The French-built nuclear reactor that was demolished Sunday in Iraq by Israeli warplanes was to be loaded with uranium fuel and begin operations in less than a month.

Sources close to the nuclear industry in the United States said the imminent loading of highly enriched uranium into the $100 million reactor, which was supposed to be for research, is one reason the Israelis chose to destroy it when they did. If Israel had waited to attack the reactor until after its uranium fuel, which was supplied by the French, "went critical," enough radiation would have built up inside the reactor to contaminate the nearby region for months.

Even so, American nuclear experts suggest there might be more than minimal radiation damage to the Iraqi site at Tuwaitha, only 12 miles from the capital city of Baghdad. They give two reasons.

One is that the 25 pounds of highly enriched uranium fuel that was to be loaded into the reactor is weapons-grade and apparently had been pre-irradiated by the French to "poison" it with radiation so it would be too hot to tamper with.This is to prevent the Iraqis, who had enough to make the equivalent of at least one atomic bomb, from diverting the fuel from power generation to weapons manufacture.

The uranium fuel was believed to be housed in underwater storage tanks in special cages to keep it from starting a chain reaction before being loaded into the reactor. Whether or not the tanks were damaged and leaking radiation into the air could not immediately be determined.

The other possible cause of radiation was that some of the bombs dropped on Tuwaitha might have hit a smaller research reactor, also built by the French Foreign Ministry said last night that the smaller reactor, which is in operation, had not been damaged.

Once a reactor undergoes a chain reaction and begins operations, it builds up large quantities of radiation in the fission products the uranium fuel makes when it "burns." The worst of the fission products are strontium-90 and cesium-137, which emit penetrating gamma rays and persist for hundreds of years.

The Israelis apparently chose to destroy the large reactor because it is capable of producing enough plutonium in its fission products to make as many as two nuclear bombs every year.

The Israelis said they attacked on Sunday to make sure no French technicians were working at the site. However, French sources reported that French technicians in Iraq follow a Moslem work week, with Friday off, and the French government said a French technician was killed in the raid.

The Israelis also did not want to wait until after the reactor was operating, sources said, because the Iraqis could shut it down and secretly remove its plutonium in as little as three months after it bagan operating.

The best plutonium for weapons is produced in the first three months. After that, it starts to be contaminated by isotopes of plutonium that are not as effective in weapons.

The Israeli attack came less than a week after French President Francois Mitterrand assured Iraq that France would supply the new reactor with uranium fuel through its lifetime. At least one source said this assurance by the French socialist may have convinced Israel to destroy the reactor. CAPTION: Picture, French-built nuclear reactor near Baghdad, demolished Sunday, cost $100 million and was scheduled to be loaded with highly enriched uranium. UPI