London ? It feels pretty strange to sit opposite a man who once worked at one of Saddam’s biological weapons plants.

Ahmed left Iraq in 1999 for an Arab country; he spoke to me while visiting London and disguised his name because he still has relatives in Iraq. His story reveals how hard it will be to get Iraq to come clean about weapons of mass destruction — and what inspectors need if they are to succeed.

Ahmed was assigned in 1992 to help design huge vats — or fermenters — at an unmarked complex of buildings called Al Hakam, 60 miles south of Baghdad. The vats, he was told, were meant to produce specialized proteins.

He had suspicions about the project, which made no scientific sense, but he got no answers from the head of the program, Rehab Taha.

Taha, now known as “Dr. Death” for her work on bioweapons, “was very aggressive and so people avoided talking to her.” Asking questions wasn’t wise.

After the gulf war, U.N. weapons inspectors found circumstantial evidence that Al Hakam was a bio-weapons facility, though Iraq insisted it produced an animal feed supplement. “The inspectors visited quite often,” Ahmed told me, “but the regime kept the real capacity of the plant hidden.”

Scientists and engineers “were coached” on how to deal with inspectors, Ahmed says. “We were asked to hide things, to give as little information as possible, to omit things and emphasize others, then to give a detailed report on what we had said to security officials.”

Ahmed and other staff had to sign a paper swearing they possessed no documents about the program and that they recognized that if they were found to have any, “we would be killed.”

Only after the defection of Saddam’s son-in-law and mastermind of the bio-weapons program, Gen. Hussein Kamel, could inspectors confirm what they had suspected — that Al Hakam was producing anthrax and other biological horrors. The plant was razed in 1996.

“You need another defector,” insists Ahmed. “The regime knows how to hide things. They have had lots of practice.” Without a defector, Ahmed doubts U.N. inspectors can succeed.

U.S. officials think likewise, and are pushing a U.N. Security Council requirement that Iraq make weapons scientists available to inspectors — outside Iraq and with their families, if need be to give them the courage to speak.

That may be the only way to find out what happened to huge stocks of biological agents — anthrax; mustard, gangrene and VX gas; and artillery shells to deliver these poisons — that Iraq had before inspections were halted in 1998. Both chief U.N. inspector Hans Blix and Colin Powell complained Thursday that Iraq has failed to verify its claims that these weapons were destroyed.

But talking to Ahmed makes clear that getting weapons scientists to speak won’t be easy, and may not provide a silver bullet. As in his case, many scientists have limited knowledge of the programs they work on. “You would need a high level defector,” Ahmed says.

Inspectors would also have to ensure the safety of any scientist who talked. “You would have to find a safe country for scientists to live in,” Ahmed says. “Not an Arab country.” He is still fearful, even though he never gave any unauthorized information to inspectors.

And the safety of a scientist’s entire clan would have to be ensured. “Saddam Hussein would kill his brothers, sisters, uncles, cousins” if a weapons expert talked, says Ahmed. Can U.N. inspectors spirit out a busload of relatives, or find a defector who will risk his extended family?

In the end all may come down to a question of conscience. Ahmed doesn’t regret helping build facilities for Iraq’s onetime nuclear project in the 1980s — he says Israel has a bomb, so should Iraq — but his work on the bio program clearly weighs on his mind.

“I fear they will use it not against Americans but against local people,” he says, reflecting Iraqi fears that if Saddam goes down, he will gas his own people and blame the casualties on the United States.

Somewhere, today, a senior Iraqi weapons scientist may be asking himself if he could bear that on his conscience. This may be the best chance of unmasking Saddam’s weapons without a dangerous war.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer. Her e-mail address is trubin@phillynews.com.