Steam oxidation and interfacial reaction behavior of seamless SS316L/ Zr-alloy/SS316L multilayer cladding fabricated by swaging-drawing methodology

초록

Zr-alloy fuel cladding exhibits excellent neutronic compatibility and manufacturability, but its rapid oxidation in high-temperature steam remains a critical vulnerability under loss-of-coolant accident conditions. In this study, a seamless (SMLS) SS316L/Zr alloy/SS316L triple-layer cladding was fabricated by swaging-drawing methodology and evaluated as a dual-sided metallic barrier architecture with a longitudinally weld-free SMLS tube wall. The SMLS design was compared with a previously developed electric-resistance-welded (ERW) G-tube architecture to assess the high-temperature response associated with improved circumferential barrier continuity in the weld-free SMLS configuration. Under LOCA-like steam exposure from 600 to 1200 degrees C, the SMLS triple-layer cladding maintained a concentric tubular configuration without catastrophic collapse. The mass gain remained below 0.5% at 600-1100 degrees C and reached only approximately 4.26% after 1 h at 1200 degrees C, indicating substantial suppression of rapid Zr alloy oxidation. The apparent mass-change-derived oxidation response, interpreted together with the post-exposure morphology and phase-analysis results, was consistent with a multilayer-mediated degradation pathway rather than unrestricted direct steam attack on the Zr-alloy core. SEM-EDS line profiles revealed that the SS316L/Zr alloy interfaces remained chemically distinguishable up to 1100 degrees C, whereas Fe/Cr redistribution, interfacial oxygen enrichment, and Zr-Fe-rich reaction products became more pronounced at 1200 degrees C. Site-specific TEM-SAED and STEM-EDS analyses further revealed that the 1200 degrees C reaction zone was crystallographically heterogeneous, containing Fe-Zr-related intermetallic domains, Zr(Fe,Cr)2-related Laves-phase domains, and discrete Fe-Cr-rich bcc precipitates. However, no clear XRD evidence for extensive ZrO2 formation was observed in the SMLS specimen, indicating that the Zr alloy core was not converted into a continuous oxide-dominated region. In addition, the SMLS cladding retained dimensional stability throughout 20 thermal-shock cycles between 25 and 500 degrees C, with only small absolute deviations in outer diameter, inner diameter, specimen length, and mass from their respective initial values and no systematic cumulative drift. These results demonstrate that the SMLS triple-layer architecture functions as a finite, longitudinally weld-free kinetic barrier that delays radial steam ingress and preserves tubular continuity under severe steam exposure. However, the thermally activated Zr-Fe-rich reaction regions indicate that future multilayer ATF claddings require additional diffusion-barrier or buffer-layer strategies to suppress interfacial embrittlement while retaining meter-scale manufacturability.

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