Improvement of Nuclear Grade Graphite Based on Isotropic and Highly Crystalline Natural Graphite by Generation of Silicon- or Zirconium-Carbide in Situ

Following the fabrication technique originally developed for HTR-molded block fuel elements a process was introduced to fabricate nuclear graphite with the aim to attain the improved irradiation stability above 3 × 10²²n/cm², E > 0,1 MeV and to increase corrosion resistance. Nuclear highly crystalline natural graphite is used. A phenol formaldehyde resin with additives of silicon or zirconium oxide powder serves as binder. The mixture thus obtained is isostatically consolidated into spheres and spheres are crushed to granules from which the 0.3 – 3 mm fraction is obtained. The granulate is hot molded into graphite bodies. The green bodies are heated to about 800 °C to carbonise the resin and subsequently annealed at 1900°C in vacuum. The key feature of the proposed process is based on the chemical affinity of binder coke with the structure obtained by carbonisation of green bodies. Consequently it reacts selectively in situ with the added SiO₂, or ZrO₂to carbides in vacuum at 1900°C. Silicon carbides and zirconium carbides are characterised by high mechanical strength and very good resistance to corrosion. The properties of reactor graphite, such as density, mechanical properties and in particular stability to fast neutron irradiation are considerably improved.