TONY JONES, PRESENTER: For the first time Japan has suggested the situation at the Fukushima nuclear plant has gone beyond a meltdown.

Japan says that nuclear fuel in three reactors has possibly melted through the pressure vessels and accumulated in the outer containment vessels.

Japanese media report this melt-through is far worse than a core meltdown and the worst possibility in a nuclear accident.

In a report to be submitted to the United Nations nuclear watchdog, Japan has also admitted it wasn't prepared for the scale of the Fukushima disaster.