Article from NHK News :
" An advisor to Prime Minister Naoto Kan says no one in the government knew of the risk of a hydrogen explosion in the initial stages of the emergency at the Fukushima Daiichi nuclear plant.
The disclosure was made on Monday by Goshi Hosono, who is a governing party lawmaker and senior member of the government's nuclear taskforce.
Hosono referred to a hydrogen blast that shattered the No.1 reactor building one day after the March 11th earthquake and tsunami. The blast occurred after workers began venting air from the reactor containment vessel to reduce pressure inside.
Hosono said he was not aware of a single nuclear expert who warned of the risk of a hydrogen blast following the venting operation. He said nitrogen inside the reactor container was supposed to prevent such explosions.
Plant operator Tokyo Electric Power Company also told reporters that hydrogen is supposed to be processed within the containment vessel, and that such an explosion is not assumed in a reactor building.
Large amounts of radioactive substances were released into the environment as a result of the hydrogen blast."