It's hard to say what specific event Chomsky was meaning since he didn't provide a specific reference, but he may have been referencing what's explained in this NYT article from 1992:
How did the Iraqis learn to use such specialized equipment? In large part from the United States Government. In August 1989, the Pentagon and the Department of Energy invited three Iraqis to attend a "detonation conference" in Portland, Ore. Financed by American taxpayers, the meeting brought together experts from around the world to explain to the Iraqis and others how to produce shock waves in any desired configuration. There were even lectures on HMX, the high explosive of choice for nuclear detonation, and on flyer plates, devices that help produce the precise shock waves needed to ignite A-bombs. Both HMX and flyer plates have turned up at Al Atheer, which should surprise no one. The three Iraqis who attended the conference came from the laboratory that eventually provided Al Atheer with its first shaped charges. [...]
Altogether, the Iraqis carried out 20 detonation tests before May 31, 1990 -- the date of the last Iraqi progress report on Al Atheer found by the United Nations. The Iraqis had worked their way through five versions of the bomb design, cutting the weapon's total weight from one ton in the first version to about half a ton in the last -- light enough to go on a missile.