Fuel clad chemical interaction of U-Mo fast reactor fuel

Abstract

U-33 at.% Mo (16.8 wt.%) metallic fuel is a candidate material for metallic fuel Fast Reactor. One of the life-limiting factors of fast breeder reactor clad is the fuel clad chemical interaction due to formation of low melting eutectics between U and Fe. This chemical interaction should be avoided or minimized to increase the fuel burn-up. Fuel clad chemical interaction between U-33 at.% Mo metallic fuel with T91 (9 Cr-1Mo) clad has been studied at 650 °C, 675 °C and 700 °C, for different soaking time, through diffusion couple experiments. Development of microstructures, phase constituents and compositions during thermal treatment were examined by scanning electron microscopy and X-ray energy dispersive spectroscopy. In the fuel side, U6Fe phase is formed along with bcc-Mo in lamellar morphology through cellular precipitation. Due to slower diffusion of Mo compared to uranium from U-33 at.% Mo fuel, a Mo rich layer is formed at the slug surface. This Mo rich layer subsequently acts as a diffusion barrier layer and minimizes further growth of clad-wastage zone and slug-interaction zone. Multi-phase layer growth constants and activation energies have been calculated and compared with interactions reported in literature of other U-based fuels and cladding elements. The study indicates that U-33 at.% Mo/T91 fuel reduces fuel clad chemical interaction significantly compared with U-23Zr/Fe and U-23Zr/Fe-Cr.