CHAPTER 4 - The Natural Uranium Reactor

Abstract

This chapter discusses the natural uranium reactor. The heavy isotope of hydrogen–deuterium does not capture neutrons to any appreciable extent and may be obtained in a concentrated form as heavy water, D2O. Although expensive, it is one of the two possible moderator materials for natural uranium reactors. Helium is chemically inert and can, therefore, only be obtained in its gaseous form, while lithium, beryllium, and boron must be rejected on the grounds of chemical reactivity, toxicity, high capture cross-section. The second feasible moderating material, carbon, is used in the form of graphite. Heavier nuclei produce insufficient energy transfer per collision. The use of natural uranium therefore imposes two basic limitations in reactor design. The reactor must be heterogeneous—that is, the fuel and moderator must be separated from each other to minimize neutron capture and the moderator itself must be either heavy water or graphite. These restrictions are imposed because of the need to reduce the percentage of neutrons removed from the fission reaction chain to a minimum. In the natural uranium reactors, it is also necessary to select the constructional materials used in the reactor with care, as any substance capturing neutrons strongly must be excluded. The situation is made more difficult by the fact that some of the nuclei produced by the fission process themselves capture neutrons strongly.