Liquid-metal-cooled reactors are third behind water-cooled and gas-cooled reactors in terms of global commercial reactor experience. Test and demonstration reactors using

sodium, lead, or lead-bismuth coolant have been constructed in the United States, the Russian Federation, France, Japan, the United Kingdom, and most recently in China and India. Interest in liquid-metal coolants is driven by the desire to develop fast-spectrum reactors, i. e. reactors that generate most of the power from fissions resulting from high-energy neutrons unlike water-cooled reactors, which generate power from thermal neutron-induced fissions. The key advantage of fast-spectrum reactors is that there are more neutrons produced per fission and these ‘excess’ neutrons can be used for purposes other than sustaining the basic chain reaction. Initially, the extra neutrons were intended to be used to ‘breed’ fuel, i. e. produce new fissile fuel faster than it is consumed. As more and more uranium reserves were discovered worldwide, interest in fast-spectrum reactors turned away from the breeding function to a resource recovery, i. e. producing energy from the unburned fuel discharged from water-cooled reactors, and waste management, i. e. consuming the associated long-lived waste products from that fuel.

Another advantage of liquid-metal-cooled reactors is that the metals have high boiling temperatures, which allow the reactors to operate without pressurization of the primary coolant. Also, the coolant can be heated to a moderately high temperature, typically around 500 °C. Although higher than the 300-325 °C outlet temperature in a water-cooled reactor, it is still lower than the 750-850 °C outlet temperature in gas-cooled reactors. The higher temperature increases the power conversion efficiency relative to water-cooled reactors and can allow for more compact power conversion systems using supercritical Rankine or Brayton cycles.

Listed in this section and summarized in Table 2.4 are the three liquid-metal — cooled reactor designs that have near-term deployment potential by virtue of commercial support and some level of engagement by a licensing authority. Many other designs and projects are underway, but either are limited to research studies or are experimental/test reactors intended to be precursors for large commercial plants, such as the Traveling Wave Reactor being developed by TerraPower.