Solidification-induced nonuniformity in U–Zr–RE metallic nuclear fuel rods

www.nature.com/scientificreports OPEN Solidification‑induced nonuniformity in U–Zr–RE metallic nuclear fuel rods Seung Uk Mun 1,2,3, Gun Oh 1,2, Jun Hwan Kim 3, Sang‑Gyu Park 3* & Byung Mook Weon 1,2* Metallic fuel is being developed for the next generation of sodium-cooled fast reactors due to its safety and fuel cycle economics through spent fuel reprocessing technology. After spent fuel reprocessing, rare earth elements that are immiscible with Uranium and Zirconium are included in the raw material for metallic fuel. Achieving a uniform composition and microstructure of RE inclusions is challenging because phase separation and temperature inhomogeneities during solidification lead to non-uniform composition and microstructure. This study explores the solidification and inclusion formation of U–Zr–RE metallic fuel rods. We show how RE inclusions are formed in the U–Zr matrix and how temperature inhomogeneities during solidification control the inclusion size distribution and edge migration of RE inclusions along the U–Zr–RE rods. Understanding the inhomogeneity of inclusions due to solidification can provide hints for making homogeneous nuclear fuel rods. Energy supply is a global concern, immediately driven by the impacts of climate change and the need for development with sustainability. Reducing carbon emissions has recently led to exploring innovative energy supply technologies. Nuclear power is essential to ensure a stable energy supply and reduce carbon emissions. Based on sustainability, the Sodium-cooled Fast Reactor (SFR), a promising Generation-IV (GEN-IV) reactor, demonstrates higher stability and efficiency performance than the previous generation r eactors1–8. SFR is being developed in conjunction with a spent nuclear fuel reprocessing process called pyroprocessing. SFR recycles nuclear fuel by pyroprocessing, enhancing uranium utilization and reducing high-level waste. Metallic fuel rods are the strongest candidates for nuclear fuel for SFRs, with many advantages such as superior thermal conductivity, reduced local hot spots, higher maximum temperatures, and low heat capacity, making heat dissipation from the fuel easier9–12. Metallic fuels provide a more significant operational margin against fuel failure than oxide fuels because they have mechanical strength and resistance to failure, except for a low melting point. Metallic alloys have high uranium mass ratios (fissile = U-235 density), which are favorable to reactor performance4,6,13,14. However, after pyroprocessing, the recycled metallic fuel raw material will contain rare earth (RE) elements that are immiscible with minor actinide elements and u ranium15,16. In particular, highly reactive rare earth elements cause many problems during the casting process of metallic fuel rods. The high reactivity of the U–Zr–RE melt with crucibles, mold materials, and cladding materials can adversely affect the composition of fuel rods, enlarging nuclear w aste17. Notably, RE elements can readily react with the reaction prevention coating layer and the cladding tube, probably generating the surface reaction layer18. Microstructure control of the highly reactive RE element in U–Zr–RE alloy is essential to prevent the problems mentioned above. Moreover, the hydrodynamic behavior of U–Zr–RE melt causes the problem of uranium nonuniform distribution due to phase separation. Rare earth elements are immiscible in the U–Zr–RE liquid alloy and undergo phase separation19–22. Phase separation causes non-uniformity in the distribution of uranium elements and makes predicting nuclear fuel performance difficult. Mixing all elements in melts, essential for uniform distribution of elements, is dynamically affected by melt hydrodynamics. The dynamic motion of immiscible RE elements can be nonuniform and directional by hydrodynamic forces such as Marangoni and Stokes forces23. Marangoni forces due to temperature inhomogeneity (inducing interfacial tension gradients) and Stokes forces due to density differences in fluids may hinder the uniform distribution of elements in immiscible alloys24–29. Understanding melt hydrodynamics is critical to developing U–Zr–RE alloys. 1 Soft Matter Physics Laboratory, School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu‑ro, Jangan‑gu, Suwon, Gyeonggi‑do 16419, Republic of Korea. 2Research Center for Advanced Materials Technology, Sungkyunkwan University, 2066 Seobu‑ro, Jangan‑gu, Suwon, Gyeonggi‑do 16419, Republic of Korea. 3Advanced Nuclear Fuel Technology Development Division, Korea Atomic Energy Research Institute (KAERI), 111 Daedeok‑daero beon‑gil, Yuseong‑gu, Daejeon, Chungcheongnam‑do 34057, Republic of Korea. *email: ; Scientific Reports | (2024) 14:19402 | https://doi.org/10.1038/s41598-024-69935-x 1 Vol.:(0123456789) www.nature.com/scientificreports/ This paper shows how phase separation and temperature inhomogeneity affect elements’ final microstructure and compositional distribution by solidifying U–Zr–RE alloys for nuclear fuel rods. We mainly investigated the RE inclusions separately formed in the U–Zr matrix and the temperature inhomogeneity induced by the cooling rate difference. The RE inclusion size distribution along a U–Zr–RE rod and the edgeward migration of RE inclusions are attributed to hydrodynamic effects caused by temperature inhomogeneity during solidification. This work provides practical information on making homogeneous nuclear fuel rods based on the behavior of nuclear fuel components in the liquid state and the effects of a non-uniform temperature distribution. Materials and methods Materials The U metal (99.8% purity) was polished using a standard polishing process to remove the oxide layer on the surface. U and Zr (Hunan High Broad New Material, 99.9% purity, sponge-type) were cleaned using ethanol in an ultrasonic cleaner for surface cleaning. The RE elements were Nd, Ce, Pr, and La as highly reactive elements (Alfa Aesar, 99.9% purity) with each composition of 53, 25, 16, and 6 wt.%, respectively.30 The RE elements were alloyed by arc melting to prevent oxidation through rapid heating. Modified injection casting The modified injection casting (MIC) process was used to manufacture the U–Zr–RE fuel rods in a chamber, as described in Fig. 1. The prepared materials (U metal 85 wt.%, Zr metal 10 wt.%, and RE 5 wt.% alloy) were loaded in a Y2 O3-coated graphite crucible.23 The composition of RE in this experiment was set according to the expected conditions of RE in TRU ingots after the pyroprocessing of spent nuclear fuel from light water reactor (LWR)31. In addition, the RE composition was chosen to determine the effect of RE under more definite conditions and to consider the economics of nuclear reprocessing. The Y2 O3 coating aims to prevent reactions between the molten alloy, the graphite crucible (by a plasma spray coating method), and the quartz mold (by a slurry coating method). The chamber was used to co (...truncated)