4.10 Introduction

This section gives a brief description of the more important commercial nuclear power reactors used in the western world. For detailed information on their design and performance there is a wealth of excel­lent textbooks, reviews and reports readily available in the literature.

Commercial nuclear power reactors (Fig 1.27) are principally thermal reactors — that is, the fast neu­trons released by fission are slowed down in a mod­erator to speeds which correspond roughly to the moderator temperature; the neutrons are thus in ther­mal equilibrium with the moderator. Thermal neutrons are much more likely than fast neutrons to induce further fission enabling natural, or perhaps only slight­ly enriched, uranium fuel to be used. This contrasts with the alternative design approach of the fast re­actor in which the chain reaction is sustained by fast neutrons. There is, therefore, no requirement for a moderator but the fuel used must have a high pro­portion of fissile material, usually plutonium.

All the three possible moderator materials — carbon in the form of graphite, ordinary light water H20 and heavy water D20 — have been exploited commercially. The fuel used is either metallic uranium or the dioxide U02 to give much superior thermal and dimensional stability in service. The coolant is either gaseous (carbon dioxide, or helium where a high degree of inertness is required) or water (light or heavy). The fuel is encased in a cladding to prevent the highly radioactive fission products from entering the coolant and also to act as a chemical barrier between the fuel and coolant. Stainless steel or mag-

nox, an alloy of magnesium, is used in the case of the gaseous coolant and a zirconium alloy, zircaloy, for water coolant.

Table 1.8 lists some of the main design features of a number of commercial nuclear reactors. The table includes the fast reactor because of its future potential. Also included is the steam generating heavy water reactor SGHWR because of its UK interest, although there is now little likelihood of any commer­cial application of this design.