There is no water left in the spent fuel pool of reactor 4 at Japan’s Fukushima nuclear plant, resulting in “extremely high” radiation levels, the chair of the US Nuclear Regulatory Commission warned Wednesday.

“In addition to the three reactors that were operating at the time of the incident, a fourth reactor is also right now under concern. This reactor was shut down at the time of the earthquake,” said NRC chairman Gregory Jaczko.

“What we believe at this time is there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the spent fuel pool,” he said, noting the explosion happened several days ago but its effects were cause for concern.

“We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures.”

Jaczko briefed lawmakers about the latest developments as part of a House of Representatives hearing on the US energy budget, after meeting with President Barack Obama at the White House.

He said that if the United States were facing a similar situation, it would order a much larger evacuation zone than Japan has (20 kilometers), and so the US has called on Americans within (80 kilometers) of the Japan nuclear plant area to leave.

“Recently the NRC made a recommendation that based on the available information that we have that for a comparable situation in the United States we would recommend an evacuation for a much larger radius than has currently been provided in Japan,” he said.

“As a result of this recommendation the ambassador in Japan has issued a statement to American citizens that we believe it is appropriate to evacuate to a larger distance, up to approximately 50 miles.”

France’s Nuclear Safety Authority (ASN) earlier Wednesday described the No. 4 rod pool as “the major concern” in the crisis.

“The next 48 hours will be decisive,” said Thierry Charles, director for factory, laboratory, transport and waste safety at the French Institute for Radiological Protection and Nuclear Safety (IRSN).

In an assessment issued at 0800 GMT on Wednesday, the IRSN said the water in the No. 4 pool was “boiling.”

“Without water replenishment, the fuel-rod assemblies will start to be exposed in a few days. If the pool runs dry, this would eventually lead to the meltdown of the fuel.

“In such an event, the corresponding releases of radioactivity would be far higher than those that have occurred up till now.”

Japan’s nuclear accident ‘out of control’: EU energy chief

The situation at Japan’s disaster-hit nuclear power plant has spun out of control, threatening a deeper catastrophe that could cost more lives, the European Union’s energy chief said on Thursday.

“The site is effectively out of control,” energy commissioner Guenther Oettinger told a European Parliament committee, one day after he said Japan was facing “apocalypse.”

“In the coming hours there could be further catastrophic events which could pose a threat to the lives of people on the island,” he said.

Scrambling to prevent a nuclear meltdown, Japanese crews have been dumping water on the stricken Fukushima No.1 power plant, which has been hit by a series of explosions after Friday’s quake knocked out reactor cooling systems.

US warns citizens near Japan nuclear plant to leave

US officials on Thursday warned citizens living within 50 miles (80 kilometres) of a crippled Japanese nuclear plant to evacuate or seek shelter, amid mounting concern of a catastrophic meltdown.

The evacuation order came as the chair of the US Nuclear Regulatory Commission (NRC) warned there was no water left in the spent fuel pool of reactor 4 at the Fukushima nuclear plant, resulting in “extremely high” radiation levels.

The US set up a much wider no-go zone than the 20-kilometre radius perimeter set up by the Japanese, after US authorities reviewed scientific and technical data about the nuclear emergency.

“We are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometers) of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical,” the US embassy in Tokyo said in its statement.

NRC chairman Gregory Jaczko told lawmakers that a fourth reactor at the nuclear complex was giving cause for concern following a hydrogen explosion there a few days ago.

“We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures.”

The White House admitted the advice it was giving to its citizens went beyond the advice Japan had given, but declined to evaluate the Japanese data.

“This crisis is happening in Japan. Obviously, the Japanese government has the lead here with regards to dealing with the crisis and advising its people,” said White House spokesman Jay Carney.

“There are a lot of Americans in Japan. When the data that we receive is analysed independently, we are going to give advice based on our analysis.”