Emptied of most of its innards, filled with concrete and encased in a 40-foot steel canister, a decommissioned nuclear reactor here is headed for a nuclear graveyard in South Carolina.

Or is it?

Officials here at the San Onofre Nuclear Generating Station, where for 24 years the reactor produced electrical power, have been stymied in every effort to find a path across the country for the reactor's shell, which weighs 770 tons and displays mild radioactivity readings. Railroads, ports, environmental groups and antinuclear coalitions -- even the Panama Canal -- have thrown up roadblocks.

Initially set to depart last month, the reactor might not leave until the end of the year, possibly on an 11,000-mile sea voyage around Cape Horn, at the southern tip of South America, and northward to whichever East Coast port chooses to accept it. So far, none have. And that trip will happen only if the Chilean government permits the reactor's passage through its territorial waters. Another option is to go west from California, around the world.

''This thing is the proverbial garbage barge,'' said Mark Massara, the San Francisco-based director of coastal programs for the Sierra Club. ''This is a dangerous coastal transport project of unprecedented proportions.''

While that assessment may be overblown, the debate over moving the San Onofre reactor is a sign of trouble to come as huge amounts of nuclear waste -- most of it spent uranium fuel, far more radioactive than the shell of an old reactor -- travel from 70 sites nationwide to the Yucca Mountain nuclear waste disposal site in Nevada if it opens, as projected, in 2010.

Normally licensed by the Nuclear Regulatory Commission to operate for 40 years, at least a dozen commercial-scale nuclear reactors have reached or are nearing the end of their useful lives. In all, 103 remain in operation out of 123 built since the late 1950's. Only a handful have been dismantled, in most cases quietly. But as more reactors reach retirement age and are decommissioned, the quandary of how to get them to their graves is becoming increasingly pressing.

''There's a phobia because people don't have much scientific grounding in nuclear power,'' said Ray Golden, a spokesman for the San Onofre plant's majority owner, Southern California Edison. ''We move radioactive material in this country every day: We fly it, we drive it in vehicles, we ship it by rail, by boat. There's really nothing novel in this.''

Mr. Golden said the plant, which still operates two newer reactors that provide electric power to 2.3 million homes and businesses in Southern California, had already shipped 10 million pounds of pipes, pumps, valves, motors and concrete from the site to a low-level nuclear waste dump in Clive, Utah, by road and rail, without incident. The Clive site also received five large components of the reactor: its 90-ton head; a pressurizer of similar heft; and three steam generators weighing 250 tons each.

The reactor itself, which was shut down in 1992, will become part of a 950-ton assembly once it is bolted to a 192-wheel trailer pulled by a truck in front and pushed by another from behind, all of which will travel together, whether by train, road or ship. The package must go to Barnwell County, S.C., where a nuclear waste dump is licensed to handle more elevated radiation levels than those at Clive. As it awaits a route, the canister holding the decommissioned reactor barely shows signs of radioactivity, Mr. Golden said.

''A CAT scan in a hospital will give you at least 1,000 millirems of exposure in a matter of minutes, whereas a person straddling this package for an hour will get the equivalent of 5 millirems -- like X-raying a tooth,'' he said. ''Of course, no member of the public will be anywhere near it, so there will be no exposure.''

Opponents of the move fear several possible situations, including terrorism that they say could cause the reactor to contaminate a large area.

''An attack could more easily sink the barge in the river or the ocean,'' said Daniel Hirsch, head of the Committee to Bridge the Gap, a nuclear watchdog organization. Mr. Hirsch said he was dismayed that Southern California Edison was struggling to get the reactor off its oceanfront site and across the country, a $10 million job.

''It's the kind of thing you'd think they would have thought about before they built this thing,'' he said.

But plant officials insist the obstacles are not insurmountable.

''This has been contemplated from the day we started,'' said James T. Reilly, who oversees the reactor's decommission. ''The technical and engineering problems are fairly straightforward. We have permit and governmental approval issues, not engineering issues.''

Edison's first choice for transporting the reactor involved loading it onto a train to Houston and shipping it from there on a barge around the Florida Keys and up the Eastern Seaboard. But that plan fell apart last fall in a conflict with the Burlington Northern Santa Fe Railroad over who would assume liability in case of an accident.

Next, the utility devised a trip through the Panama Canal, but the authorities there said regulations limited the weight of radioactive cargo to 150 tons, and they refused to let it through.

There was opposition closer to home, too.

Since the Panama Canal plan and the more recent proposal to ship the reactor around Cape Horn involved putting it on a boat, the package would have to be trucked along eight miles of beach to a Marine Corps dock in Oceanside. But environmentalists expressed concern about the beach and two species of birds that nest there.

''Nobody's ever tried to maneuver something like this across such a wide swath of significant coastal habitat in Southern California,'' said Mr. Massara, the Sierra Club official.

In Charleston, S.C., where Edison hoped to unload the reactor for a 300-mile train trip to the burial ground, the South Carolina State Ports Authority told a dock operator who had won a bid to handle the cargo that his lease prohibited the presence of hazardous materials.

''The key issue is safety and security,'' said Byron Miller, a spokesman for the ports authority. Edison officials are looking into alternative ports, including at least one in Georgia. For now, the impasse means that the reactor can neither leave here nor arrive there.

In a rare piece of good news for the utility, the California Coastal Commission last month approved Edison's plan to move the reactor along the beach to Oceanside. But Pedro Nava, a panel member, said, ''This is like the guy who built a yacht in his basement and can't figure out how to get it out the door.''