It appears likely that the operator of the Fukushima Daiichi nuclear power plant did not follow the procedures to prevent a hydrogen explosion, NHK World reports.

NHK obtained the manual for the No.1 reactor, where the hydrogen blast occurred on March 12th, one day after the tsunami destroyed the reactor's cooling system.

A failure of the cooling system causes the pressure inside the reactor's container vessel to rise and generates the risk of a hydrogen explosion.

The manual calls for releasing air from the vessel when the pressure is projected to rise to 853 kilopascals - double the operating limit.

A venting operation is necessary to prevent the vessel from being damaged, which could lead to the leakage of a large amount of radioactive substances.

The manual NHK has obtained shows that the pressure inside the vessel was close to the level that requires a venting operation 13 hours before the explosion occurred.

But the plant operator, Tokyo Electric Power Company (TEPCO), did not start the operation until 6 and a half hours before the explosion, and the operation was carried out just one and a half hours before the blast because it was hampered by high-level radioactivity.

A former nuclear plant engineer, Masashi Goto, says the utility should have released air when the pressure rose close to twice the operating limit.

Goto says if the company had done this, the amount of hydrogen leaked from the reactor core to the container vessel would have been smaller, reducing the risk of an explosion.

TEPCO declined to comment, saying it is evaluating its decision to release the air.