The temperature requirements for different heat applications vary considerably. Temperatures of the order of 100°C are required for district heating and seawater desalination whereas for some process heat applications and hydrogen production temperatures of the order of 1000°C and above are required. Different reactor types supply different temperature ranges of output, typical ranges are shown below in Table 14.1.

There is a wide range of applications, and different applications have different requirements, particularly temperature requirements (Table 14.2). The lower temperature end with water reactors and the higher end with high-temperature gas reactors have received the most attention to date (IAEA-TECDOC-923, 1997). A standard requirement for most users is reliability and availability. This is particularly so for the process industry, where production depends on energy supply to continue. In industry, energy must usually be available as a base-load commodity. This contrasts the load requirements for district heating where the demand is dependent on climatic conditions. Consequently load factors for energy producers for district heating applications may be much smaller than those required for industrial applications.

For reactors operating in co-generation mode for electricity and heat, there are issues of balance that need to be considered. For large power reactors, the main output may be electricity and these reactors will be optimised for base-load electricity generation.

For small reactors, a higher proportion of their output may be heat; therefore, significant fluctuation of the heat demand could result in fluctuation of electricity output. Thus the technology needs to ensure that the electricity production and the grid load are compatible.

In this chapter, some of the various reactor designs that are being considered for heat and other applications are discussed. To date, most of the operational experience has been on water-reactor systems. Some operational experience has been gained with liquid sodium. Lead and particularly lead-bismuth systems are being examined in Russia, both for district heating and for seawater desalination. As discussed in Chapters 12 and 13, various innovative reactor systems are being considered for high-temperature applications.