The reactor designer requires a high-density, very pure graphite, with a high scattering cross-section, a low absorption cross-section, and good thermal and mechanical properties, both in the unirradiated and irradiated condition. The purity is important to ensure not only a low absorption cross-section but also that during operation the radioactivity of the graphite remains as low as possible for waste disposal purposes.

Artificial graphite is manufactured from coke obtained either from the petroleum or coal industry, or in some special cases (such as Gilsocarbon, a UK grade of graphite) from a ‘graphitizable’ coke derived

from naturally occurring pitch deposits.9 The raw coke is first calcined to remove volatiles and then ground or crushed for uniformity, before being blended and mixed with a pitch binder. (Crushed ‘scrap’ artifi­cial graphite may be added to help with heat removal during the subsequent baking. For nuclear graphite, this should be of the same grade as the final product.) This mixture is then formed into blocks using one of various techniques such as extrusion, pressing, hydro­static molding, or vibration molding, to produce the so-called ‘green article.’ The ‘green’ blocks are then put into large ‘pit’ or ‘intermittent’ gas or oil-fired furnaces. The blocks are usually arranged in staggers, covered by a metallurgic coke, and baked at around 800 °C in a cycle lasting about 1 month to produce carbon blocks. These carbon blocks may be used for various industrial purposes such as blast furnace liners; it has even been used for neutron shielding in some nuclear reactors. (Care must be taken as the carbon blocks are not as pure as graphite and may lead to waste disposal issues at the end of the reactor life.)

To improve the properties of the graphite pro­duced from the carbon block, the carbon block is often impregnated with a low-density pitch under vacuum in an autoclave. To facilitate the entry of the pitch into the body of the block, the block surface may be broken by grinding. After impregnation the blocks are then rebaked. This process of impregna­tion and rebaking may be repeated 2, 3, or 4 times. However, the improvement in the properties by this process is subject to diminishing rewards.

The next process is graphitization at about 2800­3000 °C by passing a large electrical current at low voltage through the blocks either in an ‘Acheson furnace’ or using an ‘in-line furnace.’ In both cases, the blocks are covered by a metallurgical coke to prevent oxidation. This graphitization cycle may take about 1 month. If necessary, there may be a final purification step. This involves heating the graphite blocks to around 2400 °C in a halogen gas atmosphere to remove impurities. The final product can then be machined into the many intricate com­ponents required in a nuclear reactor.

For quality assurance purposes, during manufac­ture the blocks are numbered at an early stage and this number follows the block through the manu­facturing process. This is clearly an expensive manufacturing process and therefore, at each stage, quality control is very important. Many samples will be taken from the blocks to ensure that the final batch (or heat) is of appropriate quality compared to previ­ous heats. It is important that the reactor operators retain this data in electronic form as it may be required to investigate any anomalous behavior as the reactor ages. Samples of ‘virgin’ unirradiated graphite blocks should also be retained for future reference. Records should include information on the batch or heat, property measurements, nonde­structive testing (NDT) results, and measurements of impurities. It is not enough just to have the ‘ash’ content after incineration and the ‘boron equivalent’ as some impurities, such as nitrogen, chlorine, and cobalt, will cause significant issues related to reactor operation and final waste disposal. It is important that the reactor operator takes responsibility for these measurements as in the past it has been found that reactor designers and graphite manufacturers close down or merge, and records are lost.

Final inspection will uncover issues related to damage, imperfection, quality, etc. Therefore, a ‘con­cessions’ policy is required to determine what is acceptable and where such components can be used in reactor. Again, the reactor operator will require an electronic record of these concessions.