Chemical compatibility of uranium based metallic fuels with T91 cladding

Studies related to development of fast reactor fuels based on ternary U–Pu–Zr and binary U–Pu alloys has been initiated in India for building a data base on thermo-physical and thermodynamic properties, fuel-clad compatibility etc. which are very useful to the fuel-designer to optimize the design feature and to predict the in-reactor fuel behaviour. Fuel-clad chemical compatibility is considered as one of the major concerns for metallic fuels. In the present investigation, the performance of Zr as fuel-clad chemical interaction (FCCI) barrier layer between U and T91 was evaluated by diffusion couple experiments. The growth kinetics of reaction layers at U/Zr and Zr/T91 interfaces were established. The growth kinetics of the reaction zone at both the U/Zr and Zr/T91 interfaces were determined at 973 K from the plot of log (width) versus log (time). The value of reaction index n was found to be around 2 at both the U/Zr and Zr/T91 interfaces. The reaction constant (k) for the growth of reaction layer at the U/Zr interface was determined to be 2.07 × 10−8 m s−1/2 at 973 K. Similarly, the rate constant at the Zr/T91 interface was found to be 1.95 × 10−8 m s−1/2 at 973 K. The activation energy Q for the reaction at the Zr/T91 interface was determined and was found to be 54.7 kJ mole−1. The fuel-clad chemical compatibility between U–6Zr alloy and T91 steel was also investigated in the present study by diffusion couple experiments. The interdiffusion between U–6Zr and T91 at 973 K resulted in the formation of three different layers at the interface. The mechanism of formation of these layers was analysed in detail.

► Performance of Zr as FCCI barrier layer was evaluated by diffusion experiments.
► Rate constant for reaction at U/Zr interface was 2.07 × 10−8 m s−1/2 at 973 K.
► Rate constant for reaction at Zr/T91 interface was 1.95 × 10−8 m s−1/2 at 973 K.
► Activation energy for reaction at Zr/T91 interface was found to be 54.7 kJ mole−1.
► Interdiffusion between U–6Zr and T91 resulted in formation of three layers.