Workers at the Fukushima Daiichi nuclear power plant face new threats to their health after radiation exceeding safety levels was found to have seeped into groundwater near the facility.
The plant's operator, Tokyo Electric Power (Tepco), was the target of further criticism amid reports that some workers at the site had not been provided with personal radiation monitors.
Tepco's handling of the crisis has come under closer scrutiny since three workers were exposed to dangerously high levels of radiation last week. They have all been discharged from hospital after suffering no ill effects.
Japan's nuclear and industrial safety agency, Nisa, ordered the firm to review its latest radiation measurements taken from the air, seawater and groundwater, saying they seemed suspiciously high.
Earlier on Friday Tepco reported that groundwater beneath one of the plant's six reactors contained levels of radioactive iodine 10,000 times higher than government standards.
"We have our suspicions about their isotope analysis," said Hidehiko Nishiyama, a Nisa spokesman. Tepco said that a computer software fault could be responsible for the high readings, but added that the data could turn out to be accurate.
Experts said it was unlikely that the radioactive iodine-131 found nearly 15 metres (50ft) below one of the reactors would find its way into drinking water.
For several days authorities have issued assurances that none of the radiation readings are high enough to present a threat to people living beyond the 12-mile (20km) evacuation zone.
But the risks being taken by about 600 technicians, engineers, firefighters and troops as they struggle to contain radiation leaks have only increased.
Tepco admitted it had not been able to provide enough dosimeters to teams of workers who are completing gruelling three-day shifts in an attempt to remove and dispose of radioactive pools of water and prevent damaged fuel rods from going into full meltdown.
Tepco said it had secured additional gauges to replace hundreds that were damaged in the 11 March tsunami. "We must ensure the workers' health and safety, but we also face a pressing need to get the work done as quickly as possible," Nishiyama said, adding that sharing meters up until now had been "unavoidable".
Soon after the disaster the health ministry raised the maximum radiation level to which each worker can safely be exposed from 100 millisieverts a year to 250 millisieverts a year to enable them to spend more time in contaminated areas.
Nisa said that 21 workers had so far been exposed to radiation exceeding 100 millisieverts, although tests have shown that no one has been exposed to radiation high enough to damage their health.
Sumio Imoto, a spokesman for one of Tepco's main subcontractors, said its labourers were being looked after and were not taking unnecessary risks. "The safety of our employees is our primary concern," he said, "but keeping up morale is a big challenge."
Robert Peter Gale, a US medical researcher who has been advising the Japanese government, said: "There's a huge difference between whole-body and partial-body radiation.
"One of my primary considerations is the geometry of the reactor and the likelihood that people are in a configuration that would give them full-body radiation. It's not impossible, but it's highly unlikely. They have to be exposed in their entirety from three to four metres to get a whole-body dose. This is not a kamikaze situation."
According to the few reliable descriptions of conditions at the plant, the workers are given just two meals a day – crackers and a small carton of vegetable juice for breakfast; dried rice and canned fish or chicken for dinner – and take naps in cramped corridors on lead-lined sheets to limit their exposure to radiation.
"That's where they sleep, with only one blanket each to wrap themselves around," said Kazuma Yokota, a Nisa official who spent five days at the plant.
Yokota said the rush to save the plant meant some workers had been unable to change their underwear, while high radiation levels were hampering the arrival of fresh supplies.
Conditions have marginally improved amid widespread admiration for the workers, initially nicknamed the Fukushima 50 because they worked in groups of that number. Their daily bottled water allowance has increased and the government has vowed to improve food supplies. The workers' nightly meeting ends with shouts of "ganbaro!" ("let's keep going").
The anonymous workers have little or no contact with the outside world while they are on site, and media interviews are discouraged. "They are doing their best while they aren't even able to contact members of their family," Yokota said.
After three days' work they spend three days at J-Village, a nearby sports complex, for a shower, proper food and sleep.
"It isn't perfect, but it does provide a place for the workers to pull back and get some rest before they have to go back in," said a Tepco spokesman, Hirota Oyama. "They can eat fresh vegetables, something they can't do on the site."
The prime minister, Naoto Kan, will visit the sports complex on Saturday to show his support. His spokesman, Yukio Edano, said: "I humbly bow to the workers and officials who are doing difficult jobs at the plant's frontline."
Japanese media reported that nuclear workers had been offered up to 400,000 yen (£3,000) a day to work inside the crippled reactors. Before the crisis some contract workers were reportedly being paid as little as 10,000 to 20,000 yen (£75 to £150) a day.
Tepco said it was considering using "jumpers", or workers who enter highly radioactive reactors to perform short but essential tasks, then evacuate quickly to avoid prolonged exposure to radiation.
An early return home is unlikely given the perilous condition of the plant three weeks after the tsunami. As Kan warned on Friday: "I am prepared for a long-term battle over the Fukushima nuclear plant and to win this battle.
"At the current stage we cannot say that the plant has been sufficiently stabilised. But we are preparing for all kinds of situations and I am convinced that the plant can be stabilised. We cannot say at this stage when this will happen, but we are doing our best."
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