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STOCKHOLM--Swedes may get an unwelcome foretaste of their plan to phase out nuclear power by 2010 unless computer experts crush the millennium bug.

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 the 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 recognize 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 around 50 percent of the country's total output. Testing has shown that the millennium bug 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.

Bjoern Lindquist, manager 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.

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 program,'' Stig Goethe, manager 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.

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.

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

"There is no connection between the millennium and the safety of the nuclear reactor. The problem is power supply,'' he said.

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