(From Yomiuri Shinbun article of 10:47PM JST 4/18/2011)
Japan's Nuclear and Industrial Safety Agency (NISA) finally admits to what the rest of the world has pretty much known for the past 4 weeks, that the fuel pellets melted in the Reactor 1, 2, and 3, and that it has reported to the Nuclear Safety Commission.
NISA is yet to admit to a core meltdown.
NISA has insisted that "more than 3% of the fuel rods have been damaged", but never admitted that any melt of the fuel rods has taken place, until now.
According to Yomiuri, NISA has three levels to describe the fuel core damage:
"Core damage (炉心損傷)" refers to the situation where the cladding is damaged but the fuel rods keep their shapes. [This is what NISA and the Kan government have insisted on, up till now.]
"Pellet melt (燃料ペレットの溶融)" refers to the situation where part of the fuel pellets is melted.
"Core meltdown (炉心溶融)" refers to the situation where the melted fuel drops down.
The reason for NISA to now believe "the pellets have melted" is that "radioactive materials generated by the melted pellets have been found in high concentration" for the Reactors 2 and 3, and that "there was a hydrogen explosion" in the Reactor 1.
(I can't help laughing at The Australian writer who believes the Japanese authorities are on top of the "emergency" from the beginning.)