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 Energy Agency (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.
The report includes an apology to the international community for the nuclear crisis — the world's worst since Chernobyl in 1986 — and expresses "remorse that this accident has raised concerns around the world about the safety of nuclear power generation."
The prime minister, Naoto Kan, said: "Above all, it is most important to inform the international community with thorough transparency in order for us to regain its confidence in Japan."