High-temperature air oxidation of SDM-fabricated SS316L/Zr-alloy ATF cladding: Evidence for local ZrN-ZrO2 composite diffusion barriers

초록

This study investigates the oxidation and nitridation of a double-layer SS316L/Zr-alloy accident-tolerant fuel cladding fabricated using a room-temperature, non-vacuum swaging-drawing methodology (SDM). Exposure to laboratory air up to 1,200 degrees C produced Cr-rich and Fe-rich oxide regions at the interface, which served as an initial barrier. Additionally, local gaps inherent to the SDM process promoted the formation of a composite ZrN-ZrO2 layer at the SS/Zr interface, which acted as a secondary diffusion barrier. Despite the breakdown of the external passive film and weld-induced cracking of the SS316L tube above 900 degrees C, the double-layer cladding maintained its integrity longer than the bare Zr alloy. Mass-gain measurements and X-ray diffraction analysis revealed that the bare Zr-alloy cladding rapidly formed monoclinic ZrO2 above 900 degrees C, whereas the double-layer cladding preserved alpha/alpha '-Zr up to approximate to 1,100 degrees C with delayed m/t-ZrO2 formation. Raman peaks at approximate to 165, 224, and 503 cm(-)(1), combined with SAED data showing {111}(approximate to 0.264 nm), {220}(approximate to 0.162 nm) confirmed the presence of ZrN. The parabolic rate constant K-p(mg(2)<middle dot>cm(-)(4)<middle dot>s(-)(1)) estimated at 600-1,200 degrees C was consistently lower for the double-layer cladding than for the bare Zr-alloy cladding. Arrhenius fits indicated a higher apparent activation energy for the double-layer system, demonstrating the effectiveness of the strengthened diffusion barriers. These findings provide a mechanistic explanation for the delayed oxidation observed under open-air, loss-of-coolant-accident-like conditions and highlight the protective role of discontinuous yet functional ZrN-ZrO2 composite layers.

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