The Influence of Specimen Surface Roughness and Temperature of Steam Injection on Breakaway Oxidation Behavior of Zry-4
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The Influence of Specimen Surface Roughness and Temperature of Steam Injection on Breakaway Oxidation Behavior of Zry‑4 Fuel Cladding in Steam at 1273 K Martin Negyesi1,2 · Masaki Amaya1 Received: 9 January 2020 / Revised: 20 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The steam oxidation behavior of Zr-based Zircaloy-4 fuel cladding was studied at 1273 K with two different surface roughness levels. Steam was introduced either at room temperature (RT) or at 1273 K. Weight gain kinetics were evaluated by post-test weight measurement, and the reaction products and alloy microstructure were evaluated using optical microscopy. Hydrogen pick-up was measured by the gas extraction technique. Specimen surface roughness did not affect the oxidation kinetics or the hydrogen absorption. The time to breakaway oxidation was suppressed when steam was introduced at RT, and the oxide was more adherent, suggesting superior mechanical properties. When steam was introduced at 1273 K, an undulated oxide–metal interface formed earlier and a higher amount of hydrogen was absorbed by cladding before the kinetic transition. The alloy grain grew into larger size in the condition when steam was injected at 1273 K compared to the condition when steam was injected at RT, which may affect the observed behavior. After the oxide breakaway, the rate of hydrogen absorption accelerated substantially independent of the temperature of steam injection. Keywords Zry-4 · Surface roughness · Steam oxidation · Breakaway · Hydrogen
* Masaki Amaya [email protected] 1
Nuclear Safety Research Center, Japan Atomic Energy Agency, 2‑4 Shirakata, Tokai‑mura, Naka‑gun, Ibaraki 319‑1195, Japan
2
Present Address: Centre for Advanced Innovation Technologies, VŠB-Technical University of Ostrava, Ostrava, Czechia
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Oxidation of Metals
Introduction Nuclear fuel cladding tubes may be exposed to high-temperature steam environment during the design basis loss of coolant accident (LOCA). Zr-alloys (Zry4, Zr1Nb, etc.) are used for fabrication of fuel cladding. They have outstanding corrosion resistance. Though the oxidation rate accelerates substantially under LOCA conditions due to elevated temperature, the zirconium oxide (ZrO2) forms on cladding surface and constitutes a barrier against subsequent oxidation. The parabolic law applies at temperatures higher than ~ 1273 K [1–3]. The oxide layer is dense and protective. However, the breakaway oxidation, which is related to extensive oxide cracking, may occur under certain condition. It leads to the transition in the oxidation kinetics: the oxide growth accelerates and the kinetics becomes nearly linear [2, 4–10]. The time to the onset of the breakaway is of high importance in the safety analysis of LOCA. An extensive research has been carried out to study the oxide breakaway. However, the cause of the breakaway is not fully understood, yet. Moreover, various published works showed large differences in the onset time of the breakaway of
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