On the U-Fe-C Isothermal Section at 1100 °C

J. Phase Equilib. Diffus. https://doi.org/10.1007/s11669-024-01122-x ORIGINAL RESEARCH ARTICLE On the U-Fe-C Isothermal Section at 1100 C Margarida I. Sousa Henriques1,2 • André Silva1 • Ladislav Havela3 António Pereira Gonçalves1 • Submitted: 31 October 2023 / in revised form: 16 May 2024 / Accepted: 17 May 2024  The Author(s) 2024 Abstract The energy crisis and climate change have promoted a growing interest in non-fossil sources, such as nuclear, with uranium carbides being seen as potential fuel candidates for Generation IV nuclear reactors. However, the need of accurate thermophysical data for the fuel and its compatibility with core materials during the extreme fission conditions is still an issue. Here a study of the ternary uranium-iron-carbon system performed at 1100 C using powder x-ray diffraction and Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry is presented. The U-Fe-C isothermal section is characterized by two ternary compounds, thirteen 3-phase regions and five 2-phase regions. UFeC2 and * U11Fe12C18 were confirmed to be present at 1100 C and crystallize in structures related to the binary uranium carbides. UFeC2 crystallizes in an original structure type, a distorted variant of the UCoC2 structure, with space group P4/n and This invited article is part of a special tribute issue of the Journal of Phase Equilibria and Diffusion dedicated to the memory of Thaddeus B. ‘‘Ted’’ Massalski. The issue was organized by David E. Laughlin, Carnegie Mellon University; John H. Perepezko, University of Wisconsin–Madison; Wei Xiong, University of Pittsburgh; and JPED Editor-in-Chief Ursula Kattner, National Institute of Standards and Technology (NIST). & António Pereira Gonçalves 1 C2TN, DECN, Instituto Superior Técnico, Universidade de Lisboa, Campus Tecnológico e Nuclear, 2695-066 Bobadela, Portugal 2 FZU - Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 00 Prague, Czechia 3 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic a = 3.503(5) Å and c = 7.405(5) Å lattice parameters. * U11Fe12C18, has a crystal structure related to the Th11Ru12C18 structure-type (space group I 4 3m) with the lattice parameter a & 10 Å. Furthermore, an island of a aUC2-based phase with 32U:4Fe:64C composition was found in the 1100 C isothermal section, indicating the inclusion of Fe in the a-UC2 binary compound. Keywords isothermal section
ternary phase diagrams
uranium carbides
U-Fe-C ternary system 1 Introduction The improvement in safety, sustainability, economics, and reliability required for Generation IV nuclear systems is extremely challenging. Although the fission capability of a nuclear fuel is a fundamental issue, the safe operation of the reactor is also determined by the thermophysical properties of the fuel and its compatibility with core materials during the extreme operation conditions. Binary uranium carbides do not have phase transformations between 800 and 1500 C, have dimensional stability under irradiation, and a higher fissile metal density, melting point and thermal conductivity than UO2.[1] In this context, great attention is being paid to compounds based on actinide carbides as strong candidates for fuels to be used in gas-cooled fast reactors (GFRs), very high temperature reactors (VHTRs),[2,3] and fuels for nuclear rockets.[4] However, uranium carbides exhibit lower oxidation resistance and cladding-compatibility limit than UO2.[5] Furthermore, interest in ternary actinide carbides has grown in the recent years, and theoretical investigations have been carried out on (MC)mAl3C2 (M = metal, m = 1, 2, 3, 123 J. Phase Equilib. Diffus. …).[6–8] This practical and theoretical interest in actinide carbides led to the reinvestigation of the U-Fe-C system by studying its isothermal section at 1100 C. The U-Fe-C system was first studied to provide a background description of the interaction between carbide fuels and steel canning materials.[9] Primary works were performed in the 1960 s, mostly focusing on the vertical sections UC-Fe, UC-UFe2 and UC2-Fe.[10–13] When studying the phase equilibria along the UC2-Fe line, Baldock et al.[14] found the UFeC2 ternary carbide and concluded that it forms peritectically from UC2 and liquid. This was confirmed by Briggs et al.,[11] while analysing the quasibinary eutectic between Fe and UFeC2, and Nichols and Marples,[12] who studied the U-Fe-C and Pu-Fe-C systems. Nichols and Marples,[12] tentatively suggested several invariant four-phase equilibria based on thermal analysis, but the results were preliminary and required further experimental work. They[12] also proposed solid state tie-lines for ‘low temperatures’, which were used to construct the isothermal section at 1000 C presented by Holleck.[15] Raghavan[16] reviewed the U-Fe-C system and presented a partial liquidus projection, a reaction scheme, and the phase distribution expected after solidification. Subsequently, Alekseeva[17] investigated the system by melting alloy compositions in the UC-UFeC2-UC2 region and analyzing as-cast and annealed samples with different techniques. The partial liquid surface deduced by Alekseeva[17] is similar to that constructed by Raghavan,[16,18] except that the later work assumed Fe3C as a metastable phase that does not coexist with graphite. A partial isothermal section at 1400 C and a full isothermal section at 10508C were also drawn by Alekseeva.[17] The author[17] also attempted a qualitative description of all different isothermal sections with liquid. These results were presented in the short review by Raghavan.[18] A very complete overview of the previous work on the U-Fe-C system was made by Kuznetsov,[9] covering binary systems, solid phases, quasibinary systems, invariant equilibria, liquid, solid and solvus surfaces, isothermal sections, and thermodynamics. The isothermal sections presented were taken mainly from the work of Alekseeva,[17] but redrawn slightly to agree with the accepted binary boundary systems.[9] However, despite the amount of work done, there were still discrepancies in the results, crystallographic data were missing for some of the reported ternary compounds, and their stability and phase relationships were not fully assessed. Thus, following the group’s previous work on ternary U-Fe-X systems,[19–25] here a study on the ternary U-Fe-C system is presented to evaluate the isothermal section at 1100 C. 123 2 Literature Data 2.1 Binary Systems The binary Fe-C phase diagram[26] is presented as a double diagram, showing the metastable equilibrium with Fe3C (cementite) or the stable equilibrium with graphite. In the stable binary system, the solubility of carbon in iron is limited, and the remaining carbon precipitates as graphite, preventing the formation of equilibrium intermediate phases. The peritectic formation of the soli (...truncated)