ROWE, MASS. -- How long is the useful life of a nuclear power plant? Yankee Atomic Electric Co., which operates the nation's oldest commercial nuclear reactor here, is about to find out, in a test case for the nuclear industry.

Like all U.S. nuclear plants, Yankee Atomic is licensed by the Nuclear Regulatory Commission (NRC) to operate for 40 years. Its license will expire in 2000, followed in the next 14 years by the licenses of 49 other plants that generate about 40,000 megawatts of electricity.

Because it takes five to 10 years to plan and build any full-size power plant, nuclear or not, the owners of those plants need to find out soon whether they can extend their operating lives or will have to replace them.

This week, the NRC is scheduled to publish proposed requirements for extending operating licenses by as much as 20 years. Yankee Atomic intends to be the first applicant.

The nation already faces an electricity generating shortage of as much as 100,000 megawatts -- far more than the total current output of Potomac Electric Power Co. -- in the next 10 years. Nuclear power accounts for nearly 20 percent of the nation's electricty, but no new plant has been ordered since 1978.

The NRC's proposed rules will help utilities decide whether they can keep their aging nuclear plants operating or will have to replace them, at a cost of tens of billions of dollars.

"Every utility will have to make judgments," said Yankee Atomic president Andrew D. Kadak. "Do they have any excess capacity? Is it cheaper to maintain and refurbish than to build?

A complicating factor is the rising cost of operating and maintaining a nuclear plant as it gets older. What kind of capital investment will you need to keep the plant operating safely?"

The technical, safety and operational requirements that the anticipated NRC rules impose will enable each nuclear operator to determine whether it is cheaper to refurbish an existing plant or to shut it down at the end of its licensing period and replace it with some other source of power, Kadak said.

"For us, renewal appears to be quite competitive," he said. "Everything we know about this plant, its operating performance, the ability of its components to run, says it's a go. For other utilities, it's too early to tell."

Nuclear plant operators and regulators agree that the 40-year licensing limit, adopted when nuclear power was in its infancy after World War II, is essentially arbitrary, based on the economics of depreciation rather than mechanical experience. No commercial plant has operated for the full 40 years, so nobody knows what happens after that.

"If a plant is okay for 39 years and 364 days, is it automatically unsafe on the 365th day just because somebody all those years ago picked 40 years?" said NRC Chairman Kenneth M. Carr.

Completion of the license extension regulations is "one of my highest priorities," Carr said. "If utilities can't extend their licenses, they need to know it soon enough to get into the planning cycle for something else. Some of them may decide they can't get there, to relicensing, from here."

"Only the utilities that run well and make money" will seek license extension, Carr said. Operators of nuclear plants with a history of accidents, shutdowns or expensive maintenance may let their licenses expire, he said. Others may apply for extensions of less than 20 years, hoping that a new generation of simplified, cheaper nuclear reactors will have been developed and approved in the meantime.

The NRC does not need action by Congress to approve license extension. "The law already permits it," Carr said. "The question is, is it safe and for how long? And what do we expect of the utilities?"

Publication of the license extension rule will not come as a surprise to the nuclear utilities. NRC officials have been working with the industry, including the staff of Yankee Atomic, for several years to develop requirements designed to permit extension and ensure safety.

Kadak said Yankee Atomic is a good candidate for renewal because the plant here has one of the industry's best records of safety and reliability. When it was shut down for refueling last week, it had operated for 297 consecutive days at more than 90 percent of maximum output, plant officials said. Some of its components look like exhibits in a technology museum -- including instruments with vacuum tubes -- but many pumps, valves, pipes and safety devices have been upgraded, and its reported operating performance has been improving as it ages.

Yankee Atomic is a small plant. Its capacity is 185 megawatts, about one-fourth of what newer facilities can produce. It is also a unique design in which all components, including the reactor and its radiation containment area, are encased in a steel sphere that stands on pillars 25 feet above ground.

But design is not the issue in the NRC rulemaking proceedings. The issue is which components of a nuclear plant will have to be replaced or upgraded to continue operating. Theoretically, Kadak said, any part of a plant can be replaced, even the reactor vessel itself. The question is whether the return will justify the expenditure.

According to a "briefing paper" by the American Nuclear Energy Council, "the greatest variable in the cost of refurbishing is whether the reactor pressure vessel must be replaced, which could amount to as much as 15 percent of the cost of a new nuclear energy plant."

Opponents of nuclear power are wary of license extension.

"It's basically like rotating worn-down car tires," said Scott Denman, executive director of the Safe Energy Communication Council. "Based on our assessment of the increasing costs of operation and management as a plant ages, it makes more sense to phase the plant out at the end of its lifetime and go with other alternatives."

Denman said that license extension would ensure that "some fundamental problems won't go away," including the continued generation of nuclear waste for which the Energy Department cannot find a permanent repository.