With the nuclear meltdown, Tepco said the nuclear fuel rods in the reactor are completely exposed, as large amounts of radiation are being released. The water level at the plant's No. 1 reactor was much lower than thought - as much as 5 meters (16.4 feet) below the nuclear rods - and clearly not high enough to cover the nuclear fuel.
According to reports, several holes were found at the bottom of the nuclear reactor's pressure vessel, where the melted nuclear fuel now threatens to leak out.
On a daily basis, Tepco injects almost 200 tons of water into the pressure vessel, but it is highly likely that the water has been constantly leaking from the vessel and containment chamber, eventually flowing under the reactor building.
Reuters' report on the admission by TEPCO notes that the disclosure "is the latest indication that the disaster was worse than previously disclosed, making it more difficult to stabilize the plant."
They go on to report that "U.S. nuclear experts said that the company may have to build a concrete wall around the unit because of the breach, and that this could now take years."
But it suspects the meltdown created a hole or holes in the bottom of the reactor causing water to leak into the containment vessel.
It also suspects the water is leaking into the reactor building.
UPDATE 5/13/11: We should make clear that the "meltdown" in question did not just happen yesterday. Rather, it appears to have occured in March after the quake and tsunami, but is only now being admitted to by TEPCO --- just in case that point was not clear from the initial reports above, as some have suggested in comments. Here are a few additional details now from the Wall Street Journal's coverage...