April 15 link dump... fragments at bottom of vessel

http://www.iaea.org/newscenter/news/tsunamiupdate01.html
http://www.nytimes.com/interactive/2011/03/16/world/asia/reactors-status.html
http://mitnse.com/
http://allthingsnuclear.org/
http://www.japantimes.co.jp/news/emergency-assistance.html
http://www.greenpeace.org/usa/en/campaigns/nuclear/Fukushima-nuclear-disaster/
http://www.guardian.co.uk/world/japan-earthquake-and-tsunami

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

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.

The temperature of the fragments is kept low ... (but).. 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.