Two Methods for Converting Iran’s IR-40 Reactor to Use Low-Enriched-Uranium Fuel to Improve Proliferation Resistance After Startup

This article demonstrates the feasibility of converting Iran’s 40MWt heavy-water research reactor at Arak (the IR-40) to use low-enriched uranium fuel in order to slow the production of weapon-usable plutonium. Two methods are proposed, and both retain identical power density, thermal-hydraulic, and safety profiles as the original design and require no physical alteration of the core. They can be implemented at any time during the reactor’s life. The two methods have competing effects on cycle length, and can be combined to produce an optimized core that matches the start-of-cycle reactivity and cycle length of the original reactor. The methods are flexible enough to meet politically imposed demands, such as the need to consume a pre-specified amount of domestically produced enriched uranium. The proposed optimal design produces weapon-grade plutonium at only about 19 percent of the rate of the unmodified reactor for the same power level, cannot be readily converted back to natural-uranium fuel, and retains the ability to produce medical isotopes at rates that exceed the original design through the use of LEU targets. 1 Policy Context for Conversion Iran can provide confidence that its nuclear program is peaceful by adjusting its nuclear infrastructure so the time needed to produce a weapon quantity of nuclear-explosive fissile material is longer than some minimum, diplomatically negotiated period. In this respect, Iran’s program contains two areas of primary concern: a capability to produce high-enriched uranium using centrifuges installed at Natanz and Fordow, and an emerging plutonium capability associated with nuclear reactors, and in particular the IR-40 heavy-water reactor under construction at Arak. ∗Department of Nuclear Science and Engineering, Massachusetts Institute of Technology Address correspondence to [email protected]. The author thanks Kord Smith for assistance in modeling the IR-40 reactor with CASMO.