By Susan Sachs | Correspondent of The Christian Science Monitor

PARIS – Summer is exposing the chinks in Europe's nuclear power networks.

The extended heat wave in July aggravated drought conditions across much of Europe, lowering water levels in the lakes and rivers that many nuclear plants depend on to cool their reactors.

As a result, utility companies in France, Spain, and Germany were forced to take some plants offline and reduce operations at others. Across Western Europe, nuclear plants also had to secure exemptions from regulations in order to discharge overheated water into the environment.Even with an exemption to environmental rules this summer, the French electric company, Electricité de France (EDF), normally an energy exporter, had to buy electricity on European spot market, a way to meet electricity demand.

The troubles of the nuclear industry did not end there. Sweden shut four of its 10 nuclear reactors after a short-circuit cut power at one plant on July 26, raising fears of a dangerous design flaw. One week later, Czech utility officials shut down one of the country's six nuclear reactors because of what they described as a serious mechanical problem that led to the leak of radioactive water.

The disruptions highlight some of the vulnerabilities of nuclear power, just at a time when its future was looking brighter in traditionally nuclear-shy parts of Europe. British Prime Minister Tony Blair, for example, has just launched a drive to promote nuclear as the key to making his country self-sufficient in energy.

But antinuclear activists have seized on nuclear plants' summer troubles as evidence of the energy's limitations.

Austrian protesters, including politicians, have demanded that the Czech reactor - which is located just over the border - be closed. In Germany, influential antinuclear groups reacted to Sweden's closures by calling for the closure of the country's 17 reactors, many of the same design.

"Global warming undermines the arguments we've always heard about nuclear power, that it doesn't damage the environment," says Stéphane Lhomme, spokesman for a French group, Sortir du Nucléaire, or Abandon Nuclear. "Nuclear is not saving us from climate change. It's in trouble because of climate change."

His argument may have more resonance in France than elsewhere because, with 58 reactors, France depends on nuclear energy for 80 percent of its electricity and is criticized by some for failing to diversify its energy resources.

Concerns about global warming are central to the debate in European countries over energy. And this summer's heat wave and droughts, like those in 2003, have added a new and possibly confusing element to that debate.