A statistical analysis of pores and micro-cracks in nuclear graphite

Abstract: Microstructure characterization is of great value to understanding nuclear graphite's properties and irradiation behavior. However, graphite is soft and could be easily damaged during sample preparation. A three-step polishing method involving mechanical polishing, ion milling and rapid oxidation is proposed for graphite. Ion milling is adopted to remove the broken graphite pieces produced by mechanical polishing. Rapid oxidation is then adopted to remove irradiation-induced damage layer during ion milling. The Raman spectra show very low G peak width and ID/IG ratio after rapid oxidation, indicating a surface completely free from artificial defects. The micro-cracks which were conventionally observed via a transmission electron microscope can be observed on rapid-oxidized surface in a scanning electron microscope. By digital image processing, the micro-cracks along with the gas-escape pores in nuclear graphite IG-110 are statistically analyzed. Porosity's distributions on crack (pore) size (spanning from 10 nm to 100 um) are given, which could help to understand and simulate graphite's performances in reactors.