Fukushima reactor pressure venting system failure

The venting system designed to release pressure inside the containment vessel of the No. 2 reactor at the Fukushima No. 1 nuclear power plant likely failed during the 2011 disaster, the operator of the facility said May 20.

The discovery was made by a robot deployed last October by Tokyo Electric Power Co. to a room in the plant where the venting pipes from the reactor pass.

The magnitude-9.0 Great East Japan Earthquake that triggered towering tsunami struck on March 11. On March 13 and 14, as the pressure inside the No. 2 reactor was rising, workers at the plant opened valves in its venting system. When the valves are opened, the pressure is supposed to rupture a special disc farther down the pipes that allows it to vent.

TEPCO, until now, was never able to confirm whether the effort was successful.

The robot revealed that the radiation levels around the rupture disc were relatively low, between 0.08 and 0.30 millisievert per hour. Similarly, radiation levels around the valve near the containment vessel were also low, between 0.15 and 0.70 millisievert per hour.

However, a significant amount of radioactive material was found to have been directly released from the damaged containment vessel.

By contrast, strong radiation levels higher than 10 sieverts per hour were detected in areas around the exhaust tower where the pipes of the venting systems for both the No. 1 and No. 2 reactors release their pressure. The readings confirm that the No. 1 reactor’s venting system performed as designed.

TEPCO believes that the pressure inside the containment vessel of the No. 2 reactor was not high enough to break the rupture disc when the workers opened the valves, thus the pressure never exited through the exhaust tower.

The company suspects the hydrogen explosion at the No. 3 reactor building caused some of the valves in the No. 2 reactor to close, causing the failure.

Asahi Shimbun