Investigating the Impact of Impurity Content on Nuclear Graphite Oxidation Behavior via Synchrotron Micro-XRD Analysis

Abstract: Background: Graphite serves as the neutron moderator and neutron reflector in molten salt reactors and gas cooled reactors. Under high-temperature conditions, graphite undergoes oxidation, which includes chronic oxidation during normal operation and severe oxidation in the event of an air-ingress accident. Comprehending graphite's oxidation behaviors and the crucial factors that affect these behaviors is of great significance. Purpose: The primary objective of this research is to investigate the impact of impurities on the oxidation behaviors of graphite and the microstructures within the graphite oxidation layer. Methods: Four grades of graphite (IG-110, IG-11, NG-CT-10, NG-CT-1) with varying contents of chemical impurities were employed. The oxidation experiment was carried out in a tube furnace at a temperature of 700 °C and an air flow rate of 6L/min. Cylindrical samples, having a diameter of 25.4 mm and a length of 25.4 mm, were oxidized until the weight losses reached approximately 20%. Slice specimens were then extracted from the oxidized samples for subsequent analysis. Micro-focused X-ray diffraction (micro-XRD) was utilized to characterize the lattice structures within the oxidation layer at the Shanghai Synchrotron Radiation Facility BL17UM beamline station. Subsequently, the samples were observed under a scanning electron microscopy (SEM), and the distribution of impurities was analyzed with the energy dispersive spectrometer (EDS). Results: The weights and sizes of all graphite samples were measured before and after oxidation. When the weight loss reached approximately 20%, the sizes of purified graphite samples (IG-110 and NG-CT-10) exhibited minimal alteration, in contrast to the un-purified graphite samples (IG-11 and NG-CT-1), whose sizes diminished noticeably. The surfaces of the un-purified graphite samples were characterized by the presence of small pits. Micro-XRD spectra indicated a reduction in both the (002) peak intensity and peak width within the oxidation layer. A diminished diffraction peak intensity was indicative of weight loss. The defective structures with abundant active sites were oxidized preferentially while the graphite structures with large crystal sizes survived, leading to a decrease of the (002) peak width. Through the analysis of the profiles of the (002) peak intensity and peak width, the oxidation layer thicknesses were determined to be 1.9 mm and 3.2 mm for IG-11 and IG-110 graphite, respectively, and approximately 3.7 mm for NG-CT-10 and NG-CT-1 graphite. Significant morphological differences were observed between IG-11 and IG-110. In the oxidized IG-11, a number of oxidation pits (with sizes of around 100 μm) were detected, and they were distributed within a layer possessing a thickness of 3 mm. This thickness exceeded the layer thickness measured by micro-XRD (1.9 mm). The chemical element analysis proved that there is enrichment of chemical impurities (V, Ti, Fe and Ca) in the oxidation pits, demonstrating that chemical impurities could significantly accelerate the oxidation reaction of the surrounding graphite structures. Conclusion: Analysis demonstrated that defective structures, owing to their plentiful active sites, were oxidized preferentially, leading to very complicated oxidation paths in graphite. This is a common phenomenon for both purified and un-purified graphite. The enrichment of chemical impurities serves to catalyze the oxidation of the adjacent graphite structures, subsequently leading to the formation of oxidation pits within the graphite matrix.