The director-general of the International Atomic Energy Agency told his board of governors in closed session in Vienna last week that the Iraqi research reactor destroyed by Israeli warplanes was not capable of making plutonium for nuclear weapons.

Dr. Sigvard Eklund said it was "practically impossible" for the Iraqis to be making plutonium for weapons at the French-built reactor about 11 miles outside the capital city of Baghdad. A copy of his two-page declaration at the board's sime-annual meeting was obtained by The Washington Post.

Explaining why Israeli warplanes destroyed the reactor 10 days ago, Israeli Prime Minister Menachem Begin said the attack was made in self-defense before it began secretly to make nuclear weapons. Begin said that Israeli intelligence confirmed that Iraq would make nuclear weapons clandestinely.

Eklund said there are only two ways the Iraqis could make plutonium in the Osirak reactor. One would be to surround the reactor core of highly enriched uranium with a "blanket" of natural uranium that would produce plutonium after being bombarded by neutrons generated by the fissioning atoms in the core.

"The size and location of this blanket would be such that ordinary visual inspection would reveal its presence," Eklund said, adding that such inspections by IAEA personnel had revealed no such blanket.

Eklund said the other way Iraq could make plutonium at Osirak would be to locate a similar blanket of natural uranium directly beneath the reactor so that neutrons produced in the ractor would turn the isotope U-238 abundantly present in natural uranium into another isotope, U-239, which would then "decay" naturally to an isotope of plutonium called Pu-239 that is an optimal material for atomic weapons.

"The production of plutonium in this manner is practically impossible, since the core is placed on a thick concrete slab, which in turn is lined with heavy steel plate," Eklund said. "This provides shielding to permit maintenance work onthe drive motors located in a vault below the reactor."

Eklund said the shielding is enough to keep all but a few stray neutrons from leaking into the vault below the reactor, nowhere near enough to produce plutonium even if there were a blanket of natural uranium secretly located in the underground vault.

American nuclear experts say that natural uranium blankets must be in direct contact with fissioning uranium fuel to make any plutonium from absorbed neutrons. Both would have to be in a pool of water at least 60 feet deep to prevent them from contaminating the reactor room with radiation.

"The blanket can't be 20 feet away or even 10 feet away or the water will absorb all the neutrons," one expert innuclear proliferation said. "The blanket must be in direct contact with the fissioning uranium."

"In such a transparent pool reactor as Osirak," Eklund said, "the presence of undeclared fertile fuel elements (the natural uranium blanket) for plutonium production would be easily detected."

Eklund mentioned that a third way for the Iraqis to make nuclear weapons at Osirak would be by complete diversion of the high-enriched uranium fuel supplied by France to fuel the reactor. Eklund dismissed this possibility, too, saying that IAEA inspectors had examined the 26 pounds of fuel shipped to Osirak and found them intact without any signs of tampering. The fuel was in sealed bundles, Eklund said, whose weight can be precisely checked and seals examined to see if they have been broken.

"May I end by saying," Eklund said last Friday in Vienna, "that the presence of a large number (150) of technicians from the country which delivered the reactor could also be expected to provide some assurance."