Did rogue network leak nuclear bomb design?

An infamous atomic smuggler may have had blueprints for a compact, sophisticated nuclear warhead, and that could mean that the world's proliferation problem is even worse than many experts had thought.

US officials have long declared the nuclear technology ring run by Pakistani scientist Abdul Qadeer Khan to be shattered. But revelations that a digitized bomb design turned up on the computer of an associate of Mr. Khan's show that US and UN investigators may not yet know everything Khan did, despite the fact that he has been under house arrest in Pakistan for years.

At the least, the US should now press Pakistan for direct access to Khan, says one expert. Secondhand intelligence reports may no longer be good enough.

"People seem to have taken the Bush administration line that we have rolled up A.Q. Khan's network. I don't believe it," says Jon Wolfsthal, a senior fellow in the international security program at the Center for Strategic and International Studies (CSIS) in Washington.

The design in question is for a relatively small and light warhead, apparently of Pakistani origin. Computer files containing the design were among the items seized by Swiss authorities from Swiss nationals Freidrich, Marco, and Urs Tinner in 2004, according to a report by David Albright, a former International Atomic Energy Agency (IAEA) inspector, made public on June 16.

The computer files were encrypted and difficult to decipher, according to Mr. Albright. At least one current IAEA official doubts that the Tinners, former associates of Khan's, were the only people in possession of the design.

The compact weapon would have been ideal for Iran and North Korea, notes Albright's report.

"They both faced struggles in building a nuclear weapon small enough to fit atop their ballistic missiles, and these designs were for a warhead that would fit," says the study.

US and UN investigators have long known that the Khan network sold to Libya a nearly complete set of blueprints and instruction manuals for a relatively basic nuclear warhead of Chinese design.

But it was apparently not until sometime last year that US intelligence was able to crack the Tinner family's codes and discover the hidden, second design. They were apparently not the only ones surprised by its existence in the hands of known nuclear smugglers.

The Pakistanis, too, were upset, according to the Albright study, because they recognized that the design was most likely their own. Although Pakistani warheads are also derived from Chinese originals, Pakistan has gone much further in developing the sophisticated electronics and triggering mechanisms necessary for smaller designs.

The Pakistanis "were genuinely shocked; Khan may have transferred his own country's most secret and dangerous information to foreign smugglers so that they could sell it for a profit," writes Albright.

In some ways, possessing a sophisticated warhead design would not help would-be nuclear proliferators all that much.

For one thing, it might take only a crude bomb, even thought it might be as bulky as a truck, to provide the strategic deterrent power of nuclear weapons. For another, designing a bomb is not the hardest part of obtaining a bomb. Producing or obtaining the fissile material necessary for the weapon's explosive heart remains a more daunting challenge.

Still, nuclear engineering is difficult. North Korea's nuclear test of 2006 was widely considered by Western scientists to be a fizzle. Even US national labs have occasionally designed warhead duds.

"It's tough to really get all those parts working right," says Charles Ferguson, a nuclear expert and fellow for science and technology at the Council on Foreign Relations in Washington.

And Iran and North Korea might have special reason to desire a small warhead, one that would fit atop medium-range missiles they are already developing, threatening neighbors and turning them instantly into regional superpowers.

Clearly, the US does not yet quite understand the reach of the modern model of proliferation: nonstate actors, motivated by profit, drawing on sources and contacts in many nations.

"Even if the Khan network itself is shut down, there still are nuclear black markets going on," says Mr. Ferguson.

Ferguson and other experts say that there is no public evidence that the US yet has pressured Pakistan to allow US investigators to personally interview Khan.

Khan remains under house arrest in Pakistan. On June 16 he denied being the source of the design found in the Tinners' possession.

"This is all a lie, there is no truth to it," Khan told Agence France-Presse by phone from Islamabad.

While the US can be confident that it has gotten at least a partial picture of the Khan network through indirect sources, sitting down in a room with the mastermind for a concerted interrogation could still be invaluable, says Mr. Wolfsthal of CSIS.

The Bush administration has not pushed for such access because Khan remains a hero to many Pakistanis, and the White House does not want to do anything to further risk the position of President Pervez Musharraf, says Wolfsthal.