The six reactor types proposed by the Generation IV forum include developments of the HTR thermal reactor and the high-temperature gas-cooled fast reactor (using pebble or prismatic fuel elements as well as plate designs). The object of the GenlV designs includes development of systems giving high utilisation of fuel, reduced waste arisings and the potential for burning actinides. Development is scheduled to progress through the second and third decades of the twenty-first century.

There have been many variations proposed on the basic gas-cooled reactor. Designs have been studied for helium, or helium-xenon coolant reactors for use in space exploration. Adams Atomic Engines of the USA (now defunct) proposed designs for very small-to-medium reactors cooled with nitrogen. In the UK, designs were studied for CO2 coolant reactors loaded with graphite dust to enhance heat transfer performance, and a design for a high-temperature variant on the AGR using helium coolant (the HTAGR) was extant during the late 1970s (anticipating one of the current GenIV designs).