After the pressure inside the Containment Vessel of the Reactor 1 dropped down very, very close to the atmospheric pressure, TEPCO decided to stop the experiment of pumping extra water into the Pressure Vessel of the Reactor 1.
The idea was to pump extra water into the Pressure Vessel, which then would leak extra water into the Containment Vessel, which then would be filled with water to the level that's high enough to cool the fuels inside the Pressure Vessel.
For now, the idea of turning the "unintended" water entombment into the "intended" is out.
Snippets from Asahi Shinbun (9:58PM JST 4/29/2011):
On April 29 TEPCO reduced the amount of water being injected to the Reactor 1 [Pressure Vessel] from 10 tonnes/hr back to 6 tonnes/hr for the "water entombment" experiment to fill the Containment Vessel with water, because the temperature and the pressure dropped [more than expected]. TEPCO will continue to observe the changes.
The plan, initially, was to increase the amount of water to 14 tonnes/hr. However, the temperature and the pressure [of the Containment Vessel] dropped more than expected. The temperature of 132 degrees Celsius and the pressure of 1.5 atmospheric pressure before the experiment dropped to 113 degrees Celsius and 1.1 atmospheric pressure at 5:00AM on April 29. The steam inside the Containment Vessel is thought to have turned into water with the injection of cold water, lowering the pressure.
圧力が１気圧を下回ると、酸素を含む外気が吸い込まれ、格納容器内にたまった水素が爆発を起こしかねない。爆発を防ぐための窒素の注入も進めているため 「水素の濃度は１％未満とみられ、爆発のリスクはさほど高くない」とはいうものの、４８時間が過ぎた２９日午前１０時すぎの段階で、注水量を元に戻すこと にした。
If the pressure drops below 1 atmospheric pressure, the outside air that contains oxygen may get sucked inside the Containment Vessel, and cause a hydrogen explosion. As nitrogen gas is being injected, TEPCO thinks "the concentration of hydrogen is less than 1%, and the risk of explosion is not that high." Still, TEPCO decided to reduce the amount of water back to 6 tonnes/hr at 10:00AM on April 29, 48 hours after the start of the experiment.
It was good that the temperature dropped, but the pressure drop was the Murphy..
Here are the precise numbers for the Containment Vessel dry well pressure (from the Plant Parameter sheets from NISA, the latest is linked). It did come awfully close to the atmospheric pressure:
1 atmospheric pressure = 0.1013 MPa
April 29 11:00AM: 0.105 MPa
April 28 5:00AM: 0.125 MPa
April 27 5:00AM: 0.155 MPa (when the experiment started)