Micro-structural study and Rietveld analysis of fast reactor fuels: UeMo fuels S. Chakraborty a, * , G. Choudhuri b , J. Banerjee a , Renu Agarwal c , K.B. Khan a , Arun Kumar a a Radiometallurgy Division, Bhabha Atomic Research Centre, Mumbai, 400085, India b Atomic Fuels Division, Bhabha Atomic Research Centre, Mumbai, 400085, India c Product Development Division, Bhabha Atomic Research Centre, Mumbai, 400085, India highlights  UeMo alloys in as-cast as well as in annealed conditions have been studied using Optical Microscope, SEM, XRD.  The monoclinic a'' phase in as-cast U-10 at.% Mo alloy has been characterized through Rietveld analysis.  The dendritic microstructure of g-(U,Mo) and B.C.C. ‘Mo’ phase of 33 at.% UeMo alloy have been analysed.  Rietveld analysis has been done to optimize lattice parameters and calculate phase fractions in annealed alloys.  The Vickers microhardness of U 2 Mo phase shows lower hardness than two phase microstructures in annealed alloys. article info Article history: Received 14 June 2015 Received in revised form 27 September 2015 Accepted 30 September 2015 Available online 22 October 2015 Keywords: U-Mo alloys Fast reactor fuels Dendrite Rietveld analysis Monoclinic phase abstract UeMo alloys are the candidate fuels for both research reactors and fast breeder reactors. In-reactor performance of the fuel depends on the microstructural stability and thermal properties of the fuel. To improve the fuel performance, alloying elements viz. Zr, Mo, Nb, Ti and fissium are added in the fuel. The first reactor fuels are normally prepared by injection casting. The objective of this work is to compare microstructure, phase-fields and hardness of as-cast four different UeMo alloy (2, 5, 10 and 33 at.% Mo) fuels with the equilibrium microstructure of the alloys. Scanning electron microscope with energy dispersive spectrometer and optical microscope have been used to characterize the morphology of the as-cast and annealed alloys. The monoclinic a'' phase in as-cast U-10 at.% Mo alloy has been characterized through Rietveld analysis. A comparison of metallographic and Rietveld analysis of as-cast (dendritic microstructure) and annealed U-33 at.% Mo alloy, corresponding to intermetallic compound, has been reported here for the first time. This study will provide in depth understanding of microstructural and phase evolution of UeMo alloys as fast reactor fuel. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The UeMo alloy had been used as fuel for early metallic fuel fast reactors viz. Enrico Fermi Reactor (Fermi-I) in USA. A significant number of metallic fuel rods of UeZr, U-fissium and UeZr-X have been burnt in EBR and FFTF [1]. There are several experimental, pilot and demonstration fast breeder reactors; e.g. FBTR (India), DFR (UK), RAPSODIE (France), JOYO (Japan), PFR (Germany) and BOR-60 (Russian federation), BN600 and Super-phenix (France) [2]. The world over people has opted for more technologically mature, oxide fuel option. The rationales for extensive usage of oxide fuels are of ease of fabrication, handling and reprocessing. However, with the advancement of technology, it is now feasible to conveniently fabricate, handle and reprocess metallic fuels. The main advantages of metallic fuels over conventional ceramic fuels are high breeding ratio, high thermal conductivity and better in-reactor behavior during transient conditions. The economic competence of metallic fuel driven fast breeder reactor depends on high burneup of metallic fuels. Though currently India is in a process of commis- sioning Prototype Fast Breeder Reactor (PFBR), fueled by conven- tional (U,Pu)O 2 e MOX fuel, but upcoming FBRs are planned to be fueled by uranium based metallic alloys. Pure uranium has three stable allotropic phases, orthorhombic a-U (231  Ce667.3  C, 4 atoms/cell), tetragonal b-U * Corresponding author. E-mail address: sibasis@barc.gov.in (S. Chakraborty). Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat http://dx.doi.org/10.1016/j.jnucmat.2015.09.055 0022-3115/© 2015 Elsevier B.V. All rights reserved. Journal of Nuclear Materials 467 (2015) 618e627