Iran pledged on Thursday to allow nuclear inspectors into its newly disclosed uranium enrichment facility near the city of Qom. The Iranians insist the plant was constructed for peaceful purposes; the United States and its allies have accused Iran of pursuing nuclear weapons. How can inspectors tell whether a site is being used for good or evil?
First, they check the numbers. Inspectors from the International Atomic Energy Agency keep tabs on the amount of uranium coming in and going out of a facility, the number of centrifuges being used, and the pieces of equipment purchased. Then they compare those data with the declarations made by the country that owns the site. Any discrepancies raise a red flag, and the agency conducts a further investigation. For example, say Iran claimed it had 100 kilograms of low-enriched uranium at a particular site. If the IAEA visited and found only 50 kilograms, they would ask where the other 50 kilograms went. When a country has nothing to hide, the numbers usually add up.
Inspectors also test the uranium at a given site to measure its enrichment level. This refers to the amount of the U-235 isotope present in the fuel as compared with the amount of heavier, less reactive U-238. Natural uranium contains only a fraction of 1 percent of U-235. Low-enrichment uranium, which is used in nuclear power plants, has about 4 percent. The highly enriched stuff that goes into nuclear weapons is something like four-fifths U-235. When inspectors find enrichment levels higher than the 4 percent required for civilian purposes, they start asking questions. Sometimes it's a red herring. In 2004, inspectors found traces of highly enriched uranium on Iranian equipment. Tehran explained that the weapons-grade traces must have come from the Pakistanis, from whom the equipment was purchased. So the inspectors got a uranium sample from Pakistan that had been enriched using the same machines. The enrichment levels matched, and the Iranians were off the hook.
There are other red flags. A site that's configured to produce high-grade uranium would be set up differently, with some centrifuges arranged in tighter clusters as well as smaller "feed points" where you insert the uranium. (Of course, weapons-grade uranium is much more likely to be produced at a hidden site than a declared location.) Inspectors might also raise eyebrows at the construction of a fuel reprocessing plant. Enriching uranium isn't the only way to fuel a nuclear weapon: You can also use the plutonium generated as a byproduct of power plants and "reprocess" it—that is, make it more potent—to fuel a bomb. Inspectors therefore keep an eye on what happens to the leftover plutonium. (Plutonium is easier to produce than uranium, but harder to explode. In balance, counterterrorism officials see it as something of a lesser threat.)
The IAEA's ability to catch countries developing nuclear weapons depends on their level of access. The most basic level, known as "comprehensive safeguards," allows inspectors to monitor declared sites using cameras, maps, and prescheduled visits. More invasive is the so-called "additional protocol," which allows inspectors to do environmental testing and to show up at undeclared sites without prior warning and without overwhelming evidence. In other words, it lets them act on hunches. Some 90 countries, from France to Japan to the United States, have signed onto the additional protocol. Iran has signed the protocol but has not ratified it, making it difficult for inspectors to discover undeclared sites. *
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Explainer thanks David Albright of the Institute for Science and International Security, David von Hippel of the Nautilus Institute, and Frank von Hippel of Princeton University.
Correction, Oct. 5, 2009: This article originally stated that Iran has not signed the additional protocol. (Return to the corrected sentence.)