The World Health Organisation says it is worried about the "unacceptably high" risk of a nuclear reactor accident within the next few years.

Many of eastern Europe's reactors could be at particular risk from the millennium bug, it warns.

The risk of a reactor accident somewhere in the world before 2006 is as high as 67%, says WHO's British physicist Dr Keith Baverstock.

"There will be a lot of crossed fingers on New Year's Eve 1999," he added.

Dr Baverstock heads WHO's new nuclear emergency project based in Helsinki. The new scheme will help European countries improve preparations for coping with an accident.

Human failure likely

He says the risk lies not so much in the failure of an engineering component as in human error, with the possibility of computer failure as the year 2000 dawns making the prospect worse.

Millennium worries mean hard choices

Dr Baverstock told BBC News Online: "31 December 1999 is one occasion when we ought to be even more vigilant than we normally are.

"The millennium bug is a real problem. We shall take it very seriously indeed."

Dr Baverstock says Finland is confident its reactors will cope safely and he believes the same goes for all western European countries.

"But it may well be quite different in the east, where many countries depend heavily on nuclear power," he says.

Shutting down the reactors for a short time around midnight is not really an option because any problems might not show up until later, says Dr Baverstock.

"It will be very cold and they will need the power. What will they do ? I should not like to have to decide."

Preventing cancer

As part of its work to help countries prepare, WHO is issuing guidelines on the use of iodine tablets.

Iodine, given early enough, can help prevent childhood thyroid cancer.

Children are far more vulnerable to radiation's effects

Since the Chernobyl explosion in 1986, more than 1,000 cases of thyroid cancer, mainly in children, have been recorded in affected parts of Ukraine, Belarus and Russia.

"We used to think that iodine tablets should be available within a 5km radius of a reactor," says Dr Baverstock.

"But Chernobyl showed us that the effects spread far wider than that.