that oil may have triggered fires after the blast.

http://www3.nhk.or.jp/daily/english/16_02.html