Nuclear Engineering and Design 239 (2009) 2582–2595 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Flexible conversion ratio fast reactors: Overview Neil E. Todreas a,∗ , Pavel Hejzlar a , Anna Nikiforova a , Robert Petroski b , Eugene Shwageraus c , C.J. Fong d , Michael J. Driscoll a , M.A. Elliott a , George Apostolakis a a Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139, USA b TerraPower, LLC, Bellevue, WA, USA c Department of Nuclear Engineering, Ben Gurion University of the Negev, Israel d Nuclear Regulatory Commission, Washington, DC, USA article info abstract Conceptual designs of lead-cooled and liquid salt-cooled fast flexible conversion ratio reactors were devel- oped. The performance achievable by the unity conversion ratio cores of these reactors was compared to an existing supercritical carbon dioxide-cooled (S-CO 2 ) fast reactor design and an uprated version of an existing sodium-cooled fast reactor. All concepts have cores rated at 2400MWt. The cores of the liquid-cooled reactors are placed in a large-pool-type vessel with dual-free level, which also contains four intermediate heat exchangers (IHXs) coupling a primary coolant to a compact and efficient supercritical CO 2 Brayton cycle power conversion system. The S-CO 2 reactor is directly coupled to the S-CO 2 Brayton cycle power conversion system. Decay heat is removed passively using an enhanced reactor vessel aux- iliary cooling system (RVACS) and a passive secondary auxiliary cooling system (PSACS). The selection of the water-cooled versus air-cooled heat sink for the PSACS as well as the analysis of the probability that the PSACS may fail to complete its mission was performed using risk-informed methodology. In addition to these features, all reactors were designed to be self-controllable. Further, the liquid-cooled reactors utilized common passive decay heat removal systems whereas the S-CO 2 uses reliable battery powered blowers for post-LOCA decay heat removal to provide flow in well defined regimes and to accommodate inadvertent bypass flows. The multiple design limits and challenges which constrained the execution of the four fast reactor concepts are elaborated. These include principally neutronics and materials chal- lenges. The neutronic challenges are the large positive coolant reactivity feedback, small fuel temperature coefficient, small effective delayed neutron fraction, large reactivity swing and the transition between different conversion ratio cores. The burnup, temperature and fluence constraints on fuels, cladding and vessel materials are elaborated for three categories of material – materials currently available, available on a relatively short time scale and available only with significant development effort. The selected fuels are the metallic U–TRU–Zr (10% Zr) for unity conversion ratio and TRU–Zr (75% Zr) for zero conversion ratio. The principal selected cladding and vessel materials are HT-9 and A533 or A508, respectively, for current availability, T-91 and 9Cr–1Mo steel for relatively short-term availability and oxide dispersion strengthened ferritic steel (ODS) available only with significant development. © 2009 Elsevier B.V. All rights reserved. 1. Background and motivation for flexible conversion ratio fast reactors In the 1960s and 1970s fast spectrum reactor designs having a high conversion ratio were pursued worldwide to maximize breed- ing. However this anticipated need for breeders did not materialize in subsequent years since low cost uranium resources were abun- dant due to slowed nuclear power growth. Nevertheless concerns over accumulation of long-lived actinides in spent light water reac- tor (LWR) fuel and slow progress on a permanent waste repository ∗ Corresponding author. E-mail address: todreas@mit.edu (N.E. Todreas). have stimulated reevaluation of fuel cycle options. High among them has been the transition to a fleet of fast spectrum reactors of unity conversion ratio (CR = 1) to provide needed electricity gener- ation by a transuranic (TRU) sustainable fuel cycle regime. Reactors operating in such a closed cycle in time would also manage the legacy LWR spent fuel actinides, since these actinides would be needed to provide the large start up fuel inventory for these unity conversion ratio reactors. However the alternative path of a fleet of dedicated fertile-free fast burner (CR = 0) reactors to manage both legacy LWR spent fuel and that from the continued operation of LWRs is another option. While long-term simulations of these options performed by oth- ers as well as ourselves (Romano et al., 2006; Aquien et al., 2006) have elucidated the relative characteristics of the fuel cycle regimes 0029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2009.07.014