ScribbleLive

edano, below the tepco narrative on unit 2 in the September update (chapter 2) from NISA to IAEA: "From 04:20 to 05:00, March 12, as water level of the Condensate Storage Tank (CST) decreased and also in order to control rising of the S/C water level, the water source for the RCIC was switched from the CST to the S/C for the RCIC to continue injecting water. The reactor water level remained stable at the level sufficiently above the TAF until 11:30, March 14. From that point to 13:25, March 14, the reactor water level began to drop, at which point the RCIC was judged to have shut down. The water level dropped to 0mm (TAF) at 16:20, the same day.
SRV opening and alternative water injection operations commenced at 16:34, March 14, and the seawater injection into the reactor using fire engines was started at 19:54, the same day.
With regard to PCV vent operations to reduce pressure in the PCV, at 06:50, March 12, TEPCO was ordered by the Minister of Economy, Trade and Industry in accordance with Article 64, Paragraph 3 of the Reactor Regulation Act to contain the PCV pressure. Based on this order, TEPCO began PCV vent operations, at around 11:00, March 13 and also at around 0:00, March 15, but a decrease in D/W pressure could not be verified.
At 13:18, March 14, the reactor water level became on the declining trend, and then at 13:25, March 14, RCIC was judged to be in function failure. Reaching TAF was expected around 16:30, March 14, thus the preparatory work for seawater injection was continued, then 14:43, March 14, the work for connecting fire engines to FP was completed. After that, due to the occurrences of aftershock centered in off the coast of Fukushima prefecture from past 15:00 to past 16:00, March 14, suspension of work as well as evacuation of staff were forced, however, around 16:30, March 14, with fire engines which became workable, the arrangement for starting water injection upon the reactor depressurization was completed.
For injecting water from fine engines, parallel lines were formed at Units 2 and 3. Unit2 needed reactor depressurization via opening of SRV, however due to high temperature and pressure of S/C (As of 12:30, March14、S/C temperature 149.3°C,S/C pressure 486kPa abs), even SRV was opened, vapor condensation as well as the depressurization of AC could be difficult. In the light of this, it was decided that after arrangement of PCV vent, SRV would be opened to depressurize the reactor, and then seawater would be injected." ...to be continued.

edano, here is more:
"At 8:10 on March 13, the PCV vent valve (MO valve) was made open 25% of the stipulated procedure. At 11:00 on March 13, the solenoid valve of the large S/C vent valve (AO valve) was energized by a small generator for temporary illumination of the Main Control Room to perform the opening operation. The vent line composition was complete except for the rupture disk. However, D/W pressure was lower than the rupture disk working pressure (427 kPa gage), the state of failure in venting was retained, and the monitoring of the D/W pressure was continued.
However, at 11:01 on March 14, explosion occurred in the Unit 3 reactor building, the circuit to energize the solenoid valve of the large S/C vent valve (AO valve) was off to be closed, so the vent line composition work was needed again. After the explosion, the workers except for the duty people of the Main Control Room evacuated in the quake isolation building after interrupting all the work. It was impossible to resume the restoration work because of confirming the safety of workers and the situation on the spot.
During this time, the D/W pressure was about 450 kPa abs to be kept stable under the pressure for venting.
After the evacuation direction subsequent to the explosion was lifted, at around 16:00 on March 14, the opening operation of the large S/C vent valve (AO valve) was tried; but, at around 16:20 on March 14, the opening operation was not successful because of insufficient air from an air compressor. Because no reduction in the D/W pressure was shown, at around 18:35 on March14, the vent line restoration work was continued for not only the large S/C vent valve (AO valve) but also the small S/C vent valve (AO valve). At around 21:00 on March 14, the small S/C vent valve (AO valve) was opened slightly; it was considered that the vent line composition was complete except for the rupture disk.
The D/W pressure was lower than the rupture disk working pressure (427 kPa gage) and the state of failure of venting continued for a while; but, at around 22:50 on March 14, the D/W pressure rose to exceed the maximum working pressure 427 kPa gage, so it was judged that the situation should fall under Article 15 of the Act on Special Measures Concerning Nuclear Emergency Preparedness (hereinafter referred to as the “Nuclear Emergency Act”) “unusual rise of the pressure in the PCV.”"

@Edano , I tell you the finale is anti-climactic.

I believe the cause is quite clear. Because they could not relieve the pressure from the wet well, the torus busted.

@lillymunster Zackly

Perhaps what may be unclear is the fact the rupture disk did not appear to give way. Therefore, there is an argument to be made that the pressure in the torus was not high enough to bust it. Yet it obviously did.

Speaking of the www.tepco.co.jp server, I just saw this on their Japanese index page (Google translate):

Private consultancy office for compensation for nuclear damage in general

Fukushima nuclear power compensation consultation room ■ (Call Center) ■
　Phone number 　0120-926-404 　Sorry wrong number please note ※.

heh...oops... that was going to be:

"it's beyond expectations that the rupture disk would fail, too."

@lillymunster Under pressure, the corium would only need to thin the wall, not necessarily rupture it.