As the nation goes about the unpleasant and still unfamiliar task of prioritizing threats, consider this: The country's 103 commercial nuclear plants, including south Orange County's San Onofre, are required only to withstand an assault by several people on foot and one person operating inside. The plants are engineered to prevent meltdowns and withstand earthquakes and other natural forces, and the Nuclear Regulatory Commission compels reactor operators to comply with volumes of regulations against accidents and attacks. But those requirements did not anticipate assaults by boats, aircraft or explosives-laden trucks, and it never expected plants to fend off an attack by "an enemy of the United States." Then came Sept. 11.

The NRC has already stepped up on-site patrols with well-armed guards, asked plant operators to check all current and past employees and visitors against an FBI watch list and to search incoming vehicles for explosives. It has also launched a "top-to-bottom" review of plant security. That review, however, should be coordinated with the new Office of Homeland Security, which should simultaneously encourage immediate outside action. Our suggestions:

The Federal Aviation Administration should, at any site where it would be possible, impose a no-fly zone. Today. The agency recently asked pilots to avoid the airspace over nuclear reactors, dams, refineries and other facilities "to the extent practicable." That's not good enough.

The Coast Guard should install the protection of nuclear plants on navigable waters among its highest priorities. If it lacks the resources to do so, President Bush should place the guard under the auspices of the Navy, as happens in wartime, and add personnel and ships accordingly.

Gov. Gray Davis should follow the lead of the governors of New York and New Jersey and request prompt deployment of trained National Guard troops to patrol California's two operating nuclear sites--Diablo Canyon, near San Luis Obispo, and San Onofre--as well as two nonoperating plants that continue to store nuclear waste--Rancho Seco, outside Sacramento, and Humboldt Bay, near Eureka.

San Onofre, like all reactors, is already a "hardened" target, extremely uninviting to most would-be assailants. Which is not to say that a cell of terrorists might not see vulnerability at this plant, which overlooks a popular beach and campground and is just off Interstate 5. If a worst-case attack were to trigger a major release of radioactive material, the immediate impact on Orange and San Diego counties would be devastating. Ultimately it might cause cancers and genetic defects and render a significant swath of the region uninhabitable. It's hard to imagine a threat that merits more attention.