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 authorities now reporting that about 6:14 a.m. (Tokyo) Tuesday, March 15, there was an explosion at the Daiichi Unit 2 of the Fukushima Nuclear Station. This explosion was heard, not seen from the outside. The explosion reportedly did not blow off the roof/walls, as the explosions did at Units 1 and 3.

The explosion reportedly occurred near the containment area. Plant officials fear there may now be a crack in the reactor containment, which would allow more serious releases of radiation. A “pressure suppression pool,” the doughnut-shaped area at the bottom of the reactor vessel may have been damaged, which officials are describing as “serious.”

They are evacuating non-essential personnel in/around the plant after initial outside readings reached 965 micro-Sievers/hour. Radiation levels spiked to 8,217 micro-SV/hour, before dropping. That’s “more than eight times the 1,000 micro sievert level to which people are usually exposed in one year.” Winds are reported from the NNW.

At the time of the explosion, about one half of the reactor core — about 2.7 meters — had become uncovered. According to one analyst, at some point, the entire core was exposed. Pressure reached 3 atmospheres, but has fallen back to 1.

These pressure and radiation readings suggest the reactor pressure vessel holding the core may have been breached.

There is also an inoperable value that would otherwise allow pressure releases. That’s preventing or limiting the ability to inject cooling water.

An official is describing the event on this live tv feed, with English translation.

In a follow up news conference by TEPCO . . . utility officials said they’re continuing to inject sea water, with only a few essential personnel. They’re apologizing to the public. Reporters keep asking for details of the incident, and the officials keep apologizing!

Reporter: “we’re not asking for your feelings! Tell us the facts.”

Response: “The blast was heard; we checked parameters; pressure readings indicated some damage to the pressure pool. That caused the evacuation of the operators. Being carried out for the first time.”

“Water level was at minus 2700 mm [?} before and after; we're trying to decide what that means."

Q. What caused the damage to the suppression pool?
A. We have only confirmed the pressure went down. The Cabinet official assumed there was damage.
[Reporters obviously frustrated]
Q. Can anyone explain this? Have Unit 1 and 3 been evacuated?
A. Only those necessary are there.

Update from 11:00 a.m. (Tokyo) presser from Cabinet Official:

As of 6:00 a.m. Tuesday, all but 50 involved in water injections have been evacuated. Injections are occurring at Units 1, 2, and 3. There was a fire earlier at Unit 4, which was in cold shutdown when the quake occurred.)

Evacuations directed around 30 km radius. Everyone else urged to remain indoors.

Cabinet meetings continuing.

[note: these are paraphrases of Q and A]
Q. How are you sure the pressure vessel not been damaged?
A. Water injection has been maintained. And pressure has been stabilized; we have to continue those efforts.
Q. How serious is fire at Unit 4?
A. I mentioned it first because it occurred earlier.
Q. Is there damage to container vessel?
A. Report there is a high probability that it was.
Q. Radiation levels?
A. Very little possiblity of harm to public
Q. Status of fire at Unit 4?
A. Still working to extinguish the fire at Unit 4.
Q. Evacuation radius?
A. Out to 20 km = evacuate. Considering out to 30 km; but for now, just stay indoors.
Q. Radiation readings related to damage to Unit 2
A. Explosion at #4 reactor could have caused that [explosion?]
Q. What if fire continues?
A. We’re making every effort to put it out. The spent fire is not going to “catch fire” in general sense. Fire is in building area; but it’s best to extinguish to keep temps down and prevent release or radiation.
Q. Release possible.
A. Possible some could have been released. Ask TEPCO. But current high readings are from this fire, not from the Unit 2.
Q. What is your advice?
A. [Repeats evac and remain indoors] Minimal amounts may spread further, but further away the level if lower, so stay calm.
In another Onadaga plant, the levels will not cause damage to health.
A. For people between 20-30 km range, there are towns that overlap/inbeetween. He calls them out. [About 8 towns]
Q. Sec. Gen of IAEA says you asked for experts?
A. Not aware of any Japanese request t IAEA, but after quake, we’ve asked for support from various countries.
Q. Neutron radiation?
A. That was from Unit 3, because of its nature [fuel type]
Q. How long to extinguish Unit 4 fire, given you think it’s source of radiation?
A. Working on it.
Q. Continue to inject water?
A. Yes. Thank you for your questions.

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