London Guardian 
June 8, 2011
Molten nuclear fuel in three reactors at the Fukushima Daiichi power plant is likely to have burned through pressure vessels, not just the cores, Japan has said in a report in which it also acknowledges it was unprepared for an accident of the severity of Fukushima.
It is the first time Japanese authorities have admitted the possibility that the fuel suffered “melt-through” – a more serious scenario than a core meltdown.
The report, which is to be submitted to the International Atomic EnergyAgency (IAEA), said fuel rods in reactors No 1, 2 and 3 had probably not only melted, but also breached their inner containment vessels and accumulated in the outer steel containment vessels.
The plant’s operator, Tokyo Electric Power (Tepco), says it believes the molten fuel is being cooled by water that has built up in the bottom of the three reactor buildings.