4.7 Introduction

We have seen from a study of nuclear physics that a nuclear reactor is capable of generating heat at very high rates. The principal limitation on this rate of heat generation is the rate at which the heat can be transferred to the core coolant whilst maintaining the core components, in particular the fuel elements, within their design temperature limits. It is impor­tant therefore to be able to predict the rate at which coolants can extract heat from the core under various conditions of flow rates, pressures, geometries, etc. Hence the reason for a study of heat transfer and fluid flow in order to determine the rate of heat production in the reactor and to predict the tempera­ture patterns.

We will discuss the basic methods by which heat can be transferred and then apply them to the specific area of nuclear heat generation.

There are three methods by which heat can be transferred:

• Conduction.

• Convection.

•

Radiation.

For heat to be transferred by conduction the material through which the heat flows is essentially stationary, there being no net flow’ of atoms in solids or liquids, or of molecules in the case of gases. In reactors this mode of heat transfer is confined principally to the conduction of heat through the fuel element to the element surface.

The extraction of heat from the fuel element sur­faces by means of a gaseous or liquid coolant is prin­cipally by convection. The flow of the coolant over the element is achieved either by pumping, giving forced convective cooling, or by buoyancy forces aris­ing from differences in density. The former is by far the more important for the majority of reactor designs and particularly for gas cooled reactors; the latter is important in reactors where the coolant changes state over the fuel element, e. g., Boiling Water Reactors (BWR) and Steam Generating Heavy Water Reactors (SGHWR).

Heat transfer by radiation requires no contact or intermediate media between the two heat transfer sur­faces but since any appreciable heat transfer by this means requires high temperatures, i. e., above about 800°C, it is relatively unimportant in magnox, ad­vanced gas-cooled reactor (AGR) and-pressurised water reactor (PWR) systems.

In order that we can appreciate the principal design and operating factors affecting the rate of heat extrac­tion from the core, it is necessary to examine the laws of heat transfer, particularly those relating to conduc­tion and convection.