Mysterious cracks have appeared in the piping that supplies cooling water to atomic power plants in West Germany and Japan, raising new safety questions about similar nuclear plants in the United States.

So serious does the Nuclear Regulatory Commission consider these cracks that it has reconstituted a Pipe Crack Study Group of 10 experts it put together three years ago when 18 pipe cracks appeared in six U.S. nuclear plants.

"What surprises us about the new cracks in Germany and Japan is that they've shown up in very large diameter pipe, up to two feet across," said Harold Denton, director of the NRC's Office of Nuclear Reactor Regulation. "This is the first time we've seen cracks in such large diameter pipe,"

If cracks in large diameter pipes worsen and become leaks, so much water might flood from a nuclear plant's cooling system that it might be unable to cool adequatedly the burning uranium in the reactor core. Though the possibility is remote the core could begin to overheat under such conditions and possibly even begin to melt before an emergency cooling system could be activated to augment the leaking pipes.

The large-diameter pipes of the type that have cracked in Japan and Germany serve as the nuclear power plant's primary cooling system. The pipes carry water that circulates around the reactor vessel, cooling it down and removing excess heat generated by the burning uranium inside the vessel.

The fresh cracks have appeared in 12-inch pipes in three Japanese nuclear power plants and in 24-inch pipes in one German plant. NRC officials have expressed the most concern over the Germany cracks, in part because they're in three different sections of pipe.

The cracks are all small, not very deep, and have not resulted in any leaks, but the Japanese Agency of Natural Resources and Energy has ordered two electricity companies to close down their three plants to replace the cracked pipes. The German Reactor Safety Commission has not ordered the pipes to be replaced but it has closed down the 237,000-kilowatt plant near Munich to study the cracked pipes and ivestigate the cause.

The NRC has sent experts to Japan and Germany to study the cracks, talk with nuclear engineers about how they found them and what they believe the cause is. U.S. nuclear engineers at first expressed disbelief when the German cracks were discovered two months ago.

"There was a theory widely held that large stainless steel pipes could not crack," said Darrell Eisenhut, deputy director of the NRC's division of operating reactors, "but there's no question that the cracks in the German pipes are real. I've seen them."

Curiously, the discovery of the pipe cracks in Japan and Germany came on the heels of a fresh round of cracks found in much smaller pipes in two American plants.

Cracks were found in eight pipes of 10-inch diameter in Iowa Electric Light & Power Co.'s nuclear plant at Cedar Rapids and in one pipe at the Brunswick plant of Carolina Power & Light Co. near Wilmington, N.C. The crack in one pipe at the Iowa plant was big enough to have sprung a leak. The plant was closed to repair the leak and the cracks in the seven other pipes.

Despite the leaky pipe in Iowa, the cracks in the two U.S. plants are not viewed as seriously as the cracks in Germany and Japan. First, the U.S. pipes that have cracked are made of Inconel, a steel more susceptible to cracking tha n the stainless steel used in the cracked pipes abroad. They're also in smaller pipes that are not as essential to the cooling systems as the large-diameter pipes that have cracked abroad.

"No large-diameter pipes have cracked in the U.S.," the NRC's Denton said. "We have ordered inspections of the pipes to make sure there are no cracks but so far we've seen nothing."

All the cracks in the U.S., German and Japanese pipes have appeared near welds. Nuclear engineers note that welds are the weakest places in any pipe since they've suffered heat damage at the spot of the weld.

The cracked pipes are all in plants of the same type, the so-called Boiling Water Reactor. In these reactors, the coolant (water) circulates just at its boiling point. Partly because of this, the water carries more dissolved oxygen than in other reactors, making the pipes more subject to corrosion.

What surprises engineers is that stainless steel pipes of large diameter have cracked. They were not supposed to crack because of their strength and because their size was supposed to make them resistant to the weaknesses welding introduced to smaller pipes.