TOKYO - The amount of radiation released by the Fukushima Daiichi nuclear power plant in the days after the 11 March tsunami could have been more than double that originally estimated by its operator, Japan's nuclear safety agency has said.
The revelation has raised fears that the situation at the plant, where fuel in three reactors suffered meltdown, was more serious than government officials have acknowledged.
In another development that is expected to add to criticism of Japan's handling of the crisis, the agency said molten nuclear fuel dropped to the bottom of the pressure vessel in the No 1 reactor within five hours of the accident, 10 hours earlier than previously thought.
By the end of last week, radiation levels inside the reactor had risen to 4,000 millisieverts per hour, the highest atmospheric reading inside the plant since the disaster.
The agency also speculated that the meltdown in another reactor had been faster than initially estimated by the plant's operator, Tokyo Electric Power (Tepco).
It is not clear whether the revised account of the accident, the world's worst since Chernobyl in 1986, would have prompted Tepco to respond differently at the time.
But it is expected to raise questions about the ability of Japan's nuclear authorities to provide accurate information to the public.
According to the latest estimates, 770,000 terabequerels – about 20% as much as the official estimate for Chernobyl – of radiation seeped from the plant in the week after the tsunami, more than double the initial estimate of 370,000.
In a possible sign that the contamination is more widespread than previously thought, a university researcher said at the weekend a small amount of plutonium had been identified a mile from the front gate of the Fukushima plant.
It is the first time plutonium thought to have originated from the complex has been detected in soil outside its grounds.
However, Masayoshi Yamamoto, a professor at Kanazawa University, said the level of plutonium in the sample was lower than average levels observed in Japan after nuclear weapons tests conducted overseas.
The release of findings coincided with the start of an investigation on Tuesday into the accident by a 10-member panel.
Last week, a fact-finding team from the International Atomic Energy Agency criticised Tepco for failing to acknowledge the risk to the plant from a tsunami, despite warnings from government experts and its own scientists.
The panel, led by Yotaro Hatamura, a human error expert from Tokyo University, will issue an interim report by the end of the year. "I think it is a mistake to consider [the plant] safe," he said.
The prime minister, Naoto Kan, said he would be willing to undergo questioning in the hope that the report "stands up to scrutiny from around the world".