HAYAO said:
Oh yes, of course I know that. Only except it was after the meltdown happened.
If one looks at this post in the thread on the accident, one can find a good reference about the accident.
https://www.physicsforums.com/threa...ear-plants-part-2.711577/page-61#post-5730486
In the IAEA report, Pub1710 (IAEA Report by the Director General) page 34 (47 of 222) in report:
"Status of core cooling in Units 1 and 2
Just before the tsunami struck, the Unit 1 isolation condenser was stopped by the operators in
accordance with established operating procedures to control the reactor cooling rate. This was
accomplished by closing the valves (located outside the primary containment vessel and DC operated,
as shown in Box 2.2, p. 26 (39 of 222 in pdf)). About 2.5 hours after the loss of indications, at 18:18 on 11 March, some of the
status lamps for those valves were found to be functioning, confirming that the control valves were
closed. The operators attempted to start the isolation condenser by opening those valves. However, the
isolation condenser did not function, indicating that the AC powered isolation valves inside the
primary containment vessel were closed.
Thus, the fundamental safety function of core cooling at
Unit 1 was lost when the isolation condenser was stopped by the operators just before the tsunami,
and the Unit 1 core heated up from that time.
Additionally, local measurements (in the reactor building) at 20:07 indicated that the reactor was still
near the operating pressure of 70 bar (7 MPa), which prevented water injection by alternative methods
that would only be possible below 8 bar (0.8 MPa)."
The problem with Unit 1 was the loss of power. With various power-operated valves inoperable, the safety systems, including ECCS could not function properly. Once some valves were shut, with loss of power they could not be reopened. Attempting to pump seawater into Unit 1 would not be possible due to the high pressure within the primary system.
Footnotes on the page provide some additional insight.
The fire protection system was designed primarily for fire suppression and flooding of the containment vessel, not for injection of water into the reactor.
Cross-tie lines had been installed at the Fukushima Daiichi NPP nearly a decade earlier as a design enhancement for accident management. Sharing the functioning emergency power of Unit 6 was only possible for Unit 5, since these interconnections had been installed only between pairs of units, i.e. Units 1 and 2, Units 3 and 4, and Units 5 and 6.
The valve positions were not clear to the operators owing to the uncertain timing and sequence of each type of power loss that would determine the status of isolation valves. All the isolation condenser valves would keep their position when the AC.I don't know if the isolation valves were ever opened on Unit 1, which would have happened too late after the core was damaged anyway. Certainly, the company failed to adequately assure the design of the plant to prevent inundation by a tsunami of the magnitude that struck the plant. The auxiliary building and balance of plant were vulnerable. The designs of Units 5 and 6 were apparently adequate.
Figure 2.2 (page 29 (42 of 222 in pdf) shows the initial responses by the plant operators after the earthquake followed by the first tsunami.
Figure 2.5 (page 45 (58 of 222 in pdf) shows events after the 14.5 m tsunami arrives.
Clearly the utility failed with respect to the General Design Criteria. A 15 or even a 10 m tsunami should have been part of the design basis, since there were records of such tsunamis in the region in the past.