Invited Lecture
Interdiffusion, reactions and phase transformations observed during fabrication of low enriched metallic fuel system for research and test reactors

Monolithic fuel system with low-enriched U-10 wt.% Mo (U10Mo) fuel alloy cladded in AA6061, with Zr diffusion barrier placed in-between, is being developed for research and test reactors. Hot-rolling followed by hot isostatic pressing (HIP) are typically employed to fabricate the fuel system, wherein various metallurgical phenomena including interdiffusion, reaction and phase transformations were observed. Detailed observations and analyses were carried out for γ-U10Mo fuel alloy, and near the interface between U10Mo and Zr, Zr barrier and AA6061, and AA6061 and AA6061 claddings, as functions of temperature, time and heating/cooling rate employed in HIP process. The U10Mo/Zr interaction layer after hot-rolling prior to HIP consisted of α-U, Mo₂Zr and UZr₂, however, growth of this interaction constituents was negligible even after prolonged high-temperature HIP process. During HIP, the growth of (Al,Si)₃Zr layer at the AA6061/Zr interface was observed with an activation energy of 457 kJ/mole. The decomposition of γ-U10Mo fuel alloy into α-U and γ’-U₂Mo occurred as predicted by the time-temperature-transformation diagram of U10Mo. UC and UO₂ inclusions formed during the casting of U-Mo were associated with the limited interdiffusion and reaction with Zr barrier. The chemical bands of oxygen- and nitrogen-rich phases were observed within AA6061 particle boundaries. Mg₂Si precipitate phases along with traces of MgO were observed to develop at the AA6061/AA6061 cladding interface during HIP. These observations are presented and discussed with respect to optimized fuel fabrication for fuel performance and integrity.