compléments:

http://english.chosun.com/site/data/html_dir/2011/05/17/2011051701110.html

The number one reactor, it is now acknowledged, suffered a meltdown soon after the March 11 earthquake and tsunami devastated northeastern Japan.

Japanese experts say the fuel rods inside the reactor were fully exposed to the air and melted. However, the fuel apparently dropped to the bottom of the containment vessel, preventing it from going into a full meltdown stage.
Recent attempts to keep the reactor cool by filling the containment chamber with water have run into difficulty. The power company, known as TEPCO, says thousands of tons of highly radioactive contaminated water have leaked through holes created by melted fuel into the reactor basement.