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”.