Letting off some steam
What the Japanese powerplant chiefs decided to do at this point is vent off some of the steam from the containment vessels in order to cool the interiors down. At this point the steam is not contaminated with any long-lived nasties, but it has been well soaked in neutrons and thus it contains quite a lot of very short-lived (half-life measured in seconds) radioactive materials such as Nitrogen-16. Within a minute of being released, such steam is just steam again, but it is radioactive when it comes out.
This is obviously emotive stuff – radioactive gas leaks – even if it is harmless to anyone beyond the plant fence (the workers inside are in protected control rooms or wearing protective gear).
So the situation is being managed and the cores are being kept cool by venting off steam. Power is restored by mobile generators to most of the reactors and soon their cooling systems are running again for a smooth shutdown.
But in two cases the normal cooling systems couldn't be made to run again even once mobile power arrived on scene. The normal systems use very pure de-mineralised water, and the plant operators couldn't get a supply of this running again at these reactors. Water adulterated with other things – such as sea salt – is less desirable, as its use means that other radionuclides are generated in small quantities: also it will cause a lot of expensive equipment corrosion and so forth.
But after some time, water levels inside the three cores sank low enough from the venting that hot bits of core started to stick up out of the liquid. These parts were then being kept cool much less effectively, and trace amounts of the caesium and iodine isotopes powering the residual heat reaction were detected in the air outside the plants. This first happened on Saturday.
The plant operators thus bit the bullet and fell back on yet another backup system: they injected seawater mixed with boric acid (liquid control-rod material) into the cores. This meant a fair bit of expensive damage to the two reactors, and also that the steam emitted when venting would be slightly more radioactive due to the salt and other trace chemicals in the sea water.
This is why the Japanese operators have chosen purposely to release the steam from these reactors, not into the atmosphere, but into the interiors of their reactor buildings. These too can be made gas-tight in order to contain leaks from the containment vessel, though they aren't terrifically strong and able to hold massive pressures.
The idea was to hold the steam in the buildings for the necessary short periods until it was no longer radioactive at all before letting it out of the building – and then venting off some more steam into the building, so cooling the cores. Holding the steam in the buildings wasn't really necessary – more of a gesture than anything else – but it was done nonetheless.
Unfortunately this decision has proved to be a PR blunder rather than a bonus. Steam which has been superheated as in a reactor core can break up into hydrogen and oxygen, which is naturally an explosive mixture. At Chernobyl, this actually happened inside the containment vessel and the resulting explosion ruptured the vessel, leading to a serious release of core radioactives – though this has had basically zero effect on the world in general nor even much impact on the area around Chernobyl.