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A nuclear expert explains, in very basic language, the science at the heart of Iranian nuclear talks

Iran's controversial heavy water facility at Arak. (Hamid Foroutan/AFP/Getty Images)
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Iran's ongoing negotiations with world powers over its nuclear program, most recently this weekend in Geneva, have not yet resulted in a deal. This weekend's talks fell through, according to some reports, because French representatives worried about how the agreement would have dealt with Iran's nuclear facility at Arak.

The details of the facility, how it works and why it's so controversial can be confusing. To get a better understanding of it and the other scientific issues at the heart of this very political process, I talked to Mark Hibbs. As a senior associate at the Carnegie Endowment for International Peace's nuclear policy program, Hibbs understands both the science of the Iranian program and the politics around those scientific issues. A lightly edited and compressed transcript of our phone conversation follows.

Can you explain to me, very simply, what are the main technical issues in these negotiations with Iran? In other words, what are they negotiating over?

The two main parts that are of concern are the uranium enrichment program and the heavy-water reactor [in the city of Arak].

Can you explain the dispute over uranium enrichment first?

The question is, how much enrichment should an agreement with Iran permit Iran to do? What would be the enrichment level? Where would Iran be permitted to do the enrichment, and, finally, what happens to the enriched uranium when it comes out of the enrichment plant?

Iran has been building up an inventory of enriched uranium. Most of it is 3 percent enriched or 5 percent enriched. It's "low enriched" uranium fuel, at the level of enrichment that normally is used for normal nuclear power reactors, like the reactor Iran has at Bushehr. There is a small nuclear inventory that Iran is enriching at [its nuclear facility at] Fordow, at a second enrichment plant, and that's enriched to 20 percent. And the problem is that 20 percent enriched uranium. The amount of work that's necessary, the amount of processing of the uranium to enrich it to 20 percent, that gets you most of the way there to enrich it further to 90 percent, which is what you would want for a nuclear weapon. The concern is that there has been, in recent years, a small but growing amount of 20 percent enriched uranium, which is the focus of a great deal of tension in this negotiation, because that would be the inventory that's closest to bomb grade.

They have slowed down the accumulation of this 20 percent enriched uranium. They've slowed it down to a crawl, and they haven't crossed that line. That is an indication that Iran is aware that this is a sensitive matter. That being said, in a negotiation to try to solve the Iranian crisis, the powers negotiating with Iran want to eliminate this threat. They want all of the 20 percent enriched uranium removed, converted to other nuclear materials that can't be readily accessible. And there hasn't been a discussion on it on how best to do that.

Most of the time, since about 2006, the countries negotiating with Iran have been preoccupied primarily with schemes to get that 20 percent uranium out of the country, and more recently there has been a discussion about an alternative approach which is to take the 20 percent enriched-uranium inventory and convert it into an oxide form, which would be less accessible in the sense that Iran would have to take a number of processing steps to convert that back again into metal.

The French are involved in that; the Americans are involved in that. Politically, ultimately it has to do with what you believe to be Iran's intentions. Technically, it has to do with what your assessment is about Iran's capabilities, the question of would Iran be able to reconvert the uranium, how quickly would Iran be able to do that and would they be able to do it fast enough that they wouldn't be detected.

What about Iran's heavy-water reactor in the city of Arak? Some reporting suggests that the negotiations in Geneva fell apart because the French didn't think the agreement was tough enough on this reactor. What does a heavy-water reactor do, and why is this one so controversial?

Okay, I am going to walk you through some basic science. [Laughs] The uranium fuel in the reactor core is surrounded by what's called a "moderator." The moderator in most reactors is water. For example, in a power reactor that makes electricity, there is enriched light-enriched uranium surrounded by a water moderator, which permits the nuclear reaction to happen. In the case of this Iranian reactor, the moderator is not normal water, it's heavy water. Heavy water is water for which the hydrogen isotope has a proton and a neutron, instead of just a proton, making it denser. You can make it through several chemical processes.

Heavy water moderates the reactor less efficiently than the normal, light water does. What that means for the reaction is that the deuterium, the heavy isotope of hydrogen, absorbs fewer neutrons, which are released spontaneously by the fuel in the system. It means that there's going to be a lot more neutrons in that nuclear system, in the core of that reactor. That means that the natural uranium fuel, which is to a large extent over 70 percent of the uranium in that fuel, is isotope U-238.

The excess neutrons in that system get absorbed by that natural uranium. They absorb the neutrons and it transmutes the uranium into plutonium-239, so you're creating plutonium by doing that. That's what you're doing in your reactor. The heavy water permits the reactor in Iran, or will permit the reactor in Iran, to be very efficient at producing plutonium.

Which is used for making nuclear weapons.

Correct. The design of the reactor is considered a red flag for nonproliferators. They see the heavy water, together with the use of the natural uranium fuel, the U-238 in the fuel, as a red flag. It's a reactor that can be very efficiently used to produce plutonium for nuclear weapons.

This gets to the heart of the problem. The United States has a very firm position about this. It has taken the view that the reactor in Iran is a bomb factory. The Iranian narrative says the reactor is supposed to be used for medical isotope production and for general research.

Who's right?

In a sense the problem that the negotiators have about this project is that both of them are right. The reactor can be used to make bombs, but it's also perfectly suitable for a large number of peaceful-use applications.

What this problem reveals is a disconnect between the nonproliferation community, which sees the heavy water reactor and its neutrons as a threat because they can be used to make bombs, and then you have the nuclear research community, academic people mostly, who use heavy water reactors in many countries to produce neutrons that they need for new nuclear research. So nuclear researchers will tell you that the most valuable reactors in the world are those that create the most neutrons, and in fact these heavy water reactors do that.

But if you take the position as the U.S. government has taken, that there is no justification for this reactor in Iran other than making bombs, then there's no way that you could justify a solution that would permit Iran at the end of the day to have this reactor.

But Iran says it doesn't want a nuclear weapon, so presumably its position is that it should be allowed to keep the reactor.

Iran will make the argument that it should have the reactor, because the International Atomic Energy Agency [IAEA, the United Nations' nuclear watchdog] is inspecting everything in Iran. And Iran has agreed to implement a so-called Additional Protocol, which is an agreement that provides a far greater level of IAEA intrusiveness into the program. So the Iranians will argue, "If we're demonstrating our nonproliferation bona fides to you, to the IAEA, then you should let us have the reactor."

So what was the big disagreement over Arak? Some reports say that the deal at Geneva this weekend fell through because the French diplomats had some concerns about how it would deal with the Arak reactor.

The French were not satisfied that this agreement really addressed the future of this reactor. The French were saying, "What we want to see happen is for the Iranians to agree not to do any more work on this reactor for six months or more. And during this period of time, we will sit down with Iran and we'll discuss how to go forward about the long-term future of this project."

What we have heard is that the preliminary agreement that had been discussed was that Iran had agreed not to start up the reactor for six months. And if that's the case, that wouldn't suffice. That reactor probably cannot be started up anyway during the next six months, because the Iranians are having trouble finishing the project. They're under sanctions and there's things that are missing. So they can't finish it.

In other words, if Iran had agreed not to start up the reactor for six months, that would have been pretty meaningless since they probably weren't going to be able to do that anyway.

That's why I think that in the longer term, when we look at this in a week or so, maybe in a couple of weeks, we'll look back and we'll see the French intervention is something which was constructive and positive. And it's not something that happened because a few people in a negotiating team weren't happy for one reason or another. I think there are issues here that have to be addressed.