Japan in a report prepared for the UN nuclear watchdog on Tuesday said for the first time that fuel in its crippled Fukushima plant may have melted through three reactor core vessels.

Almost three months after the massive March 11 seabed quake and tsunami sparked the world's worst nuclear accident since Chernobyl, Tokyo also promised to tighten its nuclear industry safety standards and mechanisms.

Prime Minister Naoto Kan, approving the 750-page report, said that "above all, it is most important to inform the international community with thorough transparency in order for us to regain its confidence in Japan."

Japan is due to submit the report to the UN International Atomic Energy Agency (IAEA), which sent its own multinational fact-finding team to Japan and is due to discuss the disaster at its Vienna headquarters this month.

The document was released a day after Japan more than doubled its earlier estimate of total radiation released into the air from the plant in the first days after the tsunami knocked out its reactor cooling systems.

Japan's Nuclear and Industrial Safety Agency (NISA) now says it believes 770,000 terabecquerels escaped into the atmosphere in the first six days -- compared to its earlier estimate of 370,000 terabecquerels.

NISA's revised radiation figure is likely to fuel criticism of the initially slow and vague flow of information from the Kan government and embattled plant operator Tokyo Electric Power Company (TEPCO).

The new estimate is closer to that of Japan's independent Nuclear Safety Commission which had initially estimated a release at 630,000 terabecquerels in the first month after the 14-metre (46-foot) ocean wave crashed into the plant.

NISA also said in its review that it believes much of the nuclear fuel inside three reactors melted down faster than previously believed.

TEPCO has said it believes the molten fuel is now being cooled by water at the bottom of the number one, two and three reactors, citing the relatively low outside temperatures of the containers.

In its report, Japan's government informs the IAEA that fuel is assumed to have melted down and may have burnt through the reactor pressure vessels of units one, two and three and into their outer steel containment vessels.

Tokyo also pledged to reform its nuclear safety systems, including by separating the watchdog NISA from the ministry of economy, trade and industry, which promotes nuclear power -- echoing an IAEA recommendation.

In the report, Japan apologises to the international community for the disaster, expressing its "remorse that this accident has raised concerns around the world about the safety of nuclear power generation."

Also on Tuesday, a 10-member expert panel independent of the nuclear industry met for the first time to look into the causes of the world's worst atomic accident since Chernobyl, in Ukraine, a quarter-century ago.

Panel leader Yotaro Hatamura, a Tokyo University professor emeritus on the study of human error, said at the meeting that "nuclear power has higher energy density and is dangerous. It was a mistake to consider it safe."

Meanwhile, frantic efforts continued at the plant to meet TEPCO's stated deadline of bringing all six reactors at Fukushima Daiichi to a stable state of "cold shutdown" some time between October and January.

Months of hosing operations have left more than 100,000 tonnes of highly radioactive water in buildings, basements and ditches at the plant, and TEPCO is struggling to remove the runoff so it can resume crucial repair work.

Contaminated water has spilled or been released several times into the Pacific Ocean, and environmental group Greenpeace has warned that it has found unsafe radiation levels in marine species as far as 50 kilometres (30 miles) offshore.

TEPCO has been testing decontamination equipment, some provided by US and French companies, to remove radioactive substances, oil and sea salt from the runoff water so it can be reused as a reactor coolant from about mid-June.

With the advent of the summer rainy season, it has also started to ship to the plant 370 truck-sized water tanks with a total capacity of more than 40,000 tonnes to store excess contaminated water.

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