The Secrets of the Bomb

New York Review of Books; May 25 2006
Excepts from: The Secrets of the Bomb By Jeremy Bernstein

Review of: Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea by Jeffrey T. Richelson

The last part of Richelson's book deals with Iran and North Korea. Here events, especially in Iran, are moving so quickly that Richelson's account has to be supplemented by information released by the International Atomic Energy Agency and reported by the press. Of the two countries, in my view, the prospects of an Iranian bomb are the more serious. The North Koreans probably have a small, untested nuclear arsenal. My guess is that sooner or later, under Chinese and other international pressure, Kim Jong-il may accept an offer of economic and other rewards in return for giving up his nuclear program. Meanwhile, the principal concern about the North Koreans is that they do not try to sell their technology to terrorists. They may have little else to sell.

What makes the Iranian situation so difficult is that they have oil to sell, which makes them less vulnerable to economic sanctions. Indeed, when they are threatened the price of oil tends to go up, making the Iranians richer. The Chinese at the moment get about 14 percent of their oil from Iran, which is why they are so unwilling to apply pressure. To add to the difficulties, the Iranian president often speaks of eliminating Israel, although it is a question whether he has the authority to try to do so. In any case, if the Iranians eventually make one or more bombs, the logic of mutual deterrence would apply: use of the bomb against Israel would very likely result in a disaster for Iran. Meanwhile, the Israelis seem to mean it when they say they would not allow the Iranians to have nuclear weapons; the Iranians, one can surmise, are as aware of this as anyone else, just as they must be aware of the alleged American contingency plans for an attack on Iran recently reported by Seymour Hersh.[5]

The themes of this review have been twofold. In order to have really reliable intelligence about the atomic program of a foreign country a necessary, but not sufficient, condition is to have agents on the ground. In the examples I have given the necessity is clear. Countries can hide their nuclear programs even from satellites and other sophisticated detection instruments. The Chinese hid their program because the satellites were looking for the wrong signals. Until Vanunu, an agent on the ground, unmasked the Israeli program the Israelis hid it by deception. But even with an agent on the ground mistakes can be made. Samuel Goudsmit was selected as the scientific leader of the Alsos mission in part because he did not know anything about ours. He often said that if he had been captured by the Germans he could not have told them anything. Since he did not know about our plutonium program, he did not look for the German program and made the erroneous assertion that there was none.

The second theme is that in almost all cases the predictions have erred on the side of conservatism. Countries have acquired nuclear weapons well before they were supposed to. The example of the Russians is the most graphic. This happened because Fuchs gave them a blueprint for the US plutonium bomb and the Russians had the technological capacity to follow it more or less literally. With the Iranians we are almost in the worst possible case. We do not, it seems, have agents on the ground so not only do we not know what they have done—except for what they have chosen to tell us—but we do not even know their technological capacities. To take an example, it is rumored that the Pakistani nuclear merchant A.Q. Khan has sold them the plans for an implosion bomb. As the people at Los Alamos discovered, making an implosion bomb was a very difficult technological feat that required the enormous assembled talents of almost the entire laboratory. Do the Iranians have the people to do this, even if they have the plans? We simply do not know.


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What we do know is that the Iranians have had a nuclear energy program for many years. We know that in the fall of 2002, the Russians began assembling a large nuclear reactor at the port city of Bushehr. This reactor has not yet been put into operation. It is what is called a "light water" reactor, which means that it uses ordinary water as a coolant. Such a reactor cannot operate with natural uranium— the kind you get out of a mine. It has to be enriched. How is this done? Like all elements, uranium comes in several isotopes. The isotopes differ from each other in the number of electrically neutral particles—neutrons—in their nuclei. The number of positively charged particles in the nucleus—protons—is the same for all these isotopes. This means that it is extraordinarily difficult to separate these isotopes chemically. Methods must be employed that take advantage of the tiny differences in mass between the nuclei of the isotopes. Soon after fission was discovered in 1938 Niels Bohr realized that it was the isotope uranium-235—i.e., with 235 particles in its nucleus—that was fissioning. But in natural uranium, which consists mostly of uranium-238, only 0.7 percent consists of uranium-235. Bohr understood how difficult it would be to separate these isotopes by physical means and declared that nuclear weapons were practically impossible because it would take resources on a national scale to carry out this separation. He was roughly right about that, but wrong about the impossibility.

There are a number of ways in which the separation can be carried out, all of them difficult. During World War II most of the separation was done using electromagnetic fields. If you put a charged particle in a magnetic field it will have its orbit deflected by an amount that depends on the mass of the particle. This was what was done at Oak Ridge where the wartime separation of uranium-235 was carried out. To have an idea of what was involved, the Hiroshima uranium bomb consisted of two subcritical parts. The "target" within the bomb consisted of 38.4 kilograms of 80 percent enriched uranium while the "projectile" that was designed to hit the target inside the bomb consisted of 25.6 kilograms of 80 percent enriched uranium. This design was considered to be such a sure thing that it was never tested before it was dropped on Hiroshima. If the Iranians can make a uranium implosion bomb of the kind that the Chinese first tested, they would need only between fifteen and twenty kilograms of highly enriched uranium.

The Iranians have chosen to do this enrichment by using centrifuges. These are long cylinders stacked in parallel. The cylinders can rotate at 50,000 to 70,000 rotations per minute. Uranium is introduced in the cylinder in the form of a very corrosive gas—uranium hexafluoride—and it is estimated that by late February of 2006 the Iranians had produced about eighty-five metric tons of uranium in this gaseous form. It would take about five metric tons of this uranium gas to produce enough enriched uranium to make one bomb.[6] This assumes an implosion bomb.

In the centrifuge the heavier isotope, uranium-238, is spun to the outside leaving the lighter isotope, uranium-235, in the middle. This residue can be used as the stock for the next stage of enrichment, which is why the centrifuges are connected together in what is called a cascade. Once one obtains say a 50 percent enrichment, going to a full weapons-grade enrichment requires much less effort.[7]

So far this is mainly physics with a minimum of speculation. What is largely speculation is how far the Iranians have gotten with the enrichment process. It has been known for some time that they have had 164 so-called P-1 centrifuges in a cascade in their facility in Natanz in central Iran. "P-1" is short for "Pakistan-1." Beginning in 1997, they were supplied by the A.Q. Khan network. If remnants of this network are still operating they may still be active in Iran.

Since August of 2005 the Iranians have been running the Natanz centrifuges without external inspection or supervision. Very recently they announced that they had enriched some uranium of the type that can be used in a reactor. Normally this means an enrichment of about 3 percent. This would be consistent with the claim of the Iranians that the only use they will make of enriched uranium is for nuclear electrical power. But no external observer has been allowed to see how much and to what percent. We simply do not know. The rotors of the P-1 centrifuges are constructed from aluminum. But there is a next generation—the P-2—which is made out of a specialized steel. These are much more efficient. In some of their statements the Iranians have talked about making a cascade of 54,000 P-2 centrifuges. If this very difficult task can be achieved— and we don't know just how long it would take—and the cascade can be kept running for a year, the Iranians will have enough material to make more than one nuclear weapon.

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