Japanese authorities have suggested the situation at the Fukushima nuclear plant may have gone beyond a meltdown to a "melt-through" - the worst possibility in a nuclear accident.
The plant was badly damaged after a 9.0-magnitude earthquake and tsunami devastated northesast Japan on 11 March.
An official report which Japan will submit to the United Nations' nuclear watchdog says nuclear fuel in three reactors has possibly melted through the pressure vessels and accumulated in outer containment vessels.
Japan's Yomiuri Shimbun newspaper says this "melt-through" is far worse than a core meltdown and is the worst possibility in a nuclear accident.
It is the first official admission that a "melt-through" may have occurred, the ABC reports.
In the report, Japan also admits it was unprepared for the scale of the disaster in March and acknowledges there was insufficient communication between the government and the plant's operator the Tokyo Electric Power Company.
ABC North Asia correspondent Mark Willacy told Radio New Zealand's Checkpoint programme on Wednesday if fuel leaks through the containment vessels it would become a huge problem.
"It means there will be direct and very highly toxic contamination directly into the ground and potentially into groundwater which could then leach into the ocean.
"Those levels would be very high. It would obviously contain radioactive material such as iodine but, more seriously, caesium, plutonium and strontium ... very toxic materials."
