The Japanese media (members of Japan Press Club) can still view the video of TEPCO's teleconferences in the early days of the nuclear accident that TEPCO was willing to disclose (about 50 hours worth of video), and here's the latest from Tokyo Shinbun.
According to the newspaper, one of the reasons why TEPCO workers couldn't inject water to cool the reactor core, or open the vent line to release the pressure inside the Reactor Pressure Vessels in Reactors 2 and 3 much sooner was because the rupture disks didn't break because the set pressure was too high.
Why was the set pressure too high? Because the Japanese engineers were afraid of radioactive leak from the Primary Containment Vessels and wanted avoid the leak at all costs, says the paper quoting a nuclear researcher.
Much like TEPCO allegedly welded shut the blowout panels in each reactor building at Fukushima I Nuke Plant, except for Reactor 2.
From Tokyo Shinbun (8/29/2012):
福島第一事故 安全装置 ベント妨げる
Safety equipment prevented the vent in the Fukushima I Nuclear Power Plant accident
It was found in the teleconference video of TEPCO that the bad design of the safety equipment installed along the vent pipe delayed the response when they tried to vent in order to inject water into the reactors in the early stage of the Fukushima I Nuclear Power Plant accident in March last year. It can be said that the safety equipment designed to contain radiation ended up worsening the accident.
During the accident, the equipment to inject water at a high pressure failed in both Reactor 2 and Reactor 3. TEPCO tried to inject water using fire engines, but had difficulty doing so due to high pressure inside the reactor (Pressure Vessel).
In order to lower the pressure, they needed to open the SR valve to release [the pressure inside the Reactor Pressure Vessel) into the Containment Vessel, and at the same time do the vent. However, the rupture disk - a stainless-steel disk installed in the vent pipe - hindered the vent.
The rupture disk plugs the pipe, and it is designed not to break unless it is under certain pressure. The purpose is to prevent the leak of radioactive materials when the vent valve is accidentally opened.
However, the disk's set pressure was too high, and even when they wanted to lower the reactor pressure and vent so that they could inject water as soon as possible, the disk didn't break, delaying the response and causing the overheating of the nuclear fuel.
The teleconferencing video shows the frustrated staff at the plant. "Reactor core may be damaged before the vent can be done" (5AM on March 13, regarding Reactor 3), and "We're waiting for the rupture disk to open" (10PM on March 14, regarding Reactor 2).
Koichi Miyata, Manager of TEPCO Nuclear Safety Group, reflects, "The hardest time was when we couldn't vent when we wanted to."
As one of the lessons learned from the accident, the Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry is of the opinion that a revision is necessary so that the rupture disk doesn't get in the way of venting. The agency is to ask the [soon-to-be-created] Nuclear Regulatory Commission to devise countermeasures.
Professor Tadashi Narabayashi of Hokkaido University (nuclear engineering) says, "Some European countries have systems to bypass the rupture disks so that the vent can be carried out even when the rupture disks don't break. So far, Japan has been caught in the trap of "no leak [of radioactive materials] from the Containment Vessel", which is reflected in the design".
According to the report by the Fukushima Accident Independent Investigation Commission set up by the Cabinet Office, the set pressure of the rupture disk was "0.528MPa abs" for Reactor 2.
The same NISA and Professor Narabayashi (one of the three "Plutonium Brothers") were last heard together in December last year, when NISA officials admitted to the possibility that the very act of venting may have caused the hydrogen explosions in Reactor 1 and Reactor 3.
Further, a Tokyo University researcher (Katsumi Shozugawa) indicated in his paper (link is in Japanese; see the chart in the post) published earlier this year (online in January, published in paper in April) that it may have been the dry vent (= vent directly from the Containment Vessel, without going through the Suppression Chamber) of Reactor 2 on March 15 that released the huge amount of radioactive materials in the surrounding environment, not the explosions of Reactors 1 and 3.
Several days ago, Professor Yukio Hayakawa, speaking in front of a study group of politicians, also said that TEPCO did the vent, and NISA allowed the vent, on March 15, 2011, when it was obvious to them the wind was from the north, blowing toward Tokyo. Professor Hayakawa continued, "Was the vent necessary at that time? I don't know, as I'm not a nuclear engineer. But if the contamination is from the vent, this is a man-made contamination." (Video of the meeting is here, if you understand Japanese.)
To be sure, the wide-area contamination of Kanto region is supposed to have happened on March 20 - 22, 2011, with the rain. At that time, Reactor 3 building was smouldering, uncontrolled, white smoke then black smoke seen issuing from the wrecked operating floor. But if Reactor 3 building exploded because of the vent, the possibility of which NISA admits, it would be indeed man-made contamination as Hayakawa says.