One report said if a breakwater that extended up to 13 meters above sea level was hit by a 15-meter-high tsunami, all power sources would be knocked out--including outside electricity and emergency power generators. In such a situation, the report said, cooling functions would be lost and the reactor's core would be 100 percent damaged--a meltdown, in other words.

The breakwater at the Fukushima No. 1 plant was about 5.5 meters high, less than half the assumed height in the JNES report.

TEPCO assumed the tsunami hitting the plant would be 5.4 meters to 5.7 meters high. But the wave that struck on March 11 was 14 meters to 15 meters high.

Another report by the organization released last year predicted that if all power sources were lost due to an earthquake, fuel rods will begin melting after only 100 minutes. This report said a reactor's containment vessel would be damaged after about seven hours and a large amount of radioactive material would be released into the air.

According to an analysis by the government's Nuclear and Industrial Safety Agency, damage to the core of the Fukushima plant's No. 1 reactor started about two hours after the tsunami and its pressure vessel was damaged in about four hours--very close to what JNES had predicted.