Energy Secretary Steven Chu said Friday that roughly 70 percent of the core of one reactor at the crippled Fukushima nuclear plant in Japan had suffered severe damage.

His assessment of the damage to Reactor No. 1 was the most specific yet from an American official on how close the plant came to a full meltdown after it was hit by a severe earthquake and massive tsunami on March 11.

Japanese officials have spoken of “partial meltdown” at some of the stricken reactors. But they have been less than specific, especially on the question of how close No. 1 — the most badly damaged reactor — came to a full meltdown.

Fukushima Daiichi Nuclear Plant Hi-Res Photos - Badly Damaged

Crews 'facing 100-year battle' at Fukushima

A nuclear expert has warned that it might be 100 years before melting fuel rods can be safely removed from Japan's Fukushima nuclear plant.

The warning came as levels of radioactive iodine flushed into the sea near the plant spiked to a new high and the Wall Street Journal said it had obtained disaster response blueprints which said the plant's operators were woefully unprepared for the scale of the disaster.

Water is still being poured into the damaged reactors to cool melting fuel rods. But one expert says the radiation leaks will be ongoing and it could take 50 to 100 years before the nuclear fuel rods have completely cooled and been removed.

"As the water leaks out, you keep on pouring water in, so this leak will go on for ever," said Dr John Price, a former member of the Safety Policy Unit at the UK's National Nuclear Corporation.

"There has to be some way of dealing with it. The water is connecting in tunnels and concrete-lined pits at the moment and the question is whether they can pump it back.

"The final thing is that the reactors will have to be closed and the fuel removed, and that is 50 to 100 years away.

"It means that the workers and the site will have to be intensely controlled for a very long period of time."

But Laurence Williams, Professor of Nuclear Safety at England's University of Central Lancashire and the former head nuclear regulator for the UK, is relatively comfortable with the situation.

"I have been monitoring it for the last couple of weeks and [the] three reactors seem to be more or less unchanged from initially when they got into the seawater flowing into them," he said.

"We don't know exactly the state of the fuel in those reactors but looking at the data, the pressures and temperatures look fairly stable over the last couple of weeks.

"My view is that as there hasn't been any sort of major catastrophic release of radioactivity, if they can continue to get the fresh water into the reactors and cool them, the decay heat is now fairly stabilising.

"It will take some time before it disappears but so far, so good. But it will take some time to bring under control."

Both experts agree capping the damaged reactors with concrete is not an option.

Meanwhile the Wall Street Journal says it has obtained disaster-readiness plans which show the facility only had one satellite phone and a single stretcher in case of an accident.

The blueprints also provided no detail about the possibility of using firefighters from Tokyo or national troops - both of which have been part of the response to the Fukushima crisis - to deal with any disaster.

Levels of radioactive iodine-131 in the Pacific off the plant have been recorded at a new high of 4,385 times the legal limit.

In 2002, the plant's operator TEPCO admitted to falsifying safety reports, leading to all of its 17 boiling water reactors being shut down for inspection.

TEPCO has already vowed to dismantle the four reactors at the centre of the world's worst atomic accident in 25 years, but now Japanese prime minister Naoto Kan says the Fukushima plant must be scrapped.