As has been noted, the fierce debate over a potential nuclear agreement with Iran—which reached a fever pitch this week with Israeli Prime Minister Benjamin Netanyahu’s speech to the United States Congress—amounts to an argument over nuclear fission being deliberated in an arena dominated by politics, not by knowledge of physics. Such knowledge is essential, though, both for assessing Iran’s current nuclear capacity to the extent we can, and for gauging the likelihood that Iran’s actual intentions align with those stated.
Theoretical physicist Jeremy Bernstein, known for his accessible science writing in the New Yorker and elsewhere, wrote Nuclear Iran last year to fill the need for a brief but complete discussion of the Iranian nuclear program from its inception to the present. The book offers lay readers a history of the sort of centrifuges used to produce the weapons-grade uranium needed for bombs, a discussion of the completely different technology required to produce plutonium and to use it in a nuclear weapon, and a blending of the technical and human aspects along the way. The piece below, adapted from the postscript to Nuclear Iran, compacts that science into a concise presentation of the state of Iran’s nuclear program and the potential for any agreement to contain it.
The negotiations with Iran about its nuclear program are in process as I write this. The outcome is uncertain. But I would like to try to make the issues clear. To do this I will consider two limiting cases, neither one of which has any chance of being adopted but which serve as a useful foil for the discussion.
- Case 1: Iran agrees to give up its entire nuclear program.
- Case 2: Iran does not agree to give up any of its program but does agree to enhanced inspections.
To discuss Case 1 we need to specify what Iran’s nuclear program is. I begin with reactors. To characterize a reactor, it is useful to specify three elements:
- The power output
- The fuel
- The moderator
I begin with the power output. This is usually measured in watts—a unit of energy produced per second. The practical units for reactors are millions of watts (megawatts) and billions of watts (gigawatts). If the reactor is used to produce electricity, then two kinds of power produced are distinguished—thermal power and electrical power. The thermal power is the actual power that is produced in the fission process. This is generally converted into heat, which may boil water, making steam, which in turn runs the turbines that produce electricity. About two-thirds of the thermal energy is lost for various reasons in this process, hence the watts electric are about a third of the watts thermal.
I will deal here only with watts thermal so we can compare various reactors. The fuel for these reactors are ceramic uranium pellets placed in rods. The fissile isotope is uranium 235, so we must specify what percentage of uranium 235 there is in the pellets. In the Iranian reactors this ranges from about 3.5 percent to 90 percent, which is weapons-grade. Finally we must specify the moderator. The neutrons produced in fission move at about a tenth of the speed of light. As quantum mechanical objects they have a wave nature as well as a particle nature. If the neutrons are slowed, their wavelength increases, and this increases the probability for the production of fission. The slowing down is done by a “moderator.” We shall specify the moderators for the Iranian reactors.
At the present time there are six functioning reactors in Iran, two others under construction, and plans for others. I will begin with the Tehran Research Reactor. Nominally it generates 5 megawatts of power. The fuel rods are kept in a swimming pool of ordinary water that acts as both a moderator and a coolant. At the present time the reactor uses about 19 percent enriched uranium. The Iranians are under way in producing the fuel elements for it. This reactor was originally powered by 93 percent enriched weapons-grade uranium. It appears that about 7 kilograms remain. These are highly irradiated and are apparently stored on site. For reference, about 50 kilograms of pure uranium 235 constitutes a critical mass. Uranium taken from a reactor makes a poor explosive.
The nerve center of the entire Iranian nuclear program is, as far as I am concerned, in Isfahan. It is here that yellow cake uranium is converted into uranium hexafluoride—hex—and the enriched hex is converted into useful solids that can be used in uranium fuel pellets for reactors and the like. The construction of the fuel elements is also done here. If this center were to stop functioning, the entire Iranian program would come to a halt. There are four functioning reactors on this site, all of them small. There is a subcritical reactor—meaning that the chain reaction going on in it is not self-sustaining—that uses graphite as its moderator. It was built by the Chinese, as were the other three reactors in Isfahan. This reactor seems to be used for training purposes. There is a zero-power heavy-water-moderated reactor that presumably uses natural uranium in its fuel elements. It is used for research on heavy-water applications. In principle all work on heavy water is prohibited, but the Iranians do it anyway. There is a light-water reactor that also is subcritical and used for training purposes. The most interesting of these reactors is a 30-kilowatt light-water reactor, which produces neutrons used to make medical isotopes. What is interesting about this reactor is that the fuel elements consist of weapons-grade uranium supplied by the Chinese. About 900 grams of this uranium is required to operate this reactor, which is not much, but the fact that this is weapons-grade is something to think about. The one functioning power reactor is at Bushehr. This also uses light water as a moderator and coolant. It is producing about 1 gigawatt of power. The fuel elements are 3.5 percent enriched uranium supplied by the Russians.
Two reactors are known to be under construction. All external observers agree that the only possible function for the 40-megawatt heavy-water reactor at Arak is to produce plutonium and that any agreement must change the configuration of this reactor. Finally there is a 360-megawatt power reactor that is being built by the Iranians with external aid. It is again a light-water-moderated reactor that will presumably use low enriched uranium.
There are two underground centrifuge facilities—at Natanz and Fordow. The Natanz facility is designed to hold some 25,000 centrifuges, some of which are of the latest design, replacing the original Pakistani models. At the time of the start of the negotiations, the unit had produced about 11,000 kilograms of hex enriched up to 5 percent. This sounds like a lot, until one realizes that operating a power reactor of reasonable size requires about 75,000 kilograms of uranium, 25,000 of which are replaced every two years. If the Iranian program is designed to fuel power reactors, it looks like a very small dog chasing a very large truck. This enrichment continues during the talks. On the other hand, the enrichment to 20 percent has been suspended as part of the interim agreement, and some of the existing stock has been downblended. At Fordow there are some 3,000 centrifuges, which have produced about 250 kilograms of 20 percent enriched uranium. This production is also frozen.
In addition to this there are production facilities for manufacturing new centrifuges and producing heavy water, as well as some uranium mines. The notion that all of this is somehow going to be made to vanish seems absurd. Equally absurd is the notion that none of it is going to be made to vanish. This would be a continuation of the present unsatisfactory situation. These are the two extremes the negotiators must find a path between.