Three cable-layers at Fukushima power plant exposed to high levels of radiation after stepping into contaminated water

The dangers facing workers battling to avert disaster at the Fukushima Daiichi nuclear power plant were underlined on Thursday when three men were exposed to high levels of radiation after stepping in contaminated water.

The plant's operator, Tokyo Electric Power (Tepco), admitted that the workers had not measured radioactivity levels before beginning work, and that two, who are being treated for radiation burns, were not wearing protective boots.

The two more seriously injured men were diagnosed with possible beta ray burns and were due to be taken to a special unit at the National Institute of Radiological Sciences in Chiba, east of Tokyo, Japan's nuclear safety agency said.

The injuries are similar to regular burns, but can lead to serious complications over a period of several weeks.

Tests on the workers, who are all in their 20s and 30s, showed they were exposed to between 170 millisieverts (mSv) and 180 mSv of radiation while laying cable in the basement of the No 3 reactor's turbine building. That is above the usual legal limit of 100 mSv per year for nuclear power workers in Japan, but below a new limit of 250 mSv, introduced soon after the nuclear crisis began to enable them to spend more time inside the crippled facility.

The two hospitalised men, employees of a Tepco affiliate, were part of a team of six workers attempting to connect a water pump to the power supply and restart the supply of fresh water in an attempt to cool the reactor. Workers in the first and basement floors of the No 3 reactor's turbine building were ordered to evacuate the area after the accident.

Japan's nuclear safety agency said water had probably seeped through their protective clothing, allowing radioactive materials to stick to their skin, as they stood in a 15cm-deep puddle. The two injured men were wearing shoes, while the third had boots on and so escaped serious injury.

Radiation levels on the surface of the puddle were later measured at 400 mSv per hour, while the level in the air reached 200 mSv per hour.

The source of the water was not immediately clear. Tepco said no puddle had been spotted in the turbine building the previous day. Fire trucks have been dousing the reactor in recent days in an attempt to cool a storage pool for spent fuel rods.

The accident cast doubt on Tepco's ability to properly monitor radioactivity at the site. "This kind of exposure, from water, was unforeseen," the government's chief spokesman, Yukio Edano, told reporters.

"Atmospheric radiation levels are monitored constantly, but in this case the workers stepped into water. We are trying to find out exactly what happened so we can ensure it doesn't happen again."

Edano said the injuries were "very regrettable", but defended the health ministry's decision to raise the exposure limit for teams of workers soon after the start of the world's worst nuclear power emergency since Chernobyl.

"The decision to increase permissible radiation exposure was taken on the advice of experts, who say that workers are able to withstand up to 250 mSv per year before radiation has an effect on their health," Edano said.

He conceded that the emergency at Fukushima would lead to a review of Japan's heavy dependence on nuclear power. "It is certain that public confidence in nuclear power plants has greatly changed," Edano told Reuters. "In light of that, we must first end this situation and then study from a zero base."

More than 20 workers have been injured at the Fukushima plant, 150 miles north of Tokyo, since it was badly damaged in the 11 March earthquake and tsunami. They include 11 who were hurt when the No 3 reactor building exploded.

To date, 17 have been exposed to radiation exceeding 100 mSv at the plant, though none has been exposed to contamination exceeding the accumulative 250 mSv limit.

The condition of the No 3 unit is of particular concern as it contains plutonium-uranium mixed oxide fuel and would release highly toxic plutonium in the event of a meltdown.

On Wednesday afternoon, workers were temporarily evacuated from the plant after black smoke was seen rising from the same reactor. The smoke receded after an hour and radiation levels remained unchanged, the safety agency said.

The release of radioactive substances from the plant continued to cause anxiety in Tokyo. Wednesday's warning that radioactive iodine levels in the capital's tap water had exceeded levels considered safe for babies prompted a rush of people buying bottled water.

The warning was lifted on Thursday after iodine-131 dropped to safe levels, but they were still above the safe upper limit for infants in the neighbouring prefectures of Chiba and Saitama.

In Tokyo, a city of 13 million people, supermarkets quickly ran out of bottled water; the metropolitan government said it would distribute an extra 240,000 bottles to families with infants.

"Customers ask us for water, but there's nothing we can do," Masayoshi Kasahara, a supermarket worker, said. "We have asked for extra deliveries but we don't know when they will arrive."