Tokyo Electric Power Co. said a "major part" of the fuel rods in reactor No. 2 may have melted and fallen to the bottom of the pressure vessel 101 hours after the earthquake and tsunami that crippled the plant.
The same thing happened within the first 60 hours at reactor No. 3, the company said, releasing a worst-case scenario analysis.
The fuel is believed to be sitting at the bottom of the pressure vessel in each reactor building.
Tokyo Electric also released a second possible scenario for reactors 2 and 3, one that estimates a full meltdown did not occur. In that scenario, the water inside the reactors stayed at a higher level. Tepco estimates the fuel rods may have also broken in this second scenario, but may not have completely melted.
Temperature data shows the two reactors have cooled sufficiently in the more than two months since the incident, Tokyo Electric said.
A March 11 earthquake and tsunami knocked out cooling systems at Fukushima Daiichi, causing the three operating reactors to overheat. That compounded a natural disaster by spewing vast quantities of radioactive material into the atmosphere.
Tokyo Electric has already reported that damage to the No. 1 reactor was more extensive than previously believed. The company says the fuel rods at the heart of the reactor melted almost completely in the first 16 hours after the disaster struck, the remnants of that core are now sitting in the bottom of the reactor pressure vessel at the heart of the unit and that vessel is now believed to be leaking.
Tokyo Electric has avoided using the term "meltdown," and says it is keeping the remnants of the core cool. But U.S. experts interviewed by CNN say that while they may be containing the situation, the damage has already been done.
"On the basis of what they showed, if there's not fuel left in the core, I don't know what it is other than a complete meltdown," Gary Was, a University of Michigan nuclear engineering professor and CNN consultant, said this month. And given the damage reported at the other units, "It's hard to imagine the scenarios can differ that much for those reactors."
A massive hydrogen explosion -- a symptom of the reactor's overheating -- blew the roof off the No. 1 reactor unit the day after the earthquake, and another hydrogen blast ripped apart the No. 3 reactor building two days later. A suspected hydrogen detonation within the No. 2 reactor is believed to have damaged that unit on May 15.
In Unit 1, the remnants of the fuel rods are going to be far cooler than they were at the worst of the accident, more than two months ago. That mass likely resembles "a pile of gravel, with a central portion of it being liquid or close to liquid," said Kenneth Bergeron, a physicist and former staffer at the Sandia National Laboratories in New Mexico.
"What's going to be true is that portions of the debris bed will be at different states, different temperatures, and some of it will be liquid," Bergeron said. The liquid portion still has the potential to burn through the bottom of the thick steel pressure vessel, which he said is already likely to be leaking through damaged seals around the machinery that drives the reactor control rods.
Jack DeVine, who helped lead the cleanup of the Three Mile Island nuclear plant after the 1979 partial meltdown there, said the pressure vessel itself was "probably fine," but pipes and valves that lead into it are "softer spots" that are probably the source of the leak.
CNN
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