Japan's Fukushima city said Tuesday it would hand radiation dosimeters to 34,000 children to gauge their exposure from the crippled nuclear power plant about 60 kilometres (40 miles) away.

The city will hand the measuring devices to all children aged between four and 15 for three months from September so that they can wear them around the clock, an official at the city's education board told AFP.

The city is outside the government's 20-kilometre (12-mile) evacuation and no-go zone around the tsunami-hit Fukushima Daiichi nuclear plant, but many residents are concerned about radiation, he said.

"There have been fixed-spot radiation measurements but parents and citizens are concerned about individual exposure," said the official.

"We also believe the distribution of dosimeters will help ease parents' worries if they confirm their children's exposure does not pose health risks."

He added that radiation in the city had been below the official threshold for health risks, and said the children's dosimeters would be read out once a month to assess cumulative radiation exposure.

Japan has struggled to bring the plant under control since it was hit by a tsunami that knocked out its cooling systems, leading to three reactor meltdowns, explosions and radiation leaks into the air, soil and sea.

Since the March 11 disaster, Japan has raised the legal exposure limit for people, including children, from one to 20 millisieverts per year -- matching the safety standard for nuclear industry workers in many countries.

Environmental activist group Greenpeace called on Japan last Thursday to evacuate children and pregnant women from Fukushima town.

It said people were being exposed to 10 to 20 millisieverts per year through the air, not counting contaminants inhaled or ingested, a level Greenpeace considers unacceptable, especially for high-risk groups.

Radiation experts agree that children are at greatest risk from cancers and genetic defects because they are still growing, are more prone to thyroid cancers, and because they will have more time to develop health defects.

The city of Date just outside the no-go zone also plans to distribute dosimeters to all its 8,000 pre-school, elementary and junior high pupils.

Embattled plant operator Tokyo Electric Power Company (TEPCO) said this week that elevated levels of radioactive strontium had been detected in the sea and groundwater at the plant, aside from iodine and caesium isotopes.

TEPCO also said that six more nuclear emergency workers had received more than the permitted annual radiation dose, a limit that was raised from 100 to 250 millisieverts amid the current crisis.

Previously two male workers exceeded the limit, and two women workers topped the lower limit for females of five millisieverts in a three-month period.

Health and labour minister Ritsuo Hosokawa said Tuesday he had ordered TEPCO to relieve workers of their duties if their preliminary radiation doses for internal exposure exceed 100 millisieverts, Kyodo News reported.

Hosokawa also criticised the company's "extremely deplorable" delay in testing the thousands of workers and subcontractors at Fukushima Daiichi.

The crews have for three months hosed water into the facility to cool the reactor fuel, creating more than 100,000 tonnes of highly radioactive runoff that has prevented them from carrying out crucial repairs.

TEPCO has installed a water treatment system, using French and US technology, and plans to launch it Friday to process about 1,200 tonnes of water per day, with the aim of recycling it for reactor cooling.

Amid the crisis, Japanese public support for gradually reducing the use of nuclear energy to zero in the future came to 74 percent, the Asahi Shimbun daily said after a nationwide weekend telephone poll.

The poll also showed 64 percent of respondents believed renewable energy sources such as wind and solar power would replace nuclear power in the future.

The survey covered 3,394 voters of whom 58 percent gave valid responses.