As with the current large-rated reactors, SMR coolants can be light water, gas, or liquid metal. Key SMR examples of these primary system coolant types with their principal design parameters are presented in Table 1.2. The coolant properties which dictate the different design characteristics of these SMRs are presented in Table 1.3. Principal among them are:

• the very high outlet temperature (750-950 °C) of the high-temperature gas reactor (HTGR) possible with the use of helium as coolant and graphite as the principal core material, yielding a high plant thermal efficiency and supply of reactor heat for processes requiring high temperature heat;

• the low primary operating pressure of the liquid metal reactors permitted by the low vapor pressure of their primary coolant at their high operating temperature; and

• the high power density of the sodium-cooled reactor possible because of its operation with a fast neutron spectrum coupled with a very high heat transfer coefficient that allows tight packing of its fuel pins.

The predominant use of light water in both pressurized and boiling water large-rated reactors currently in use can be readily replicated for SMR application. The smaller primary system components of pressurized water SMRs allows their arrangement within the pressure vessel as is already done even for large power rated BWRs. This PWR configuration, the integral reactor, was pioneered (as discussed in Section 1.1.3) in the commercial merchant vessel, the German Otto Hahn, and is a principal configuration of current PWR SMRs as elaborated in Chapter 3.

Helium has been the gas coolant of SMR choice, although carbon dioxide is used in advanced gas reactors (AGRs) operating in the UK which are currently slated for retirement. The liquid metal coolants of SMR choice are sodium, lead, and lead — bismuth. Sodium has been exploited significantly for large-rated reactors based on early work with sodium-potassium and sodium, while more exotic coolants such as lithium have been used for electricity-generating space reactors, e. g., the SNAP (Systems for Nuclear Auxiliary Power) series. For SMRs attention is focused on sodium and the variants of lead cooling — both pure lead and lead-bismuth eutectic.

Differentiation among reactor types and specific reactor designs within a coolant — type design is based on their satisfaction of a selected mission and then a set of criteria including operational reliability, protection of public health and safety, and finally economic competitiveness. The salient characteristics of the SMR reactors as they relate to these factors are presented next. Chapter 2 and the chapters in Part Four elaborate the detailed technical features of SMRs covering this range of primary coolants.