[Update Friday a.m., Japan: via NKH World, based on the essential facts below, TEPCO now describes the reactor core in Unit 1 as in "meltdown."
The utility company said on Thursday that most of the fuel rods are likely to have melted and fallen to the bottom of the reactor. Earlier in the day, it found that the coolant water in the reactor is at a level which would completely expose nuclear fuel rods if they were in their normal position.
The company believes the melted fuel has cooled down, judging from the reactor's surface temperature. ]
Ever since the Japan earthquake and tsunami disabled the cooling systems at Units 1-4 of the Fukushima Daiichi Nuclear plant, each of those units has been suffering significant leaks of radioactive water through various but mostly unconfirmed sources. Those leaks could be in the cooling system pipes, damaged valves and pumps, or even related condenser equipment in adjacent turbine buildings.
More ominous would be leaks from the containment structure or pressure suppression pool, and worst of all would be leaks directly from the reactor vessel that holds the damaged, partially melted fuel core.
News reports this morning are now confirming there is a major leak from a hole, likely “several centimeters” in diameter, in the reactor vessel at Unit 1. The utility, TEPCO, discovered the leak/hole in the reactor vessel after making repairs on a related gauge. From Reuters:
Workers at the Fukushima Daiichi nuclear plant have been pumping water into four of the six reactors on the site to bring their nuclear fuel rods to a “cold shutdown” state by January.
But after repairing a gauge in the No. 1 reactor earlier this week, Tokyo Electric Power Co discovered that the water level in the pressure vessel that contains its uranium fuel rods had dropped about 5 meters (16 ft) below the targeted level to cover the fuel under normal operating conditions.
“There must be a large leak,” Junichi Matsumoto, a general manager at the utility also known as TEPCO, told a news conference.
The Reuters article is not clear about the location of this hole, and authorities may not know. We have reported previously that one theory about the continuing leaks from the Unit 1-3 reactors noted leaks could arise from damage to the seals for one or more of the water injection or steam withdrawal pipes connected directly to the reactor vessel. These occur at various levels on the vessel, relative to the height of the fuel rods. One set, for example, would be about half way down the level of fuel rods, and if a leak at that point were large, it would effectively preclude the operators from maintaining water above that level, leaving the fuel rods partially exposed. In theory, they might be able to inject water faster than it leaked out, but at the time, their makeshift pumping equipment did not have that capacity.
A worse scenario would be that the “hole” was created by melting fuel either damaging the vessel as it fell to the bottom, or worse, burning through the bottom of the pressure vessel. That could mean that both contaminated water and melted fuel could fall through onto the containment structure floor. What Reuters reports, in quoting local officials, doesn’t tell us the answer:
“The fuel pellets likely melted and fell, and in the process may have damaged … the pressure vessel itself and created a hole,” he added.
Since the surface temperature of the pressure vessel has been holding steady between 100 and 120 degrees Celsius, Matsumoto said the effort to cool the melted uranium fuel by pumping in water was working and would continue.
Based on the amount of water that is remaining around the partially melted and collapsed fuel, Matsumoto estimated that the pressure vessel had developed a hole of several centimeters in diameter.
However, this report from NHK World suggest the “hole” is below the bottom of the fuel rods, even though TEPCO doesn’t believe the fuel has melted through the reactor vessel:
The utility had suspected the gauge wasn’t working properly because the water level hasn’t been rising despite pumping in 150 tons of water daily to cool the reactor.
On Thursday morning, it was found that the water level was more than one meter below the bottom of the fuel rods, suggesting a large volume of water is leaking into the containment vessel.
The utility company also believes that the water is leaking from the containment vessel into the reactor building. This is because the estimated volume of water inside the containment vessel appears to be less than what leaked into it from the reactor.
And there’s this from Japan Times:
According to Tepco, the water-level indicators of the pressure vessel had indicated the water surface was about 1.65 meters below the top of the fuel rods. But as of Thursday morning the reading was more than 5 meters below the top. The fuel rods, if undamaged, are only 4.5 meters in height.
[Note: When Japan sources refer to the "contaiment vessel," they mean what I've been calling the "containment structure" that encloses the reactor vessel. See diagram above. They don't mean the reactor vessel that holds the core.]
Since no one can get inside the containment structure or close to the vessel to inspect it, they can only infer how large the hole is based on water injection and leakage rates and temperatures. At this point, it’s not clear whether they’ve just discovered a condition that has been there since the first days, when the core was uncovered and the melting occurred, or whether this is a later hole that is being steadily enlarged by the melted fuel. Either way, it means that keeping the remaining fuel covered is a serious challenge.
Update: the Wall Street Journal has more details confirming the basics of the story.