What use the pumps may be after the meltdown is still debatable...
If they still pump liquids they are VERY important. Any Watt of heat removed is one Watt that brings you away from a Europe Syndrome. Even of the fuel assembly gets completed destroyed, as long as you contain most damage inside the pressure vessel, you are still saving many lives.
Once the primary containment is open but the pressure vessel is intact, you have a bad situation, but are still able to prevent big damage.
If the pressure vessel bursts open, or just the primary circuit, you have a very very very bad situation.
---------- Post added at 12:40 PM ---------- Previous post was at 12:35 PM ----------
This graphic here shows the layout of a older generation of such reactors:
The torus below is likely in this Japanese reactor integrated into the primary containment structure as suppression pool and not outside it, but the part above the pressure vessel should be similar.
---------- Post added at 12:45 PM ---------- Previous post was at 12:40 PM ----------
The Japanese government now officially speaks of a meltdown.
Well, it is pretty hard to deny it now, after the symptoms of one had been already visible for hours.
Mystery:
One German expert claimed in TV (N24) that the explosion happened while workers tried to cast a column into concrete.
Not sure why they did they and how reliable this information is, but it is possible that either they tried to reinforce the containment or the building was internally much worse damaged by the Earthquake as known.