Transmission electron microscopy characterization of Fuel Cladding Chemical Interaction (FCCI) in ATR-irradiated HT9 clad U-10M (10M = 5Mo-4.3Ti-0.7Zr wt%) metallic fuel

Abstract

The pseudo-binary metallic fuel alloy, U-10M (wt%, M is the optimal combination of Mo, Ti, and Zr), has the potential to increase fuel solidus temperature, reduce the onset temperature of body-centered cubic phase, and increase the fuel's chemical stability compared with the conventional U-10Zr (wt%) metallic fuel. Post Irradiation Examination (PIE) confirmed excellent fuel performance for the U-10M (10M=5Mo-4.3Ti-0.7Zr wt%) fuel irradiated in the Advanced Test Reactor (ATR) to 2.2 at% burnup at Peak Inner Cladding Temperature (PICT) of 650 °C, an upper bound temperature for metallic fuel. But previous PIE study also observed Fuel Cladding Chemical Interaction (FCCI) on the cladding side, which is known as a fuel performance limiting issue but has not been fully understood yet. As an effort to improve the understanding of FCCI phenomenon, this study performed Scanning Electron Microscopy (SEM) and in-depth Transmission Electron Microscopy (TEM) characterization on a FCCI region. The examined FCCI region is dominated by (U, Zr)(Fe, Cr)_2, suggesting that U-Fe interdiffusion reaction played a key role in inducing FCCI. Additionally, the FCCI boundary into the cladding consists of four distinctive phases, (U, Zr)(Fe, Cr)₂, fcc-Cr, tetragonal UCr_0.1Fe_9.9Si₂, intermetallic σ-FeCr, and lanthanide fission products at concentration up to ∼5.5 at%. Another goal of this study is to verify the involvement of Ti and Mo in FCCI formation on the cladding side. Observable Ti is found halfway of the thickness in the examined FCCI region, while 0.3–5 at% Mo is detected across the entire thickness of the examined FCCI region. Neither Ti nor Mo reacted with HT9 cladding constituents despite their diffusion footmark. Therefore, alloying Ti and Mo into the U-Zr fuel should not complicate the interdiffusion reaction on the HT9 cladding side.