This article is more than 9 years old

This article is more than 9 years old

The firm at the centre of Japan's worst nuclear accident insisted on Tuesday it would bring stricken reactors at the Fukushima Daiichi power plant under control by January 2012, despite evidence that the complex is more seriously damaged than previously thought.

Tokyo Electric Power Company (Tepco) said it would try new methods to cool the reactors, but would stick with its original plan, announced a month ago, to stabilise radiation levels and bring the units to a stable condition – a process known as "cold shutdown" – within six to nine months.

Officials said the melting of fuel rods in three reactors did not raise the risk of explosions and would not affect the timeline for bringing the plant under control.

But Tepco's roadmap has looked increasingly unworkable recently, after it said uranium fuel rods in three reactors had been left exposed and had melted hours after the earthquake on 11 March.

On Friday, the company revealed fuel in the No 1 reactor had partially melted and fallen to the bottom of the pressurised vessel which holds the reactor core together.

Junichi Matsumoto, a general manager at Tepco, said reactors Nos 2 and 3 were likely to have suffered similar problems.

"The findings at the No 1 reactor indicate the likelihood that water level readings in the other reactors aren't accurate," he said. "It could be that a meltdown similar to that in the No 1 reactor has occurred."

Tepco said it will abandon plans to stabilise the reactors by filling them with water, after leaks were discovered in the main vessel of the No 1 reactor.

Flooding them would increase the chances of vast quantities of contaminated water leaking out of the complex.

Instead, workers will attempt to cool the melted fuel by circulating water that has already built up inside the reactors.

The Fukushima Daiichi complex now contains thousands of tonnes of water – enough to fill 36 Olympic-sized swimming pools – adding to fears that the liquid could find its way into groundwater and the Pacific ocean if efforts fail to store it safely.

Questions have been raised about Tepco's original explanation for the crisis. For weeks it claimed power to vital cooling systems inside the reactors was knocked out by the 15 metre tsunami that followed the earthquake.

But recently retrieved data from the plant showed the earthquake had been more powerful than three of the six reactors were built to withstand, raising the possibility that at least one of the reactors was disabled before the waves arrived.

"This was clearly a larger earthquake than we had forecast," Matsumoto said. "It would have been hard to anticipate this."

In another revelation that reflects badly on the firm's ability to manage the crisis, reports suggest a Tepco worker manually cut the power to the cooling system in the No 1 reactor, after data showed it was cooling too quickly in the immediate aftermath of the quake.

"At the time we could not have known that the tsunami was coming and that we would lose power," Matsumoto said.

Japan's earthquake and tsunami killed more than 15,000 people, while another 9,500 are still missing.

More than 80,000 people were forced to evacuate their homes after the nuclear accident, with no indication of when they might be able to return.

The crisis prompted the prime minister, Naoto Kan, to announce a review of Japan's energy policy.

He ordered the temporary shutdown of another nuclear plant deemed vulnerable to quakes and tsunamis, and abandoned plans to build 14 new nuclear power plants by 2030.

Any adjustment to Tepco's roadmap would have caused embarrassment for Kan, who has been widely criticised for his handling of the crisis.

"In terms of achieving cold-shutdown status within six to nine months, I believe we should be able to proceed without changing the timeframe," he told parliament on Monday.

Even if Tepco meets its deadline, experts predicted work to decontaminate the site and decommission the reactors would take years.