9.4. As background for the thermal-hydraulics aspects of reactor design, it is helpful to consider the path of energy transport from the thermody­namic viewpoint. In a nuclear power plant, heat flows from the fuel ele­ments to a coolant, then perhaps to a secondary coolant, as in a PWR, to form steam. Work is done by expanding the steam in a turbine. The turbine exhaust steam is then condensed and recycled. Heat extracted during the condensation process is rejected to the environment. Thus, we have a movement of energy from the heat generation at high temperature in the fuel elements progressively downward on the temperature scale until the environment is reached. Along the way, a portion of the original energy is converted to useful work by a steam expansion in the turbine that is almost thermodynamically reversible. However, at each heat flow step, an irreversible temperature “drop” is needed to accomplish the heat transfer at a practical rate. As we shall see, this temperature difference acts as the driving force for the thermal transport rate process.

9.5. The thermodynamic efficiency of the conversion to work is im­proved if the heat input from the steam to the turbine is at as high a temperature as possible and the heat rejection in the condenser is at as low a temperature as possible. Since the “high” temperature in a water — cooled reactor is usually limited by materials and pressure considerations, and the “sink” or rejection temperature by environmental and other fac­tors, the irreversible temperature “drops” required for heat transfer be­tween these two limits must be expended carefully by the thermal system designer.

Design Methods

9.6. Engineers have used design methods describing the transfer of heat and the movement of fluids for many years. In a typical reactor core, the transport of heat from the fuel to the moving coolant involves the traditional processes of conduction and convection. Further steps in the thermody­namic energy path described above also involve the behavior of fluids in motion, particularly the effects of a second phase, as is produced by boiling. Over the years, a substantial body of literature has been developed de­scribing the principles relevant to thermal system design. Early, largely empirical design methods have been supplemented by models having some theoretical basis. Finally, the general availability of powerful computers has made a reasonably sophisticated description of transport processes practical for design purposes.

9.7. Since it is not our purpose here to treat the principles of conductive and convective heat transfer from the beginning, readers who have not been introduced to this area are advised first to consult an elementary text. Of importance is the concept of a rate process and the role of a mathe­matical representation of the conservation of mass, energy, and momentum in the description of such processes. With this background, we are then able to introduce the highlights of nuclear reactor thermal-hydraulics. Dis­cussions of sophisticated analysis methods are beyond the scope of this book but are available in standard sources [1].

9.8. Our objective here is merely to provide a picture of energy trans­port considerations in a reactor system using simple analysis methods. Such methods may be useful for preliminary design or scoping, but appropriate computer codes are needed for subsequent more detailed design. However, it is important to recognize that every method has a level of confidence or range of error that should be identified by the user.