Workers braved the high radiation (up to 100 millisievert/hour) in the Reactor 3 reactor building on June 9 and found water in the basement. TEPCO says it expected to find water there, but didn't bother to tell us until the water was actually found.
Have I seen this before? Yes, the Reactor 1.
To state the obvious, if there's water in the basement of the reactor building, that means 1) the Reactor Pressure Vessel (RPV) is broken; 2) the Containment Vessel is broken.
The only question is: Where's the corium?
From Asahi Shinbun (10:45PM JST 6/10/2011):
東京電力は１０日、福島第一原発３号機の原子炉建屋地下でたまり水を確認したと発表した。東電は原子炉を冷却する注水量などから、５月末現在で６４００トンの水がたまっていると推計していたが、実際に存在が確認された。
TEPCO announced on June 10 that they confirmed the presence of water in the basement of the Reactor 3 reactor building at Fukushima I Nuclear Power Plant. TEPCO had estimated that 6,400 tons of water would be in the basement as of the end of May, based on the amount of water injected into the RPV. Now its existence has been confirmed.東電によると、９日に作業員が原子炉建屋内に入り、１階から地下に行く階段を下りて、水がたまっているのを確認した。地下の床から５．８メートルのところまで水が来ていたという。量は東電の推計よりやや多い模様。水の近くの放射線量は毎時５１ミリシーベルトだった。
According to TEPCO, workers entered the reactor building and descended the stairs from the 1st floor to the basement, and saw the water. Depth of the water was 5.8 meters [19 feet] from the basement floor, and the amount of water looked to be slightly more than TEPCO's estimate. The radiation level near the surface of the water was 51 millisieverts/hour.
１号機同様、炉心溶融によって原子炉圧力容器が破損し、水が外に漏れ出している可能性があるという。近く、水を採取して放射性物質を調べる。
According to TEPCO, it is possible that the RPV has been damaged by the core meltdown, leaking the water. TEPCO plans to test the water for radioactive materials.
１階を確認したところ、計器盤の扉がゆがんだり、はしごが上層階から落下したりしていた。また、機器の一部がすすのようなもので黒くなっていた。東電によると、水素爆発の影響とみられるという。
Survey of the 1st floor found distorted doors of the instrument panel and a ladder dropped from the floor above. Part of the equipment was blackened with what looked like soot. TEPCO thinks they were caused by the hydrogen explosion.