Second core damage and major radiation discharge occurred in reactor1&3 from 3/20 to 3/23/2011
[link to fukushima-diary.com]

[snip]

In the afternoon of 3/20/2011, the temperature around RPV reactor1 was nearly 400℃. In the morning of 3/22/2011, they reported reactor1 was boiling with no water again, the core damage was on-going.