Tokyo Electric Power Company now admits that meltdown occurred, at least with Reactor 1, on 11 March.  They base their claims on their most recent data.

That means high levels of toxic radiation have indeed been spewing from the plant for at least two months now.  There is concern that fuel rods in reactors 2 and 3 have also melted down.  TEPCo admits it could be the cause of the massive leaks of highly contaminated water coming from the the buildings.

The latest check of cesium levels in the ocean, show that Reactor 3 is pouring even more cesium into the ocean, than it was a few days ago.

TEPCo says that after the 9.0 quake, and then the tsunami, it took too long for back up power to start pumping coolant through the system; the meltdown had already occurred by 06:50 on 12 March.  The melted fuel rods were starting to crack the vessel when the coolant hit, which seems to have stopped, or slowed the melting process.  But the reactor vessels are now damaged enough to cause the water being pumped in to simply leak out, almost as fast as they can pump in the water.  Of course the water became highly contaminated.