In Situ Thermal Oxidation Process of Tungsten Under Fusion Relevant Accidental Conditions

Tungsten has been selected for use in the divertor of the International Thermonuclear Experimental Reactor (ITER) and will also be likely applied for the future Demonstration fusion reactor (DEMO). However, tungsten is susceptible to corrosion by oxidation, which often accentuates quickly at moderately high temperatures above 500 °C accompanied by cracks. In the cases of loss-of-coolant accidents (LOCAs) and loss-of vacuum accidents (LOVA), air ingress and nuclear decay heat could trigger aggressive tungsten oxidation, leading to uncontrollable first wall decomposition and emission of volatile radioactive oxide dust, posing a severe fusion safety hazard. However, significant knowledge gaps exist in the understanding of tungsten thermal oxidation and how the coupling of irradiation-induced damages affects tungsten oxidation and decomposition, which are essential for tungsten-based PFM sustainability.<br/>In-situ and/or <i>operando</i> environmental transmission electron microscopy (ETEM) is an emergent paradigm that enables a direct correlation of kinetic data and structural evolution in real-time during such material transformations as oxidation in a controlled gaseous environment. In this talk, we present the first in-situ ETEM observation of thermal oxidation of a pair of high purity tungsten samples at temperatures from 500 °C to 900 °C, using a novel MEMS-based gas cell under atmospheric pressure. These in-situ characterizations provide direct insights into the genuine phases and stress states in tungsten oxide scales under the oxidation conditions and shed light on the dynamic changes in the oxide scale phase, microstructure, thickness, and orientation dependence at the early stages of oxidation. We also discuss the effects of irradiation-induced displacement damage and surface restructuring on the tungsten oxidation process and kinetics.