The six were all employed at the Tokaimura uranium processing plant at the time of the accident there in September 1999. They include former plant manager Kenzo Koshijima and a deputy manager in charge of processing operations.

More than 400 people were exposed to radiation

Yutaka Yokokawa, the only survivor of the three workers who caused the accident, was also among those arrested.

They face maximum penalties of five years in prison or fines of over $4,600.

They are the first to be arrested since the accident at the plant run by JCO Co, which killed two workers and exposed more than 400 people to radiation.

"After thorough investigations, we have arrested six JCO officials on suspicion of professional negligence," a police official said.

Systematic violations

Police believe that systematic violations by JCO were to blame for the accident, some 140km (90 miles) northeast of Tokyo.

They will also seek charges against JCO as a corporate body and former company president Hiroharu Kitani for the alleged violation of laws regulating nuclear reactors.

It took 20 hours for the nuclear chain reaction to be harnessed

The accident was caused by three workers at the plant who mixed excess amounts of liquefied uranium in steel buckets, setting off a nuclear chain reaction.

They were believed to be unaware of the dangers involved.

And newspapers have alleged that JCO approved this highly dangerous method in 1996 to cut costs.

In March, the Japanese authorities withdrew JCO's operating licence.

Government under fire

The Japanese Government has also come under heavy criticism for failing to enforce safety standards at Tokaimura and other nuclear plants.

The BBC Tokyo correspondent says the government has barely begun the task of rebuilding public confidence in nuclear power.

Japan has 51 nuclear reactors, which provide 30% of its electricity.

Nuclear power is essential to Japan because of its lack of natural resources.