Melted fuel rod fragments have sunk to the bottoms of three reactors at the Fukushima No. 1 power plant and could theoretically burn through the pressure vessels if emergency water-pumping operations are seriously disrupted, the Atomic Energy Society of Japan said Friday.

If too many of the melted fuel fragments puddle at the bottom, they can generate enough concentrated heat to bore a hole in the pressure vessel, which would result in a massive radioactive release to the environment..

via Fuel rod fragments at bottom of vessels | The Japan Times Online.

I’m not sure about the accuracy of that last sentence, because of what Dr. Joseph Oehmen  (a research scientist at MIT, in Boston. He is a PhD Scientist, whose father has extensive experience in Germany’s nuclear industry) wrote:

The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure.  This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment.

Of course, what does he know? some might think. He’s not there on the ground in Fukushima. Plus he’s relying on design plans and on info from TEPCO, and how trustworthy are they? Also, he wrote the above in March, a few days after the quake. Also, he’s probably pro-nuclear power.

BNC is also pro-nuclear power, as is IAEA and World Nuclear News and the Nuclear Energy Institue. But I trust these sites more than the newspapers, and I will be visiting them regularly to get informed updates and comments on the situation.

The IAEA update IAEA Briefing on Fukushima Nuclear Accident (15 April 2011, 14:30 UTC) includes this report:

The 141st Session of the FAO Council was briefed by representatives of the Joint FAO/IAEA Division on the nuclear emergency in Japan on Friday, 15 April, at the FAO Headquarters in Rome, Italy. The briefing included a general background summary of the emergency, the current situation in Japan, FAO/IAEA/WHO responses and actions taken to date, and future challenges. The Member States expressed their appreciation for the IAEA/FAO/WHO inter-agency collaboration and coordination during the Japanese nuclear emergency and called for strengthening cooperation in future remediation actions.