Scientists and weapons experts, surprised that Iraq secretly used a method for enriching uranium that the West abandoned half a century ago, say Iraq's feat is a blow to international efforts to stem the spread of nuclear arms.
In a single stroke, it has overturned decades of assumptions about which procedures and materials need to be safeguarded. Until now, the control effort has focused on keeping certain techniques secret and limiting export licenses for high-technology equipment that can be used in making bombs.
But Iraq has shown that a low-technology method openly described in scientific literature can be readily used to circumvent the restrictions, making the Iraqi weapons effort far more ingenious and dangerous than had been thought.
The clandestine Iraqi method is reported by a defector to have produced about 90 pounds of highly enriched uranium, enough for two bombs.
Experts say the 30 enrichment machines that Iraq has admitted to using could make enough fuel for one warhead a year.
Iraq probably has the skill to perfect an atomic bomb, weapons experts say. They stress that it is unlikely Iraq has already done so, though some suggest it might be able to build one on short notice.
Before the Persian Gulf war, many intelligence analysts said Iraq's engineers might start limited production of nuclear warheads in five to 10 years. The estimates were based on how rapidly the Iraqis might build high-technology devices for uranium enrichment.
Concern soared in May when the Iraqi defector reportedly told U.S. officials that Iraq used the antiquated, low-technology method to produce 90 pounds of enriched uranium. The technique uses electromagnetism in machines known as calutrons.
Under international pressure, Iraq told the United Nations last week that it had used the old method to produce a pound of enriched uranium. Experts think that much more was produced.
The fact that the Iraqis quietly used the old method at all to produce weapons fuel showed great cleverness, the experts say.
"It's astonishing," said Dr. GlennT. Seaborg, a Nobel laureate in physics and former chairman of the Atomic Energy Commission. "It represents quite a technical effort."
Dr. Edward J. Lofgren, a physicist at the Lawrence Berkeley Laboratory in California who helped develop calutrons during World War II, said Iraq's choice of enrichment method was surprising but sensible.
"The other methods are very efficient but take lots of capital and big plants," he said. "A calutron, on the other hand, in one stage enriches a large amount. It's not energy-efficient, but it doesn't take a lot of capital."
Dr. J. Carson Mark, a former official of the Los Alamos National Laboratory in New Mexico who has studied the Iraqi nuclear program, criticized U.S. intelligence agencies for apparently failing to learn about the clandestine Iraqi effort at uranium enrichment.
"Why spend all that money on intelligence," he asked, "when it apparently and evidently learns nothing?"
Paul L. Leventhal, president of the Nuclear Control Institute, a private group in Washington that studies the spread of nuclear technology, said the re-emergence of the old enrichment technique will force the development of a whole new set of international safeguards and precautions.
"We can't put the genie back in the bottle," he said. "The main thing is to try to improve our intelligence-gathering ability" so that existing calutrons can be tracked down.
Leonard S. Spector, an expert on the spread of nuclear arms at the Carnegie Endowment for International Peace in Washington, said news of the Iraqi enrichment success had toppled the international program to stem weapons proliferation, which has focused on limiting advanced methods.
"It's cataclysmic," he said. "All this was being done in Iraq without anybody knowing it. So who else is doing it? Everybody in the community knew this kind of thing was a possibility, but to be confronted by an example is devastating."
Though slow and costly, experts say, the electromagnetic process has many virtues from the Iraqi point of view.
For one thing, it has been declassified for decades. Detailed blueprints showing how it works have been published by the federal government and academic scientists. More advanced uranium-enrichment methods, in contrast, have been surrounded by secrecy.
Most important, experts say, calutrons are relatively easy to build. They use few exotic materials, in principle allowing them to be constructed largely without Western aid.
The main drawback is that their bulky electromagnetic coils have a large appetite for electrical power.
But experts note that Iraq, with large oil supplies, can generate electricity inexpensively.
The bulky and cumbersome calutrons achieve one of the most delicate tasks of science: separation of isotopes. Isotopes are varieties of an element that differ only in the number of neutrons in their nuclei.
The scientific challenge, on which untold billions have been spent, is to separate the extremely rare isotope from the common one.
The man who came up with the idea for the calutron and promoted it extensively in the early 1940s was Dr. Ernest O. Lawrence, inventor of the cyclotron particle accelerator and director of a physics laboratory at the University of California at Berkeley. The name calutron derives from California University cyclotron.
The process worked experimentally at Berkeley, producing minuscule amounts of fairly pure U-235, the material needed for nuclear weapons. In 1943, the government embarked on a huge project at Oak Ridge, Tenn., to expand its scale. At its peak, the program had nearly 25,000 employees.
The electromagnetic coils were 15 feet in diameter and weighed thousands of tons. The magnetic fields were so powerful that hammers and screwdrivers were pulled from workers' hands if they came too close. The U-235 produced by the calutrons helped power the bomb that destroyed Hiroshima, Japan in August 1945.
By the end of World War II, other enrichment methods were working better than the calutrons, and the costly machines were abandoned.
Scientists say the electromagnetic method might have been refined and simplified by the Iraqis.
Mr. Leventhal estimated that 30 Iraqi calutrons, if they were fairly modern, probably could produce up to 44 pounds of bomb-grade fuel a year. That, he said, would be enough for one bomb.
Experts on the spread of nuclear weapons say they will now have to completely rethink their field.
"How are we going to find this stuff?" Mr. Spector asked.