Radiation said leaking from holes in bottom of container vessel...
By Brad Friedman on 5/12/2011, 4:36pm PT  

From BNO News tonight:

The Tokyo Electric Power Co. (Tepco), which operates the troubled Fukushima nuclear plant, on Thursday admitted that a nuclear meltdown occurred at the plant.

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."

NHK World adds:

The company believes the melted fuel has cooled down, judging from the reactor's surface temperature.

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...

Previously, Tepco officials had said they believed there had been "damage" to the fuel rods but didn't specify what that meant. On Thursday, for the first time, officials conceded that the fuel rods likely had "melted," crumbled or changed shape, and that the fuel had probably fallen from its casings.

The nuclear industry lacks a technical definition for a full meltdown, but the term is generally understood to mean that radioactive fuel has breached containment measures, resulting in a massive release of fuel.

Tepco engineers estimate that 90% of the fuel is still in the inner pressure vessel and that there are no cracks or obvious ruptures to the outer containment vessel, where Mr. Matsumoto said the rest of the fuel is likely contained. So the risk of a large radioactive release of fuel is minimal, he said.
Soon after the March 11 quake and tsunami knocked out the plant's cooling systems, temperatures in the No. 1 reactor likely rose to more than 2,000 degrees Celsius, experts have said, well above the point at which the metal casings of the fuel rods would begin to melt. The fuel pellets inside would start melting at 2,800 degrees, potentially fusing into a dangerous large mass. Tepco estimates the fuel rods in the No. 1 reactor have been 55% destroyed, making it the worst-damaged of the plant's six reactors.
The tops of the four-meter-long fuel rods reach a little more than nine meters from the bottom of the pressure vessel; past data suggested the water was eight meters deep, enough to mostly submerge them. When workers entered the reactor building this week and corrected the water gauge, it told them the depth was just four meters.

If the rods were intact, that would leave them dry. But Mr. Matsumoto said he believes that some of the fuel slid down and is likely sitting in water.

At the same time, we should also point to the report yesterday, coincidentally, from the U.S. Nuclear Regulatory Commission finding "serious problems" in the backup diesel generator cooling systems at nuclear plants in this country.