Beginning with Chicago Pile I, graphite has been used as a moderator material in nuclear power stations and is considered a potential material for use in future Generation IV advanced reactors. The microstructure of graphite is responsible for much of its mechanical and thermo-physical properties, and how it responds to irradiation. To understand graphite microstructure, it is necessary to understand its porosity at the macro- and micro-scales; and to understand its porosity, it is necessary to characterize the morphological connectivity of the void content and the two main phases of graphite: filler and binder. Here, using several microscopy and analytical techniques, a detailed examination of the heterogeneity, microstructure and pore structure of different graphite grades and their binder and filler phases is presented. Significant differences were found between coarser and finer nuclear grades. Coarse grades have a more diverse range of filler particles, pores and thermal cracks. Finer grades have a more well-defined pore size distribution, fewer variations of filler particles sizes and do not contain as many large thermal cracks. Fine grades tend to have a well-connected network of pores whereas coarser grades contain a larger content of closed porosity. The framework developed within this work can be applied and used to assess the various graphite grades that would down-select materials for specific use in graphite moderated reactor designs.
Micrographs of multiple nuclear graphite grades were captured using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), complementing the data contained in the related manuscript, “A multi-technique image library of nuclear graphite microstructures of historical and modern grades.” The SEM micrographs show the differences among filler particles, binder, and thermal cracks contained in nuclear graphite. This library of microstructures serves as a baseline of as-received material and enables understanding the phases and differences between nuclear grades. TEM micrographs included in this manuscript elucidate the content of a common material contained in the binder phase known as quinoline insoluble (QI) particles. These particles are a phase of graphite that can be used as a forensic fingerprint of the neutron irradiation effects in graphite. The manuscript also contains some data of glassy carbon, an allotrope of carbon that shares similarities with some of the chaotic structures in nuclear graphite. Combined, these micrographs provide a detailed overview of the microstructures of various graphite grades prior to neutron irradiation.
