This cutaway diagram shows the central reactor vessel and thick concrete containment in a typical boiling water reactor of the same era as Fukushima Daiichi 1 (image:

Japanese responders continue to battle rising heat and pressure and falling water levels in the damaged reactors, Units 1 and 3, at Fukushima I (Daiichi) Nuclear Power Station.

But the big news is that Unit 2 lost cooling and the core was left uncovered, allowing a likely partial meltdown. There has not yet been an explosion at Unit 2, and they’re trying to relieve pressure to prevent that. More on that below.

And there is concern about the condition of spent fuel rods in pools located above the reactor. They too must be continuously cooled, but the cooling systems are also disabled.

The New York Times interviewed US industry and regulatory officials who had been briefed on the Japanese efforts and reports several interesting facts mentioned in FDL threads here but not previously summarized. Also note the photo at the top of the Times’ article, showing the damage to the Unit 3 reactor building from yesterday’s explosion.

– In addition to the reactors themselves, they’re worried about the condition of the spent fuel holding ponds, which are inside the reactor buildings. The spent fuel has ceased fission reactions, but residual radioactive decay continues and must be continuously cooled.

. . . there was deep concern that spent nuclear fuel that was kept in a “cooling pond” inside one of the plants had been exposed and begun letting off potentially deadly gamma radiation.

– The reason they not only lost the back-up generators when the tsunami hit, but can’t easily replace them with portable generators brought to the site is because the connection points, with the generators, were completely flooded by the tsunami.

[The tsunami] easily overcame the sea walls surrounding the Fukushima plant. It swamped the diesel generators, which were placed in a low-lying area, apparently because of misplaced confidence that the sea walls would protect them.

– The core in Daiichi Unit 1 suffered significant exposure when water levels fell:

While estimates vary, several officials and industry experts said Sunday that the top four to nine feet of the nuclear fuel in the core and control rods appear to have been exposed to the air — a condition that that can quickly lead to melting, and ultimately to full meltdown.

– Official reports of pressure readings inside the reactors are, as we’ve suspected, not necessarily reliable.

Workers inside the reactors saw that levels of coolant water were dropping. They did not know how severely. “The gauges that measure the water level don’t appear to be giving accurate readings,” one American official said.

– With all the normal/backup water cooling systems inoperable, the responders where attempting to inject sea water using fire-fighting equipment, but with limited success.

To pump in the water, the Japanese have apparently tried used firefighting equipment — hardly the usual procedure. But forcing the seawater inside the containment vessel has been difficult because the pressure in the vessel has become so great. . . . it was “not clear how much water they are getting in, or whether they are covering the cores.”

– The outer structure of the Daiichi units was reportedly designed to be blown away in an explosion, to relieve pressure but preserve the reactor vessel and containment structure inside.

The walls of the outer building blew apart, as they are designed to do, rather than allow a buildup of pressure that could damage the reactor vessel.

We’ll be updating as needed.

Update I: (h/t lobster) Last night (our time) the core at Unit 2 became uncovered when sea water cooling efforts failed for a time, so we’ve got another meltdown in the works:

Kyodo (22:15) reporting Unit 2 fuel rods were fully exposed for about 2.5 hours.

This would be Fukushima Daiichi Unit 2, where the fuel rods were completely exposed for a time when the fire equipment pumping sea water into the reactor ran out of fuel. (h/t lobster)

Fuel rods at the quake-hit Fukushima No. 1 nuclear power plant’s No. 2 reactor were fully exposed at one point after its cooling functions failed, the plant operator said Monday, indicating the critical situation of the reactor’s core beginning to melt due to overheating. . . .

The seawater injection operation started at 4:34 p.m., but water levels in the No. 2 reactor have since fallen sharply with only one out of five fire pumps working. The other four were feared to have been damaged by a blast that occurred in the morning at the nearby No. 3 reactor.

The utility firm said a hydrogen explosion at the nearby No. 3 reactor that occurred Monday morning may have caused a glitch in the cooling system of the No. 2 reactor.

. . . To prevent a possible hydrogen explosion at the No. 2 reactor, TEPCO said it will look into opening a hole in the wall of the building that houses the reactor to release hydrogen.

Apparently they only have one operable fire pump available to pump sea water at Daiichi, so they’re focused now on Unit 2, even though Units 1 and 2 also need sea water.

Update 2, 12:00 p.m. EDT: Reports now indicate that despite efforts to inject sea water into Unit 2′s reactor, the core became exposed again. Officials are now conceding that partial meltdowns of exposed fuel are likely occurring at all three units at Fukushima Daiichi.