Why Iran’s agreement to modify the Arak reactor is more good news than meets the eye

R. Scott KempR. Scott Kemp explains the science behind Iran's proposed compromise on the final status of the heavy water reactor at Arak. The plan, he argues, offers a framework for limited, peaceful uranium enrichment under a comprehensive nuclear deal.


By R. Scott Kemp

On April 19, 2014, Iranian vice president Ali Akbar Salehi announced that Iran would redesign the Arak heavy water reactor to limit greatly the amount of plutonium it could make. The announcement marks a major turn for the emerging plutonium program toward a more peaceful orientation. Depending on the design changes implemented, plutonium production should be reduced by at least 80%, and possibly by 95% or more if the reactor’s power is also reduced at the same time.

Started before Iran’s uranium enrichment program was successful, the Arak reactor was probably meant to be a backup means of making nuclear-explosive material for nuclear weapons. The Iranian regime has pursued progress on the reactor with less vigor than they have with its enrichment program, and the Arak facility has not come under as much scrutiny as those at Natanz and Fordow. Rather, it has remained as a lurking threat in the background.  In spite of sanctions, Iran made steady progress and brought the reactor to the threshold of operation this past winter.

Heavy water reactor at Arak
October 14, 2012 - IR-40 heavy water reactor at Arak. (Photo via Nanking2012)

Shortly after his election, President Rouhani named Mohammad Javad Zarif as his foreign minister and the new chief negotiator for the nuclear program. This marked a dramatic shift in the formation of policy, moving the portfolio away from the more contentious Supreme National Security Council to an expert group of diplomats familiar with the West and with nuclear technology. Zarif surprised Western interlocutors last September when he quietly signaled a willingness to consider options to reduce plutonium production. This was reinforced again when he agreed to the French government’s insistence that Iran suspend work on the Arak reactor as part of the interim deal. 

The magnitude of Zarif’s decision is perhaps missed by the non-technical community. Had Iran simply continued with its plans, Zarif could have argued that the reactor had become a fait accompli. Unlike a centrifuge program, in which the number of machines can be throttled up or down depending on the state of politics, the monolithic reactor is open for easy re-design only up until the point it is activated. After that, it becomes highly radioactive and considerably more difficult to modify. Zarif could have used this technical argument to pocket the plutonium capability. Instead, he chose to support negotiations by allowing the possibility the reactor might be modified into a more peaceful orientation. This decision is also remarkable because Zarif’s primary technical advisor is one of the reactor’s designers, and very likely this advisor would have preferred to see the reactor brought into operation as soon as possible.

Although modifying the reactor constitutes a major concession in Iran’s weapons capabilities, Iran would not be losing out on the peaceful benefits of the reactor. The modified reactor should still be able to produce medical isotopes at design levels and meet or exceed previous scientific performance. This is enabled by the shift to low-enriched uranium fuel, a design option that was not available prior to the existence of Iran’s enrichment program.  Low-enriched uranium, made in Iran’s centrifuges, contains less of the plutonium-producing uranium-238 isotope. Conversion does not require changing from heavy water to light water, a popular misconception. Several designs have been put forward by American academics, although the final design will have to be an Iranian effort, or possibly a joint effort between Iran and the West.

The process of converting reactors is well established, and many dozens of reactors have been converted to low-enriched fuel around the world. The most probable course will be to choose a more compact core design that increases the neutron flux at the center of the reactor but reduces the amount of fuel needed. The advantage of this design is that it reduces plutonium production while also leading to improved scientific performance.

An unusual alternative design has been proposed by scholars at MIT. Their design requires no modifications of the physical hardware in the reactor, but rather modifications to the composition of the fuel and water moderator. By adding aluminum powder to the fuel and a small amount of ordinary water to the heavy-water moderator, the MIT design reduces plutonium production by more than 80%, about the same as the compact core.  The MIT option is less interesting from the perspective of the reactor’s scientific users, but is notable because it offers a conversion pathway that can be implemented at any point in the reactor’s life, even after the reactor becomes radioactive. The existence of this option means Iran can no longer argue that it is too late for conversion and therefore has no incentive to bring the reactor online anytime soon in an attempt to present it as a fait accompli.

While the exact details of conversion remain to be worked out, it is interesting to note that, under any likely comprehensive nuclear deal, the converted reactor will become Iran’s only means of domestically consuming the low-enriched uranium produced by its centrifuge program for at least the next decade. Conveniently, then, the modified Arak reactor could provide a natural basis for establishing just how much enrichment capacity should be left in Iran. By sizing the centrifuge program to match Arak’s uranium needs, there would be no accumulating stockpile of enriched uranium, and neither the enrichment or reactor programs would pose a meaningful weapons threat.  The entire nuclear program will be regularized, with all elements of the program supporting legitimate peaceful activities in an internally self-consistent way. Having scientific basis for an end-state that also meets the stated political goals of all parties is a rare luxury, but just such a basis is enabled by the conversion of the Arak reactor.

R. Scott Kemp is an Assistant Professor of Nuclear Science and Engineering at the Massachusetts Institute of Technology and an Associate at the Project on Managing the Atom.