4.11.1 Introduction

This chapter aims to address that need by explaining the influence of microstructure on the properties of nuclear graphite and how irradiation-induced changes to that microstructure influence the behavior of graph­ite components in reactor. Nuclear graphite is manu­factured from coke, usually a by-product of the oil or coal industry. (Some cokes are a by-product of refining naturally occurring pitch such as Gilsonite.9) Thus, nuclear graphite is a porous, polycrystalline, artificially produced material, the properties of which are de­pendent on the selection of raw materials and man­ufacturing route. In this chapter, the properties of the graphite crystal structures that make up the bulk poly­crystalline graphite product are first described and then the various methods ofmanufacture and resultant properties of the many grades of artificial nuclear graphite are discussed. This is followed by a description of the irradiation damage to the crystal structure, and hence the polycrystalline structure, and the implica­tion of graphite behavior. The influence of radiolytic oxidation on component behavior is also discussed as this is of interest to operators or designers of graphite­moderated, carbon dioxide-cooled reactors, many of which are still operating.

Nuclear graphite has, and still continues, to act as a major component in many reactor systems. In prac­tice, nuclear graphite not only acts as a moderator but also provides major structural support which, in many cases, is expected to last the life of the reactor. The main texts on the topic were written in the 1960s and 1970s by Delle et al.l Nightingale,2 Reynolds,3 Simmons,4 in German, and Pacault5 Tome I and II, in French with more recent reviews on works by Kelly6,7 and Burchell. This text is mainly on the basis of the UK graphite reactor research and operating experi­ence, but it draws on international research where necessary.

During reactor operation, fast neutron irradiation, and in the case of carbon dioxide-cooled systems radiolytic oxidation, significantly changes the graph­ite component’s dimensions and properties. These changes lead to the generation of significant graphite component shrinkage and thermal stresses. Fortu­nately, graphite also exhibits ‘irradiation creep’ which acts to relieve these stresses ensuring, with the aid of good design practice, the structural integ­rity of the reactor graphite core for many years. In order to achieve the optimum core design, it is important that the engineer has a fundamental under­standing of the influence of irradiation on graphite dimensional stability and material property changes.