The cold North Pacific waters lap quietly against the rocks a few yards from the Humboldt Bay power plant's now-defunct Unit 3.

The sea air chills the surrounding marsh and tall grass, but inside the square, windowless building, under 20 feet of water, thin rods of uranium 235 still give off lethal radiation, an unstoppable -- if now useless -- fact of nature that has drawn the attention of the national antinuclear movement to this bleak northern California coast.

With U.S. politicians in a quandary over where to put the used fuel from still-functioning atomic power plants, the nuclear age finally has reached the day when some plants are too old to operate at all, leaving hundreds of tons of contaminated concrete, steel and fluids that must be disposed of.

According to Department of Energy spokesman Dan Butler, more than 15 plants will be ready for permanent shutdown by the year 2000, increasing to 70 by 2010.

The old-fashioned, 63-megawatt reactor outside this city 275 miles north of San Francisco is the first major investor-owned U.S. nuclear plant to face dismantlement, or what nuclear engineers call "decommissioning."

Its case is all the more difficult because federal rules bar disposal of the most radioactive wastes from private facilities such as this one, no matter how threatened they might be by earthquakes or how fearful their neighbors are.

Humboldt Bay No. 3 closed in 1976 after 13 years of operation, sooner than the expected 30-year lifespan of most large commercial plants. But other plants, including the 27-year-old Shippingport plant near Pittsburgh, have also reached retirement age and must be disposed of along with the dangerous radiation they will carry for several thousand more years.

At a public hearing in early December, about 200 residents from this area of fishing boats, lumber mills and universities gathered to tell the Nuclear Regulatory Commission what they wanted done with Humboldt Bay Unit 3. Nearly all demanded that the entire structure, much of it underground, be dug up and carted away as soon as possible. Suggested disposal sites ranged from outer space to the ocean to the basement of the White House.

The Pacific Gas and Electric Co. proposed leaving the plant where it is for up to 30 years until a safe disposal spot is available. "There is no other option," said PG&E attorney Louis Vincent.

The plan, called SAFSTOR, would keep Unit 3 -- attached to four operating gas and diesel power plants -- under careful watch for leaking radiation and terrorist attacks until the Energy Department found a place to bury its contaminated fuel and equipment.

Because a national dumping spot is not expected to be found for two decades, many Humboldt County residents have expressed displeasure with the PG&E plan. Carl Zichella, 30, an energy policy analyst who has campaigned for years to have the unit removed from the area, complained of "artificial distinctions" in the federal government's ban on dumping of private industry waste.

The problem, he said, is that the Nuclear Regulatory Commission controls the future of the Humboldt Bay unit, but the Energy Department controls the rules for dumping nuclear wastes.

"We can't let something as important as Humboldt Bay fall into the middle of a turf fight," he said. "It has nothing to do with reality."

Several groups, including Zichella's Redwood Alliance, another antinuclear group called the Acorn Alliance, the Sierra Club Legal Defense Fund and the Friends of the Earth, have joined to push for Unit 3's immediate dismantlement. Prompt action is necessary, Zichella said, "because there are three earthquake faults within 4,000 feet of the plant."

More than 14,000 metallic-gray fuel rods, each six feet long and a half-inch thick, are stored in racks at the bottom of a 26-foot-deep underground pool inside Unit 3. The steady decay of the unstable uranium provides radiation and enough thermal energy to heat the rods to about 112 degrees Fahrenheit if they were left dry. But they no longer glow in the dark as they once did fresh out of the reaction chamber.

The plant's work force has dropped from about 90 to 79 since Unit 3 shut down, but one of four operators on each shift still monitors the unused unit's radiation. Its dormant turbines and other equipment are still maintained in case they are needed again, operations supervisor Brian Getty said.

Plant manager E.D. Weeks, who has been at the facility since its nuclear unit went into operation in 1963, argued that the plant was built to withstand strong earthquakes, and that any leaks that did occur would be trapped automatically. During the most severe seismic test of the unit's life, a 7.0-magnitude earthquake centered 30 miles offshore in 1980, Unit 3 emerged unscathed.

But earthquake fears forced PG&E to put aside any hope of restarting the plant last year, and they remain a lively issue in an area wrinkled with fault lines.

PG&E shut down the reactor in 1976 to refuel and to conduct more seismic studies to meet new earthquake-protection standards. The research was still dragging on when the 1979 accident at Three Mile Island in Pennsylvania produced a deluge of new federal regulations.

Reopening Unit 3 became more difficult and expensive than anticipated. The price of oil was dropping, reducing the advantages of nuclear power, and the company felt it was not getting adequate federal guidance on what was needed to make the old plant sufficiently earthquake-proof. Last year PG&E announced that it was not economically feasible to restart Unit 3.

Andrew Archibald, a part-time computer consultant working with the Redwood Alliance, said this history should convince the company of the need to remove the plant immediately.

"The seismic safety issue was not resolved enough to restart the plant, so why is it resolved enough to let the material sit there for 30 years?" he said.

While Unit 3 and PG&E's much larger and more modern nuclear plant at Diablo Canyon have come under fire from antinuclear activists, the company has insisted that atomic power still has its uses. Customers saved about $11 million in fuel costs during the 13 years of Unit 3's operation, said PG&E spokesman George Sarkisian.

Some critics charge that the utility wants to delay dismantling Unit 3 to protect the nuclear industry's tattered image. Once consumers realize the costs and difficulty of dismantling old nuclear plants, critics say, they will never again support construction of new ones.

Vincent, the PG&E attorney, called this argument "nonsense." The utility said it will spend, in 1984 dollars, $14 million to prepare Unit 3 for storage, $650,000 in annual storage costs and $54.8 million for eventual dismantlement. This would add less than 5 cents a month to the bill of an average residential customer, Sarkisian said.

Some advocates say the nuclear industry may someday pull out of its slump. Use of electricity jumped 5.5 percent nationally in the last year, a sign of new demand for power that may require more nuclear plants, said Donald Winston, spokesman for the Atomic Industrial Forum, which represents utilities, unions and investors.

Excess generating capacity dropped from 40 percent to 34 percent, Winston said. If it drops to 25 percent, he said, power company officials and politicians who ruled out any expansion of nuclear power "are going to start getting sweaty palms."

Although no new nuclear plants have been ordered since 1978, the United States still has 89 nuclear plants in operation and 38 more under construction. More than 70,000 metric tons of nuclear waste await federal approval of a national dumping site, now narrowed to proposed locations in Texas, Nevada and Washington.

At the Shippingport plant, engineers are preparing to ship the defunct plant's parts for burial at the Hanford Nuclear Reservation in Washington, the only place in the country where residents show any enthusiasm for the role of nuclear garbagemen. The plant is federally owned, and so is eligible for dumping of its most radioactive wastes in a federal facility. The plant's spent fuel will go to another federal facility in Idaho for research testing.

Unlike Eureka residents, some neighbors of the Shippingport plant have complained about the dismantlement, fearing that it may carry a greater risk of spills than leaving the defunct plant alone.

Zichella and other Eureka activists say they understand this view, but prefer dismantlement for Humboldt. They say they are encouraged both by Shippingport's demonstration of the feasibility of dismantlement and by Hanford's willingness to accept a contaminated, broken-down nuclear plant.

"I'm glad someone wants it," Zichella said.