France, which built Iraq the nuclear research reactor attacked by Israeli fighter-bombers last week, says it had no secret chamber for developing an atomic weapon.

"There are two things underground in the Osirak reactor," Bertrand Barre, chief nuclear attache of the French Embassy in Washington, told The Washington Post. "One is the drive mechanism for the control rods in the reactor; the other is something new called the neutron guide chamber. We think what Mr. Begin referred to the other day is the neutron guide chamber and there is nothing secret about it."

Israeli Prime Minister Menachem Begin has said Israel destroyed the $350 million reactor when it learned a secret installation had been built 120 feet below ground to evade detection by the International Atomic Energy Agency. Begin gave no other details about the chamber except to imply it would be used to begin work on an atomic bomb.

Barre said the neutron guide chamber, next to the reactor, goes about 10 feet undergrond. He said it is used in the plant's research on effects of neutron radiation. Technicians in the chamber target materials placed in a reactor at the bottom of a swimming pool with neutrons produced by the fissioning uranium fuel in the reactor.

Barre said the neutron guide chamber is new to the Osirak reactor, first of its type to be built outside France.

The drive motors for the control rods that start, speed up and slow down the chain reaction inside the swimming pool reactor are also built underground, Barre said, which is cheaper and not unusual for a research reactor. Since there are only 26 pounds of high-enriched uranium, a relatively small amount, in a large pool of water, and not under pressure at the Osirak reactor, there is no need to locate the drive rods in a special facility on the top.

Barre said he had no idea why Begin said there was a secret chamber below ground at this reactor where the uranium fuel or any plutonium produced by the unranium could be diverted to make an atomic bomb.

"We have built this reactor and every auxiliary piece of equipment that goes with it and there is nothing secret about it," Barre said. "If there was anything secret going on, our technicians would know about it, believe me."

Barre said Iraq signed a contract with France to have 150 technicians at the site for the next 15 years. He said all but a dozen were called home to France for their safety after the Israeli air attack killed one French technician.

Barre said many of the technicians had received anonymous threatening letters posted in France before the attack.

Barre confirmed that the concrete cupola covering the reactor was destroyed. He said damage to the reactor manchinery, pumps, valves and electronic controls was "extensive" but not so bad the reactor would have to be completely rebuilt.

"We expect to get a full damage report in about two weeks," Barre said, "but it appears that if Iraq wants us to rebuild it that it might take about three years and cost between $150 million and $200 million to do so."

Barre said there was no damage to the unranium fuel in the reactor, mostly because it was stored in a smaller reactor [called Iris] alongside the Osirak reactor. Just days before the Israeli attack, the fuel had "gone critical" to test its efficiency.

Technicians had planned to move the fuel by remote control late next month through a water channel to the larger Osirak reactor, where it would have built up large doses of radiation. The reactor was due for full start-up in August.

The French nuclear attache acknoledged that highly enriched unranium could be used to make nuclear weapons but insisted the French had tight safeguards over its use, had pre-irradiated it in the smaller reactor to make it too dangerous to handle for someone wanting to make a weapon out of it and that the Iraqis had never hinted of such a move.

Barre said the plutonium that could be produced would be so negligible that it would not be enough for a bomb. He said plutonium is best made in a power reactor that uses low-enriched uranium (6 percent U-235, the rest U-238) and not high-enriched uranium that is 93 percent U-235.