This satellite picture shows Reactor 3 just after it exploded. There is steam venting clearly visible. This venting is almost certainly not a design feature. It is coming from an area which has been totally destroyed by the blast. I suspect it shows a breach in containment which was caused by the explosion. Various reports on the web indicate high radiation levels near Reactor 3.
There have been many reports of intentional venting of the containment. When the plant does that, the gas is passed through filters in order to remove as much radiation as possible. Then it is emitted from a tall smokestack.
AN EVEN BIGGER PROBLEM?
Somewhere in the radioactive ruins of Reactor 3 there is a swimming pool like tank called a spent fuel pond. It has been reported that there is some fuel in the pond. Such ponds often contain many reactors worth of radiation.
Heat from the spent fuel evaporates the water, and under normal circumstances several percent a day of the volume would be lost. If the explosion has damaged the tank and it is leaking then levels could fall more quickly. If all the water were to be lost, then the fuel rods would overheat and probably catch fire. That would lead to a Chernobyl style release of radiation.
It is essential that the water in this pool is kept topped up, but how will this be done? The pool is high up in the reactor structure, in an area subject to intense radiation. Normal access routes to the pool may be impassable. Pipes feeding the pool may have been destroyed in the explosion.
Here is the question. Do the Japanese know what the water level is, or were the instrument cables destroyed in the explosion? Do they have the ability to top up the water level?
If not, then Japan has a very big problem.
A LITTLE GOOD NEWS
Latest reports are that the fire in Reactor 4 had nothing to do with spent fuel. It is now said that an oil leak was to blame.
UPDATE: There are conflicting reports on the fire.
UPDATE: Latest photographs make it clear that Reactor 4 has suffered a massive explosion. The damage is almost as bad as Reactor 3. I wonder if hydrogen from #3 somehow got into #4, perhaps via the ventilation system?