AN CLEMENTE, Calif., May 11 — A Sunday on San Onofre State Beach is a step into the idyllic 1960's Southern California of Gidget movies. Below a low sandstone bluff, a half-mile of cars, many of them classics, line a palm-fringed shore. Around thatched-roof huts, surfers strum ukuleles, grill burgers or prepare to ride the celebrated waves.

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This vision of paradise almost obscures another vestige of the 1960's rising from the surf a few hundred yards south. There, two nuclear reactors quietly split atoms and churn out 20 percent of Southern California's electricity.

It has been like this since the San Onofre Nuclear Generating Station opened in 1968. The surfers, campers and residents of San Clemente and other nearby towns have largely accepted the plant as an unobtrusive, if unwelcome, neighbor. But since Sept. 11, security concerns and a proposal for a long-term repository for spent nuclear fuel have raised alarm.

"We want to believe San O is safe, and that the palm trees, blue sky and waves are the reality," Steve Netherby said on a recent walk around the plant. "Unfortunately, the reality is a lot more dangerous."

Mr. Netherby is a former editor at Field & Stream magazine and co-founder of San Clemente's Coalition for Responsible Ethical and Environmental Decisions. He points out that San Onofre lies amid six miles of popular state beach and south of growing population centers of southern Orange County. A quarter-mile to the east runs Interstate 5 and a coastal rail route. Beyond that sprawls Camp Pendleton, a Marine base.

The plant's owner, Southern California Edison, and the Nuclear Regulatory Commission say the plant is safe and secure.

At a public meeting, the regional chief of the commission, Kriss Kennedy, said of plant security: "There have been examples in the past where we've been very critical of facility operations, but in this case, San Onofre has done a good job."

Yet Mr. Netherby remains skeptical. Despite the presence of guards wielding M-16's, he walks unchallenged through an unsecured parking lot overlooking the site, past several employees. He points out the enormous turbines and transformers, and the functioning Unit 2 and 3 reactors, and what appears to be a hole in the side of the decommissioned Unit 1.

He wonders what would happen if a van drove into the lot and a terrorist launched a shoulder-fired missile. "It's a target down there. And that makes all of us here in Southern California a target," Mr. Netherby said.

Unit 1 is being demolished at a cost of $600 million. Its site is now proposed for a "dry cask" waste storage system that would hold spent nuclear fuel.

A San Onofre spokesman, Ray Golden, said the dry casks offer far greater security and earthquake protection than the system used now, adding: "The spent fuel is moving from a pool, which requires human intervention, electricity and other features, to a completely passive design with no mechanical components. If you painted that scenario, I think most people would say, `Hey, it sounds like you should put it in the passive design.' "

Project opponents agree that the dry casks are somewhat safer, but question assertions by the Nuclear Regulatory Commission that these systems can withstand earthquakes. They also worry that the project would lead to a vast, long-term increase above the several hundred tons of stored waste already on site.

Mr. Netherby's group is beginning "an extensive effort" to make residents aware of the security threats at San Onofre, the dangers of stored fuel, and the risks posed by earthquakes and earthquake-spawned tsunami waves.

The group is also asking that local towns begin storing potassium iodide pills as a radiation antidote, that Camp Pendleton troops be assigned to San Onofre to augment security, and that a loudspeaker system be placed on area beaches alongside existing sirens. They also want the Federal Aviation Administration to revisit its recent lifting of a 10-mile no-fly zone around the plant.

Meanwhile, on the beach the party is in full swing. But it appears that after passing an unattended State Beach guard kiosk and driving to the south end of the beach, the only thing that would prevent an attacker from reaching the sea-wall road fronting the plant is a "no vehicles" sign.

Are beachgoers concerned?

Daniel Dowden, a San Onofre Surf Club member, points to two recent security breaches at the plant and accidents involving a fire and a construction crane.

"It's a plant run by human beings who've made a lot of mistakes already," Mr. Dowden said. "I don't say they're dumber than anybody else, but they're certainly as dumb as the rest of us, and they're going to make mistakes. I'd rather those mistakes be out in the desert somewhere where nobody's around than right here on the beach where we're completely exposed."

Paul Strau is a Hawaiian surfer who holds a mini-luau with his friends here every Sunday.

"Even with the danger, you still come down to the beach to enjoy the ocean," Mr. Strau said. "It takes your mind off the stresses of the day-to-day world.

"But looming right over the bluff is this edifice that says, `I could take all of you out real quickly.' It's scary."