May 21, 2015

TEPCO robot finds Fukushima No. 2 reactor pressure venting system failure

http://ajw.asahi.com/article/0311disaster/fukushima/AJ201505210037

By MASANOBU HIGASHIYAMA/ Staff Writer

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.

Faulty part may have caused Fukushima nuke plant venting failure

http://mainichi.jp/english/english/newsselect/news/20150520p2a00m0na011000c.html

Venting of the No. 2 reactor at the Fukushima No. 1 Nuclear Power Plant may have failed due to a part inside the pipes failing to operate, it was learned from the results of an investigation by Tokyo Electric Power Co. (TEPCO) on May 19.

During the March 2011 disaster at the Fukushima plant, the No. 2 reactor no longer had power for its cooling system and pressure inside it rose to abnormally high levels. Plant operator TEPCO attempted to vent the reactor to lower the pressure, but the venting did not go smoothly. The company has been investigating what went wrong with the venting in order to clear up the cause of the disaster.

The part that may have failed to operate was a "rupture disk," located inside a pipe leading from the nuclear reactor's containment vessel to an exhaust pipe. Meant to keep radiation from leaking outside, the disk is supposed to break after a certain level of pressure is applied to it. However, in its investigation TEPCO found no evidence of radioactive contamination inside the pipe around the rupture disk, suggesting that the disk did not rupture and the vented gas was trapped in front of it.

Radioactive gas is thought to have leaked directly from the No. 2 reactor's containment vessel, and it is estimated as having leaked the most radiation out of the No. 1 through 3 reactors.

More Fukushima disaster findings revealed

http://www3.nhk.or.jp/nhkworld/english/news/20150520_20.html

May 20, 2015 - Updated 07:22 UTC+2

Tokyo Electric Power Company says it is highly likely it had failed to vent a vessel containing one of the reactors at its Fukushima Daiichi plant in the days after the March 2011 disaster.

TEPCO attempted to vent the Number 2 reactor container to release water vapor and radioactive materials and prevent it from being damaged.

The reactor suffered damage 4 days after the accident, as the pressure inside did not drop. The damage is thought to have caused the release of a massive amount of radioactive materials, forcing most plant workers to temporarily evacuate.

TEPCO has been investigating the accident, focusing on dozens of points that had remained unconfirmed.
The finding on the venting of the Number 2 reactor is one of the 4 points the utility revealed on Wednesday.

TEPCO now says the venting procedure may have failed because in its investigation no high levels of radiation were detected in piping connected to a device called a “rupture disk". Water vapor and radioactive materials were to pass through the disk after venting.

This was the first time for the likelihood of the venting having failed to be substantiated by the actual situation at the site.

TEPCO says it will continue to investigate how the radioactive release occurred at the Number 2 reactor.
It also says it will continue on-site investigations and find out the details of the situation more closely, and use the findings to secure the safe decommissioning of the reactors.