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Iran’s Plutonium Game

Jeremy Bernstein
To anyone who has been following the Iranian nuclear program, it was almost a forgone conclusion that negotiations with Iran would hit a road block when it came to the so-called IR-40 heavy water reactor located in Arak.
Arak nuclear reactor.jpg


The IR-40 heavy water reactor, Arak, Iran

There have been conflicting reports about why the much-watched negotiations in Geneva failed to produce an interim agreement about Iran’s nuclear program. On Monday, senior US officials said that the Iranian delegation was not ready to sign on to a draft agreement, which called for a six-month freeze in Iran’s uranium enrichment activity to allow time to produce a comprehensive accord. But over the weekend, French officials gave another reason: the French government is concerned about the continuing construction of a heavy-water nuclear reactor at Arak.

In fact, to anyone who has been following the Iranian nuclear program, it was almost a forgone conclusion that negotiations with Iran would hit a road block when it came to the so-called IR-40 reactor located in Arak. The “40” here refers to the projected power output of forty megawatts of thermal power. To convert this into electric power involves a cumbersome process. The thermal power, which is generated in the form of energetic fission fragments in the reactor, must be converted into steam to run inefficient steam turbines. Thus much of the original reactor generated energy is dissipated; something like only a third of this power could be converted into electricity. And since one large building alone can use several megawatts of power, it is hard to imagine generating much electricity from a forty-megawatt reactor. Whatever the IR-40’s intended use, it is not to produce electric power. A reactor designed for that purpose—such as the one at Bushehr—produces billions of watts.

Moreover, there is nothing about the reactor’s declared purpose that would require it to be a heavy water reactor. According to the Iranian government, the IR-40 reactor is supposed to make medical isotopes. But a light water reactor would have served the stated purpose just as well—and generate the same amount of power.

What makes the Arak reactor suspicious, then, is the design. To understand this we need to understand how a heavy water reactor works.

The isotope of uranium that is fissioned in a nuclear reactor is uranium-235. The fission happens when a neutron strikes a uranium-235 nucleus causing it to split and also producing additional neutrons. These neutrons in turn can cause other uranium 235 nuclei to split and it is this chain reaction that produces the power. One of the oddities of quantum mechanics is that the probability of fission increases when the neutrons are slowed down. Unlike breaking a window with a baseball, it is not the case that speeding up the ball makes it more likely to break the window. In quantum mechanics, it is rather as if the baseball gets bigger when it travels at a slower speed, causing a larger collision with the window.

Hence in order to cause fission, the neutrons, which are initially moving very rapidly, must be “moderated” in their speed and this is done by having them collide with the nuclei of some “moderator” such as ordinary water. There is now a balancing act. Moderators can swallow neutrons taking them out of the chain. Ordinary water does that. To compensate one must enhance the percentage of uranium 235 in the fuel. In a so-called light water reactor, which uses ordinary water , the uranium 235 must be enhanced to produce enriched uranium containing about four percent uranium 235. So a supply of enriched uranium is necessary to power a light water reactor.

This is not the case with a heavy water reactor. A heavy water nucleus consists of one proton and one neutron plus oxygen as opposed to light water where the neutron is missing; this means there is less of a chance of neutron swallowing. So one can use natural uranium, which has less that one percent of uranium 235. But this method also produces plutonium as a byproduct—something that is useful for making a bomb.

To make plutonium you need to maximize the percentage of uranium 238. The reason is the chain of reactions that produce plutonium: uranium 238 absorbs a neutron and become uranium 239; this is unstable and decays into neptunium 239; this is also unstable and decays into plutonium 239. For a reactor of the type being built at Arak, a rough rule is that in the course of a day, for each megawatt of thermal power generated one gram of plutonium is produced. Thus the IR-40 could produce forty grams of plutonium per day. If it ran constantly for a year, it could generate 365 x 40 = 14,600 grams =14.6 kilograms of plutonium. Realistically it might operate about 75 or 80 percent of the time, so 11 to 12 kilograms is probably a better estimate.

This amount of plutonium is enough for one or two bombs. Once Iran has reprocessed it, the plutonium could replace high-enriched uranium as the explosive, as it did in the plutonium bomb dropped on Nagaski. The Iranian government has announced that the Arak reactor will go on line in 2014. What still has to be verified is that there are no facilities at this site for reprocessing any spent fuel elements to extract plutonium.


On August 28 of this year, the International Atomic Energy Agency (IAEA) produced its latest report on Iran. Of the Arak reactor it said:

Contrary to Iran’s obligations under the modified Code 3.1 of the General Part of the Subsidiary Arrangements to its Safeguards Agreement and although the Agency has made repeated requests, Iran has not provided the Agency with an updated DIQ [Design Infomation Quetsionnaire]for the IR-40 Reactor since 2006. At that time, the IR-40 Reactor was in a very early stage of construction. As the commencement of the IR-40 Reactor’s operation approaches, the lack of up to date design information is having an increasingly adverse impact on the Agency’s ability to verify the design of the facility and to implement an effective safeguards approach. The Agency requires this information as early as possible in order, inter alia, to ensure that all possible diversion paths are identified, and appropriate safeguards measures and customized safeguards equipment are put in place.

On Monday, the IAEA and Iran issued a joint statement saying that they had reached a new “framework of cooperation” to gain information on Iran’s nuclear program. But all the agreement says is that Iran will provide “information on all new research reactors.” (The second clause in the annex of the agreement says that Iran will provide to the IAEA “mutually agreed relevant information and managed access to the Heavy Water Production Plant.” This is a reference to a plant, also near Arak, where heavy water is produced, one imagines, for the IR-40 and any successors.) What is left unspecified is the time frame in which this information is going to be supplied.

It is hard to imagine any legitimate reason for not converting the Arak reactor into a light water reactor. The Iranians have enough enriched uranium fuel to power such a reactor, and surely it would be worth the while of the countries that are now negotiating with Iran to offer to help in this endeavor. If the IR-40 became a light water reactor, this would end all the suspicions about it.

By going ahead with a heavy water reactor, Iran seems to be saying it is determined to have the capacity to produce plutonium—and leave open a path to making a bomb. But it is very difficult to read the real intentions of the Iranians. Perhaps the fact that real negotiations have begun offers some hope that a tragedy can be avoided.

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