The microstructure of a typical nuclear graphite is described with reference to Gilsocarbon. This product was manufactured from coke obtained from a naturally occurring pitch found at Bonanza in Utah in the United States. To understand the microstructural properties, one has to start with the raw coke. The structure of Gilsonite coke is made of spherical parti­cles about 1 mm in diameter as shown in Figure 4. This structure is retained throughout manufacture and into the final product. In Figure 4(b), the spher­ical shaped cracks following the contours of the spherical particles are clearly visible. This coke will be carefully crushed in order to keep the spherical structures that form the filler particles and help to give Gilsocarbon its (semi-) isotropic properties.

At a larger magnification in a scanning electron microscopy (SEM), the complexity of these cracks is clearly visible, Figure 4(c), and at an even larger magnification, a ‘swirling structure’ made up of graphite platelets stacked together is discernable between the cracks. In essence, the whole structure contains a significant amount of porosity.

After graphitization, the Gilsonite coke filler par­ticles are still recognizable (Figure 5(a) and 5(b)). From the polarizing colors, one can see that the main V axis orientation of the crystallites follows the

(a)

Figure 4 Gilsonite raw-coke microstructure. (a) Photograph of Gilsonite coke, (b) Scanning electron microscopy (SEM) image of polished Gilsonite coke, (c) detail in an SEM image showing the region around cracks that follow the spherical shape of the coke particles, and (d) a higher magnification SEM image showing the intricate, random arrangement of platelets. Courtesy of W. Bodel, University of Manchester.

200 pm

Figure 5 Polarized optical and scanning electron microscopic images of Gilsocarbon graphite. (a) Optical image, (b) optical image, (c) SEM image, (d) SEM image. Courtesy of A. Jones, University of Manchester.

spherical particles circumferentially, as does the ori­entation of the large calcination cracks. The crystal­lite structures in the binder phase are much more randomly oriented, and this phase contains signifi­cant amounts of gas-generated porosity. There are also what appear to be broken pieces of Gilsonite filler particles contained within the binder phase.

The bulk properties of polycrystalline nuclear graphite strongly depend on the structure, distribu­tion, and orientation of the filler particles.12 The spherical Gilsonite particles and molding technique give Gilsocarbon graphite semi-isotropic properties, whereas in the case of graphite grades such as the UK pile grade A (PGA), the extrusion process used dur­ing manufacture tends to align the ‘needle’ type coke particles. Thus, the crystallite basal planes that make up the filler particles tend to align preferentially, with the ‘c’ axis parallel to the extrusion direction and the V axis perpendicular to the extrusion direc­tion. The long microcracks are also aligned in the extrusion direction. The terms ‘with grain (WG)’ and ‘against grain (AG)’ are used to describe this phe­nomenon, that is, WG is equivalent to the parallel direction and AG is equivalent to the perpendicular direction. Thus, the highly anisotropic properties of the crystallite are reflected in the bulk properties of polycrystalline graphite (Table 1).

A graphite anisotropy ratio is usually defined by the AG/WG ratio of CTE values. For needle coke graphite, this ratio can be two or more, while for a more randomly orientated structure, values in the region of 1.05 can be achieved by careful selection of material and extrusion settings. A more scientific way of defining anisotropy ratio is by use of the Bacon anisotropy factor (BAF).

Other forming methods are usually used to pro­duce isotropic graphite grades such as the Gilsocar — bon grade described above. In this case, it was found that Gilsocarbon graphite produced by extrusion was not isotropic enough to meet the advanced gas — cooled reactor (AGR) specifications. Therefore, a

‘molding’ method where the blocks were formed by pressing in two directions was used. This had the effect of slightly aligning the grains such that the AG direction was parallel to the pressing direction and the WG was perpendicular to the pressing direc­tion. However, Gilsocarbon has proved to be one of the most isotropic graphite grades ever produced, even in its irradiated condition.

Another approach is to choose an ‘isotropic coke’ crushed into fine particles and then produce blocks using ‘isostatic molding’ process. The isostatic mold­ing method involves loading the fine-grained coke binder mixture into a rubber bag which is then put under pressure in a water bath. In this way, high quality graphite can be produced mainly for use for specialist industries such as the production of elec­tronic components. This type of graphite (such as IG — 110 and IG-11) has been used for high-temperature reactor (HTR) moderator blocks, fuel matrix, and reflector blocks in both Japan and China. However, even these grades exhibit slight anisotropy.

The final polycrystalline product contains many long ‘thin’ (and not so ‘thin’) microcracks within the crystallite structures that make up the coke particles. Similar, but much smaller, cracked structures are to be found in the ‘crushed filler flour’ used in the binder, and in well-graphitized parts of the binder itself. It is these microcracks that are responsible for the excellent thermal shock resistance of artificial polycrystalline graphite. They also provide ‘accom­modation,’ which further modifies the response of bulk properties to the crystal behavior in both the unirradiated and irradiated polycrystalline graphite. Typical properties of several nuclear graphite grades are given in Table 2. One can see that polycrystalline graphite has about 20% porosity by comparing the bulk density with the theoretical density for graphite crystals (2.26 gcm~ ). About 10% of this is open porosity, the other 10% being closed.