On January 11, an Iranian nuclear scientist named Mostafa Ahmadi Roshan was assassinated by the detonation of a magnetized bomb attached to his car in Tehran. He was thirty-two years old, and, it appears, the deputy director of the Iranian enrichment facility in Natanz. No one has taken responsibility for the assassination, but the Israelis have expressed a certain satisfaction and Western intelligence officials suggest the Mossad was involved. This is the fourth such killing of a top Iranian scientist in the past two years—all in attacks that seem to have been carefully planned and bear striking similarities. Of course, the Iranian government has blamed the Israelis and the United States and there has been a call for revenge. It has also stepped up security for its other scientists. But even more important than this disturbing attack may be what we are now learning about the current state of Iran’s nuclear program.

One thing that struck me in reading about Roshan is that he achieved a position of seniority in the nuclear program as a graduate of the Sharif Technical University in Tehran rather than by study abroad, as members of the older generation typically did. This suggests that Iranians are now able to produce their own scientists and engineers rather than having them trained elsewhere. Indeed, the day after Roshan’s assassination, three hundred students at Sharif and a thousand students elsewhere in Iran are reported to have changed their major to nuclear physics and engineering. I am not surprised since there seems to be a broad support among Iranians for their nuclear program, even among many who are opposed to the Iranian government in other ways. It is a source of pride.

The second thing that struck me was that a day or so after the assassination the Iranians announced that they had manufactured the first fuel elements for the Tehran Research Reactor, or TRR, the small research reactor that was built by the Shah in the 1960s as part of the Atoms for Peace program and went critical in 1967. Along with many other reactors in the Atoms for Peace program, the TRR was initially powered by weapons-grade uranium. This is uranium that has a content of some 90 percent of the fissile isotope of uranium U-235. Uranium from a mine has less than 1 percent, the rest being the non-fissile isotope U-238. Achieving higher grades of uranium requires separating out the U-235 from the U-238, usually using centrifuges. At the time, the reason for using weapons-grade uranium was probably that there was left over supply from the American bomb program and no one thought that selling it to underdeveloped countries for use in civilian research reactors could possibly pose a threat. A good deal of this uranium is now unaccounted for.

In 1987 the TRR—now under the control of Ayatollah Khomeini’s Islamic Republic—started to run out of fuel and, to keep it going, the Iranian government decided to retool it so that it could operate with 20 percent enriched uranium. By this time enriched uranium of lower grades could be produced in other countries and no one wanted to proliferate the weapons-grade variety. Iran bought fuel elements from the Argentinian company Investigaciones Aplicadas for a price of some $5.5 million. Now it needs to be refueled again. It will be recalled that there has been a sort of dance of the deaf between the Iranians and various countries like the United States and Russia about providing Iran with new fuel elements in exchange for Iran giving up uranium that it had enriched on its own. None of this came to anything and now the Iranians say it does not matter because they have enough enrichment capacity themselves to produce what they need. If this is true it is a very considerable technological achievement. But the claim has been nearly impossible to verify, so who knows.

While much mystery about Iran’s enrichment capabilities remains, however, several facts about the program are particularly worth scrutiny. One is somewhat paradoxical: the way in which the Iranians are proceeding seems to be first to enrich uranium to about 4 percent, which is the level you would need to run a civilian power reactor. Then, they are reprocessing this 4 percent uranium and enriching it further to 20 percent. Twenty percent is a very important boundary. Below it, the fuel is “low enriched uranium” not useful for weapons; above, it is “highly enriched uranium” which can be used to make bombs. A weapons designer would want the enrichment as great as possible and 90 percent is certainly weapons-grade; but a very skilled designer could make a device with much less enrichment.

Now here is the paradox. It takes less centrifugal activity to enrich a kilogram of uranium hexafluoride from 20 to 90 percent than it does from 4 percent to 20; and it takes less to go from 4 percent to 20 than from 1 percent to 4. Thus, if the Iranians are able to get to 20 percent, and have a goodly stock of 20 percent enriched uranium—as they now claim they do—they should be able to reach weapons-grade pretty quickly. The same centrifuges are used at every stage but less separative power is required since the more the uranium has been separated the less work is needed to do the additional separation.

So far the IAEA inspectors have been able to monitor these enrichment activities to a degree. But the Iranians have a penchant for building secret enrichment sites, and for all one knows they may have one or more of these operating. Of one thing I am quite certain. They have plans that can be used to make a nuclear weapon. They bought the same package from A.Q. Khan as the Libyans did and that contained the design of the Chinese implosion device.

So what happens now? It appears that the US has sent the Iranian government some sort of communication warning that certain actions would be met with responses. Closing of the Strait of Hormuz, as Iran has threatened to do, would be one such action. There would be no ambiguity if this happened and I am sure that the US would respond. But the nuclear situation is far more subtle. We have announced that we will not permit the Iranians to make a nuclear weapon. But where is the red line? If the inspectors are told to leave that is surely one. But what about the possession of enriched uranium that is not bomb-grade but that could quickly be enriched to 90 percent? What if we only know after Iran has tested a device?

None of the current options seem very helpful. I am pessimistic that sanctions will work. There is too much pride at stake. I also do not see how to shut the program down by force without unleashing a demon we may not be able to control. Nor does killing Iranian scientists seem productive; to the contrary it may strengthen resolve and support for the program among Iranians. Perhaps political changes in the region (or even in Iran itself) could induce Iran to consider talks again, though negotiations have not worked very well in the past. In short I am not sure what to do.