Oxidation of nuclear- and matrix-grade graphite for VHTR air ingress accident scenarios
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One of the most severe accidents anticipated for the Very High Temperature Reactor (VHTR) is an air ingress accident caused by a pipe break, where the reactor vessel and core become fully immersed in air as the core temperature rises potentially reaching 1873 K. Graphite oxidation is predicted to be severe under these conditions. Gasification of graphite impacts its geometry and reduces thermal and mechanical properties, thus affecting the safe performance and shortening the service-life of components constructed of graphite. We have studied the oxidation rate of several nuclear-grade and matrix-grade graphites including NBG-18 and IG-110 as well as GKrS, respectively. Oxidation data was collected thermogravimetrically for air ingress accident scenarios from 873 to 1873 K using a Thermax700[registered trademark symbol] thermogravimetric analyzer. A semi-empirical Arrhenius rate equation was developed for the kinetic regime for each graphite grade. The activation energy for the matrix-grade graphite was within the limited historically reported values while the activation energy of nuclear-grade graphite was determined to be well within literature values. The surfaces of oxidized graphite samples were further characterized by SEM, EDS, FTIR and XPS.
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