ADSWORTH, Tex., April 30 — Reactor experts around the country hope that there is something unique about Reactor No. 1 at the South Texas Project here. If not, the little crust of white powder that technicians found at the bottom of the reactor vessel, a discovery that has brought operations here to a halt for the indefinite future, could be the beginning of a broad problem for the nuclear power industry.

The powder, which managers here repeatedly compare in volume to about half an aspirin tablet, is boric acid, which is used in reactor cooling water to soak up excess neutrons, and its presence under the vessel presumably means there is a leak.

Highly corrosive when damp, boric acid has been found in the last few years on the lids of reactor vessels around the world. A plant near Toledo, Ohio, accumulated 900 pounds, some of which ate away a football-size chunk of steel in the vessel lid, leaving only a thin stainless-steel liner and bringing the plant uncomfortably close to accident.

But until the discovery here, on April 12, nobody had ever seen a leak on the bottom. A leak in that location is far harder to repair, and would be harder to control if a significant hole developed in the vessel, although the chances of accident seem far smaller than they did in Ohio.

"It is something different," said Gary Parkey, vice president of the South Texas Project Nuclear Operating Company, which runs the two reactors here.

Measuring the problem and then resolving it will take new applications of technology, he said, adding with no evident pleasure, "We are at the cutting edge of this issue."

Until the discovery, in an inspection during a routine shutdown for maintenance, the Nuclear Regulatory Commission believed that it understood the mechanism for leaks in reactor vessels. It assumed that such leaks were caused by an occurrence called stress corrosion cracking, which, after long periods of operation, develops in hard metals that are under strain and high temperature.

Not long ago, the commission developed a formula combining temperature and years of operation, and used it to tell reactor operators around the country whether they needed to shut down promptly for inspection or could do the job at a more convenient time.

But the South Texas Project, here amid beef cattle and wildflowers 90 miles southwest of Houston, is only 15 years old, and its reactors operate at a relatively low temperature. That raises the possibility that there may be a problem even with plants that scored well in the regulatory commission's formula, and have not been inspected.

"If this turns out to be stress corrosion cracking, and there's nothing unique about it, then it raises questions about the validity of that equation," said Brian W. Sheron, the commission's associate director for project licensing and technical analysis.

That would be bad news for the nation's 102 other commercial power reactors, which despite vast electricity deregulation have prospered in the last few years, by achieving new levels of reliability.

The South Texas Project boasted last year that its Reactor No. 1 generated more electricity than any other in the nation in 2001, and ranked eighth among the 433 power reactors worldwide. A majority of the plant is owned by two commercial companies: Reliant Energy and an American Electric Power subsidiary, AEP Central Power and Light; an additional 44 percent is owned by the municipal utilities of Austin and San Antonio.

The plant's operators underscore that they caught the problem early.

"There was no puddle, no buildup of boric acid on the bottom," said the general manager, Ed Halpin.

David Lochbaum, a nuclear safety engineer at the Union of Concerned Scientists, a group often sharply critical of nuclear operators, also pointed out that the leak had been discovered early, but in an inspection, he said, that was more thorough than the regulatory commission requires.

"It does show the prudence of looking periodically in places you don't expect to have problems," he said, "rather than blindly assuming you're not going to have problems except where you're looking."

One difficulty at the Ohio plant, Davis-Besse, was that management delayed taking the time to remove thermal insulation around the vessel lid to check for leaks. As a result, corrosion continued unnoticed for years. That corrosion has been a nightmare for the Davis-Besse owners, keeping the plant shut for 14 months so far and probably some months to come.