Experts throw doubt on container screening plans.

As the Department of Homeland Security continues efforts to devise efficient ways to detect nuclear material or bombs entering U.S. ports through shipping containers, two experts said it is more likely that such a bomb would be assembled inside the nation's borders.

Peter Zimmerman and Jeffrey Lewis raised eyebrows in November when they published an article in Foreign Policy Magazine that described how such an operation would take place.

While a crude, but effective nuclear bomb could be constructed in the United States, it's still likely that a terrorist group would have to smuggle uranium 235 through the border, they said. That may not be as complicated as it sounds.

"Highly enriched uranium is probably one of the most difficult to detect radioactive materials that we know," Zimmerman, a professor of science and security at the department of war studies, King's College, London, told a gathering of military writers.

"It has very little neutron, very little gamma emission ... You can shield it with a few thicknesses of aluminum foil basically." The amount needed to build a bomb wouldn't even upset the balance of a cargo container, he said.

Its high density will stand out, but that requires an active x-ray machine to scan every container.

"I have to say that I am not optimistic that current efforts to inspect and scan will have any payoff against highly enriched uranium," said Zimmerman, who once served as the chief scientist at the U.S. Arms Control and Disarmament Agency.

DHS Secretary Michael Chertoff said during a press conference that scanning for density, along with the radiation leakages, will give Customs and Border Protection a new tool that "dramatically enhances our ability to identify containers" that need to be inspected.

"At the end of the day, though, here's the kicker," Chertoff added. "When in doubt, we pull it out. And then we open it up and we look at it. And this is basically a way of selecting among the millions of containers that are on the way to the U.S., those that require a closer look."

Zimmerman said using x-ray scanners may pose a health hazard, and slow down commerce.

Lewis said the United States should put more resources in its efforts to secure overseas facilities where uranium is enriched. While there has been progress in upgrading security at such facilities, the threat may come from within. A lab worker, for example, could be bribed to smuggle out such material.

Email your comments to [e-mail]