The invention of an electric furnace2 capable of reach­ing temperatures approaching 3000 °C by Acheson in 1895 facilitated the development of the process for the manufacture of artificial polygranular graphite. Detailed accounts of the manufacture of polygranular graphite may be found elsewhere.2-4 Figure 2 sum­marizes the major processing steps in the manufacture of nuclear graphite. Nuclear graphite consists of two phases: a filler material and a binder phase. The pre­dominant filler material, particularly in the United States, is a petroleum coke made by the delayed coking process. European nuclear graphites are typi­cally made from a coal-tar pitch-derived coke. In the United Kingdom, Gilsonite coke, derived from natu­rally occurring bitumen found in Utah, USA, has been used. Both coke types are used for nuclear graphite production in Japan. The coke is usually calcined (thermally processed) at 1300 °C prior to being

crushed and blended. Typically, the binder phase is a coal-tar pitch. The binder plasticizes the filler coke particles so that they can be formed. Forming pro­cesses include extrusion, molding, vibrational mold­ing, and isostatic pressing. The binder phase is carbonized during the subsequent baking operation (800-1000 °C). Frequently, engineering graphites are pitch impregnated to densify the carbon artifact, fol­lowed by rebaking. Useful increases in density and strength are obtained with up to six impregnations, but two or three are more typical.

The final stage of the manufacturing process is graphitization (2500-3000 °C) during which, in sim­plistic terms, carbon atoms in the baked material migrate to form the thermodynamically more stable graphite lattice. Nuclear graphites require high chemical purity to minimize neutron absorption.

Moreover, certain elements catalyze the oxidation of graphite and must be reduced to an acceptable level. This is achieved by selecting very pure cokes, utilizing a high graphitization temperature (>2800 °C), or by including a halogen purification stage in the manufac­ture of the cokes or graphite. Recently, comprehensive consensus specifications5,6 were developed for nuclear graphites.

The electronic hybridization of carbon atoms (1s2, 2s2, 2p2) allows several types of covalent bonded structure. In graphite, we observe sp2 hybridization in a planar network in which the carbon atom is bound to three equidistant nearest neighbors 120° apart in a given plane to form the hexagonal graphene structure. Covalent double bonds of both s-type and я-type are present, causing a shorter bond length than in the case of the tetrahedral bonding (s-type sp3 orbital hybri­dization only) observed in diamond. Thus, in its per­fect form, the crystal structure of graphite (Figure 3) consists of tightly bonded (covalent) sheets of carbon atoms in a hexagonal lattice network.7 The sheets are

weakly bound with van der Waals type bonds in an ABAB stacking sequence with a separation of 0.335 nm.

The crystals in manufactured polygranular graph­ite are less than perfect, with approximately one layer plane in every six constituting a stacking fault. The graphite crystals have two distinct dimensions, the crystallite size La measured parallel to the basal plane and the dimension Lc measured perpendicular to the basal planes. In a coke-based nuclear graphite, values of La ~ 80 nm and Lc~ 60 nm are typical.8 A combination of crystal structure bond anisotropy and texture resulting from forming imparts aniso­tropic properties to the filler coke and the manufac­tured nuclear graphite. The coke particles become preferentially aligned during forming, either with their long axis parallel to the forming axis in the case of extrusion, or with their long axis perpendicu­lar to the forming axis in the case of molding or vibrational molding. Consequently, the graphite arti­facts are often attributed with-grain and against-grain properties as in the American Society for Testing and Materials (ASTM) specifications.5,6 The degree of isotropy in manufactured graphite can be controlled through the processing route. Factors such as the nature of the filler coke, its size and size distribution, and the forming method contribute to the degree of isotropy. Nuclear graphites are typically medium or fine grain graphites (filler coke size <1.68 mm)5,6 and are considered near-isotropic. Fine grain graphites (grain sizes <100 pm) formed via isostatic pressing often exhibit complete isotropy in their properties.

In response to the recent renewed interest in high — temperature gas-cooled reactors, many graphite ven­dors have introduced new nuclear graphites grades. Table 1 summarizes some ofthe grades available cur­rently, although this list is not exhaustive. The graphite manufacturer is listed along with the coke type and comments related to the given graphite grade.