WASHINGTON — President Bush signs into law tomorrow a port-security law that doesn’t address what security experts and American lawmakers fear the most: terrorists placing a nuclear or “dirty” bomb in a shipping container and detonating it upon arrival in America.

The law, passed by Congress on September 29 with bipartisan support, requires incoming cargo at the 22 largest American ports to be scanned upon arrival by the end of next year. What isn’t in it is more significant than what is, Stephen Flynn said.Mr. Flynn is a senior fellow at the Council on Foreign Relations in New York and a former National Security Council official under President Clinton. Mr. Bush and Congress are “creating the illusion of an important new layer of security, when in fact they do not address the worst-case scenario,” Mr. Flynn said.

He and others say the only protection against an in-port attack before scanning is to check containers while they’re still overseas — a massive undertaking, given that 12 million containers are shipped to America every year from 704 ports in 147 countries. The director of the Domestic Nuclear Detection Office in the Homeland Security Department, Vayl Oxford, said it would take between five and 10 years to install radiation detectors and image scanners in the 100 biggest overseas ports. That would allow America to scan 90% of the incoming cargo.

The Bush administration is caught between competing pressures — to improve homeland security on one hand, and on the other to avoid imposing standards that would tie up trading routes on which companies such as Wal-Mart Stores Inc., Nike Inc., and Lowe’s Cos. rely.

“You could cause a total constipation of the system,” Mr. Flynn said. That would inadvertently hand terrorists a victory — the slowdown of the $1.1 trillion in trade that moves through American ports annually — without ever exploding a bomb. The Congressional Budget Office estimated in March that an incident closing the ports of Los Angeles and Long Beach for a year would reduce America’s gross domestic product by $70 billion, or 0.5%.

While the measure Congress passed requires American officials to establish pilot projects within a year to scan all American-bound cargo in three foreign ports, the Homeland Security Department hasn’t announced the design of the programs, or what companies will help implement them. Oxford said it isn’t clear how quickly such programs, even if successful, could be duplicated elsewhere. “This could either blossom or it could be a long slog,” he said.

A separate pilot project already under way in Hong Kong’s port provides a glimpse of the promise and problems American officials face.

SAIC Inc., a San Diego-based defense contractor, teamed up with Hutchison Port Holdings Ltd. and Modern Terminals Ltd. to install radiation detectors and gamma-ray imaging machines flanking two of the port’s 40 incoming lanes. Trucks laden with goods drive through the scanners at 10 miles an hour, allowing the machines to scan 3,500 to 4,500 containers a day. Senator Schumer, a Democrat of New York, was so enthusiastic after he toured the Hong Kong operation that he tried to add an amendment to one piece of port legislation requiring all cargo to be scanned overseas in four years. With the backing of shippers and port-authority lobbyists, congressional Republicans defeated the requirement on the grounds that the Hong Kong system needs to be more thoroughly tested before it’s expanded. It’s an assessment some security experts agree with.

The Hong Kong program “should be examined to see if it could be made sufficiently rugged to be useful,”a senior research scholar and director of the Center of Integrated Security Logistics at the University of Maryland in College Park, Kenneth Gabriel, said. “We really need to take our time on this.”

The program was designed only to see if the containers could be scanned; no one is looking at the thousands of images the machines have created.

American officials also face more mundane problems. Some ports, for instance, don’t allow truck drivers to stay in their vehicles during the current inspection process and may require a machine to tow one truck after another through scanners.

“Subtleties that are overlooked sometimes become operational nightmares,” Oxford said. He said a plan for a U.S. overseas scanning system, which will be outlined in a broad strategy the Homeland Security Department will unveil by the end of the year, would be based on a mix of different technologies and pilot programs.

The plan may include radiation detectors that can specify what molecules in a container are emitting radiation. That might lower the number of false positives that delay shipments.

Scanned images and a container’s radiological profile could be transmitted to analysts in America. Rather than hire armies of analysts to examine every image, software could screen most containers flagged by the scanners, funneling only the most suspicious pictures to U.S. Customs workers.

Working out the technological and logistical kinks is more important than throwing together a security process just to make Americans feel safer, Mr. Gabriel said. “The implication of deploying the wrong system can be really bad,” he said. “You’ll have a false sense of security.”