Japan earthquake: US alarm over nuclear crisis
Increasing alarm has been voiced in the US about the crisis at the Fukushima Daiichi nuclear plant in Japan.
A top US nuclear official said attempts to cool reactors with sea water to prevent a meltdown appeared to be failing and workers could be exposed to "potentially lethal" radiation doses.
Japanese army helicopters on Thursday dumped water on the reactors to try to cool overheated fuel rods.
The plant was severely damaged by last week's huge earthquake and tsunami.
US Energy Secretary Steven Chu has said that the situation at the plant appears to be more serious than the partial meltdown at the Three Mile Island nuclear plant in Pennsylvania in 1979.
The US state department has urged Americans living within 80km (50 miles) of Fukushima Daiichi, which lies 220km from Tokyo, to leave the area - a much wider exclusion zone than the 20km advised by the Japanese government.
Some US military personnel in Japan have been given tablets against possible radiation effects.
Britain has now advised its nationals currently in Tokyo and to the north of the capital to consider leaving the area.
Engineers are racing to avert a nuclear catastrophe at Fukushima Daiichi, where the tsunami wrecked back-up diesel generators that kept the nuclear fuel cool.
Workers have been dousing the reactors with sea water in a frantic effort to stabilise their temperatures, since the first in a series of explosions rocked the plant on Saturday.
On Thursday, army helicopters dumped tonnes of water on reactor three - a day after they were forced to pull out amid concerns over radiation levels in the air above the site.
There are fears that reactor three may have released radioactive steam due to a reported damage to its containment vessel.
The helicopters soon left the site in order to minimise the crews' exposure to radiation.
Meanwhile, water trucks are now on standby to spray water on to a spent-fuel storage pond at reactor four, following fires there.
Fukushima has sent low levels of radiation wafting into Tokyo.
Plant operator Tokyo Electric Power Co. has said it is seeking to restore the power supply to the plant's cooling systems "as soon as possible".
On Wednesday, Greg Jaczko, chairman of the US Nuclear Regulatory Commission (NRC), told a congressional energy and commerce subcommittee hearing in Washington that there appeared to be serious problems with attempts to cool the reactors.
"We believe that around the reactor site there are high levels of radiation," he said.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."
The US NRC has 11 agency experts in Tokyo monitoring the situation.
The head of the UN's atomic energy agency, Yukio Amano, is travelling to Japan in person to gather more information.
Earlier, in a rare public appearance, Japan's Emperor Akihito said he was "deeply worried" about the crisis his country was facing.
TV stations interrupted programming to show the emperor describing the crisis facing the nation as "unprecedented in scale".
The 77-year-old - deeply respected by many Japanese - said: "I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times."
Fukushima prefecture governor Yuhei Sato has criticised official handling of the evacuation of the area around the stricken power plant. "Anxiety and anger felt by people have reached boiling point," he said.
Mr Sato said centres already housing people who had been moved from their homes near the plant did not have enough hot meals and basic necessities such as fuel and medical supplies. "We're lacking everything," he said.
In other developments:
- The benchmark Nikkei index fell 3.6% in early Thursday trading in Tokyo, shortly after the yen briefly hit the highest level against the US dollar since World War II
- France urged its nationals in Tokyo to leave the country or move south; two Air France planes were sent to begin evacuation
- Australia advised its citizens to consider leaving Tokyo and the most damaged prefectures
- Turkey warned against travel to Japan
Thousands of people were killed in the 9.0-magnitude quake and tsunami.
Snow has blanketed swathes of the disaster zone, where many survivors have little food, water or heat.
About 450,000 people have been staying in temporary shelters, many sleeping on the floor of school gymnasiums.
More than 4,300 people are listed as dead but it is feared the total death toll from the catastrophe, which pulverised the country's north-east coast, will rise substantially.