The steam cycle in Figure 4.9 is very similar to that of a conven­tional fossil-fuelled power plant, because the maximum steam temper­atures are similar. The similarity is to some extent fortuitous, however, because the maximum temperature is set by different considerations in the two cases. In a fossil-fuelled power plant the maximum steam temperature is limited to about 565 °C because anything higher would require the use of austenitic steel rather than ferritic, and the increased cost would not be justified by the gain in efficiency. In a sodium-cooled fast reactor plant, however, austenitic steel is widely used, as we have seen. The main temperature limits are the maximum permissible fuel cladding temperature, and the temperature of the structure above the core in a pool reactor or of the hot leg pipework in a loop reactor. The parts of the structure in contact with hot primary sodium are subject to substantial thermal shock if the reactor is shut down suddenly in an emergency, and the primary sodium maximum temperature is limited to a level at which this shock can be withstood.

The problems of withstanding thermal shocks has even led some designers to propose reduction of the primary sodium maximum temperature to below 500 °C (Anderson, 1978; Horst, 1978). Steam superheaters are omitted, and moisture separators and reheaters are incorporated between some of the turbine stages to avoid the irre­versibilities associated with high moisture content in the steam. The resulting cycle is very similar to that of a boiling water reactor.

If maximum steam temperatures of 500 °C or above are permiss­ible, a conventional steam cycle with superheat and reheat can be

Secondary Sodium

water/Steam

Heat Transferred
1 High Feed Temp., Low Saturation Temp

Low Efficiency
2 Low Feed Temp., High Saturation Temp

High Efficiency

Figure 4.15 The interrelation of plant efficiency with the feedwater temperature.

used, as shown in Figure 4.9. The details of the cycle, and in particular of the feed heating system, may be slightly different from those of a fossil-fuelled power plant because the effect of final feed temperat­ure on efficiency is rather different if heat is being transferred from a relatively low-temperature coolant than if it is transferred from high — temperature gas. The point is discussed in detail by Haywood (1975) and can be illustrated by reference to Figure 4.15.

The plant efficiency is greater the higher the mean temperature at which the working fluid receives heat. Because a large portion of the heat supplied to the fluid is taken up by evaporating it, the higher the saturation temperature, and therefore the pressure, the greater the effi­ciency. If the cycle efficiency were to be increasedby increasing the steam pressure without changing the secondary sodium temperatures, it would be necessary to decrease the feedwater temperature by means of a different feed heating system, as shown in Figure 4.15. This illustrates
the fact that the feed and saturation temperatures in a plant of this type are interdependent. In contrast in a fossil-fuelled plant, they are independent (Haywood, 1975).

In this example the increased saturation temperature could, of course, be accommodated by allowing the minimum secondary, and also primary, sodium temperatures to rise. This in turn would require an increase in the secondary and primary sodium flow-rates and would involve the disadvantages of higher coolant speeds, more likelihood of vibration of heat exchanger tubes and fuel elements, greater pres­sure differences and stresses in the core, bigger circulating pumps and so on.