Sweden's nuclear watchdog is tightening its grip on the power industry's 2000 preparations after uncovering a fault at an atomic unit which could have left many Swedes in the cold and dark.

Nuclear power workers are in for a dull and sober New Year 2000 as plants plan full staffing in case of problems. And some plants are worried the millennium bug might even strike at the end of this year.

Maintenance personnel at the three-reactor Forsmark station sounded the alarm in July when they found that the plant's data system was unable to recognise the first two digits of the number 2000, resulting in an automatic shutdown.

"If possible millennium problems are not taken care of well in advance, people could face a cool and dark New Year's eve," spokesman Anders Bjoerle at Sweden's nuclear power inspectorate (SKI) told Reuters.

SKI has said it might order a temporary shutdown on the eve of the millennium and go over to hydro-power as a back-up if it cannot guarantee there will be no computer problems.

Bjoerle said SKI's first step was to inspect Sweden's 12 nuclear units, which produce a combined 66.9 terrawatt hours (TWh), or around 50 per cent of the country's total output.

Testing has shown that the so-called millennium bug --expected to hit computer systems worldwide at a tick past midnight on December 31, 1999 -- could prove fatal to nuclear power production.

Programmers in the 1960s and 1970s saved time and money by skipping the first two numbers of a four-digit year. That means computers will read 2000 as 1900, threatening glitches in power supply and raising the urgent need for back-up sources.

"Minus 30 degrees without electricity -- that could be a cool Swedish New Year's eve," said spokesman Forsmark for Anders Markgren.

Swedes voted to phase out nuclear power by 2010 in a 1980 referendum. The government plans to spend nine billion crowns ($1.1 billion) on closing down the nation's 12 reactors.

Sweden reckons its plants are among the safest in the world, with stiffer maintenance demands than in many countries. Sweden has never had a major accident at any plant.

Manager Bjoern Lindquist for the millennium project at Vattenfall AB, Sweden's national power producer, said the company was focusing on testing to find ways to avoid shutdowns.

"We do not fear any problems after all the precautions we have taken. But if a plant shuts down, we will have alternative plants ready and we will take the electricity we need from other places," Lindquist told Reuters.

He said stand-by measures were not yet in place but that plants would be fully staffed on the night.

"We have notified personnel not to plan any great parties on that particular New Year's eve, but to leave the 2000 celebration for later," Lindquist said.

Vattenfall, the owner and operator of the Forsmark station, has been working on the 2000 bug since 1996.

"We have invested hundreds of millions of crowns in this programme," manager Stig Goethe for Vattenfall's environment and development unit, told Reuters.

But he said the costs involved represented just a fraction of the potential losses the company could face if its data system breaks down when 2000 begins.

With a total capacity of 6,640 megawatt (mw), Vattenfall's two plants, Ringhals and Forsmark, can produce 159,360 megawatt hours (mwh) in 24 hours at maximum power.

Sweden's other major nuclear power owner, Sydkraft AB says it stepped up its millennium bug efforts after discovering computer problems could arise earlier than expected.

The three-unit Oskarshamn plant, operated by Sydkraft unit OKG AB, uncovered an error which would have disturbed regulation of the reactor's feed water, likely to result in an automatic shutdown when the computer systems switched to 1999.

"We turned the clock forward in our computer system and found the reactor broke down as soon as it was confronted with the figure 1999," plant spokesman Anders Oesterberg said.

Programmers had used 999, or "zero" in computer language, to store unidentified information which interfered with the plant's operation.

Oesterberg said Oskarshamn has solved -- or postponed -- the problem by turning back clocks to make its data system believe that 1999 was 1991. In 2000, computers will be programmed for 1992, delaying bug problems by eight years.

"That will leave us plenty of time to replace the old computer systems," he said.

Sydkraft's three-unit Oskarshamn plant and two-reactor Barseback station have the capacity to produce 518,280 mwh in a 24-hour period.

SKI's Bjoerle said there was no threat to safety from the millennium bug but merely the problem of keeping Swedes warm.