Phase characteristics of a number of U–Pu–Am–Np–Zr metallic alloys for use as fast reactor fuels Douglas E. Burkes * , J. Rory Kennedy, Thomas Hartmann, Cynthia A. Papesch, Dennis D. Keiser Jr. Nuclear Fuels and Materials Division, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6188, USA article info Article history: Received 12 April 2009 Accepted 20 October 2009 abstract Metallic fuel alloys consisting of uranium, plutonium, and zirconium with minor additions of americium and neptunium are under evaluation for potential use to transmute long-lived transuranic actinide iso- topes in fast reactors. A series of test designs for the Advanced Fuel Cycle Initiative (AFCI) have been irra- diated in the Advanced Test Reactor (ATR), designated as the AFC-1 and AFC-2 designs. Metal fuel compositions in these designs have included varying amounts of U, Pu, Zr, and minor actinides (Am, Np). Investigations into the phase behavior and relationships based on the alloy constituents have been conducted using X-ray diffraction and differential thermal analysis. Results of these investigations, along with proposed relationships between observed behavior and alloy composition, are provided. In general, observed behaviors can be predicted by a ternary U–Pu–Zr phase diagram, with transition temperatures being most dependent on U content. Furthermore, the enthalpy associated with transitions is strongly dependent on the as-cast microstructural characteristics. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The development of fuel alloys for use in the transmutation of minor actinides (MA), such as americium, neptunium, and curium, is one of the top goals of the Advanced Fuel Cycle Initiative (AFCI) program. Fast reactors are poised to effectively provide long-term management of plutonium and MA, thereby minimizing prolifera- tion risks and waste depository requirements while still generating a respectable amount of heat for energy, hydrogen, or water desa- lination. Fuels for fast reactors must behave in a benign manner during core transient events, maintain integrity at high burnup, lend themselves to low-loss recycling processes, and be easily fab- ricated with minimal material loss in a remote handling environ- ment. The application of metal alloy fuels for use in fast reactors for MA transmutation is of particular interest, due to the ease in fabrication, high thermal conductivity, high fissile and fertile den- sity capability, and small Doppler reactivity feedback [1]. Because of this, metal alloy fuels are one of the common fuel types consid- ered for fast reactor recycle. Unfortunately, there is only limited fuel performance data to support the selection of a fuel type for this application. Thousands of metal fuel pins were fabricated in support of the Experimental Breeder Reactor-II (EBR-II) reactor in Idaho during the 1960s, 70s, and 80s [2]. Several fuel designs (designated as Mark) were explored, including alloys of composition U–5Fs, U–10Zr, and U–Pu–Zr. Only a few irradiation tests were carried out on a U–Pu–Zr fuel alloy that was intended for the conversion of the EBR-II driver core before it was terminally shut down in 1994. Thus, only a limited characterization and performance data- base is available on these particular alloy systems. These original EBR-II designs have been expanded in the form of Advanced Fuel Cycle (AFC) designs that are based on U–Pu–Zr ternary alloys but contain minor amounts of Am and Cm. To date, a series of compo- sitions has been characterized and irradiated, designated as the AFC-1 and -2 designs. Beginning-of-life (BOL) phase behavior is an important charac- teristic to determine and evaluate the performance and behavior of the fuel alloy as a function of irradiation (i.e., operating tempera- ture, fission rate, burnup). It is important to a fuels-development campaign to separate changes that might occur upon irradiation in order to affect fuel fabrication techniques to optimize micro- structural characteristics and to relate behavioral differences of well-characterized fuels at BOL and end of life (postirradiation evaluation [PIE]). Furthermore, adequate understanding of phase behavior can be a powerful tool in the development and verification of predictive modeling tools, both in terms of fabrica- tion processes and irradiation performance. Thus, an adequate understanding of how constituents affect the thermal and phase behaviors of metallic fuel alloys is important for continued devel- opment of fast reactor concepts. 2. Experimental materials and methods Six metal alloys were investigated based on compositions used in the AFC-1 and AFC-2 irradiation test designs. Although not 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2009.10.053 * Corresponding author. Tel.: +1 208 533 7984; fax: +1 208 533 7863. E-mail address: Douglas.Burkes@inl.gov (D.E. Burkes). Journal of Nuclear Materials 396 (2010) 49–56 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat