Thermophysical and mechanical properties of UFe$_2$ fabricated by spark plasma sintering

Abstract

Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, core meltdown produced fuel debris whose safe retrieval and management require reliable thermophysical and mechanical property data. Among the metallic phases identified in the debris, the U-Fe system is particularly important because of the abundant iron originating from in-vessel stainless steel structures. However, within this system, the high-temperature thermophysical properties of UFe$_2$ have received relatively little attention, with most prior studies focusing on its magnetic and electronic properties. To fill this data gap in the literature, we fabricated dense, nearly single-phase polycrystalline UFe$_2$ by arc melting followed by spark plasma sintering, and characterized its thermal and mechanical properties from room temperature to 1073 K. Results show that the thermal conductivity of UFe$_2$ increased monotonically from 10 Wm$^{-1}$K$^{-1}$ at 306 K to 25 Wm$^{-1}$K$^{-1}$ at 1073 K, surpassing those of the iron intermetallics Fe$_2$Zr and Fe$_2$B at high temperatures. In addition, UFe$_2$ is mechanically more compliant, displaying a Young's modulus $E$ of 69 GPa, a shear modulus $G$ of 24 GPa, and a Vickers hardness $H_{\mathrm{V}}$ of 5.6 GPa, all well below those of both Fe intermetallics. Consequently, during decommissioning, thermal-management and structural evaluations should take into account the comparatively high-conductivity and mechanically compliant nature of UFe$_2$ within the heterogeneous fuel debris.