The largest coolant temperatures are limited to 350 °C by pressure in the case of water cooled reactors and to 600 °C by corrosion in the case of liquid metal cooled reactors. Higher temperatures would allow higher efficiencies for electricity conversion, using combined cycles, as well as heat cogeneration. They might also have interesting chemical applications like thermal decomposition of water to produce hydrogen. High temperatures can be reached with a gas coolant, especially helium. These considerations were at the origin of the studies on high-temperature gas reactors (HTGRs). These reactors also have, potentially, interesting safety properties, although they use graphite as their neutron moderator like the British Wind — scale or the Chernobyl RBMK reactors. The high operating temperature would prevent the Wigner effect which led to the Windscale reactor accident. Using helium rather than water as coolant would ensure strong negative temperature coefficients, in contrast to the case of the water cooled RBMK reactors. The strong negative temperature coefficient ensures a breaking off of the chain reaction in the case of a loss of cooling. After reactor shut­down, the fuel element temperatures will rise until radiation cooling takes over. This is possible because fuel elements are designed to be able to sustain very high temperatures. The fuel is made of microspheres (TRISO spheres) of fissile and fertile nuclei surrounded by several layers of carbon, which ensure that no fission products can escape from the spheres. The microspheres are, themselves, embedded in carbonaceous materials which constitute the fuel rods. These are placed in graphite blocks, through which holes allow cooling gas circulation. Extensive tests were carried out in Germany, on the AVR reactor, to evaluate the behaviour of the fuel with temperature. The operating temperature is around 1000 °C. The fuel was tested at 1600 °C for several hundred hours and very small fission product release was observed. For moderate power reactors with around 150 MWe, calculations show that, in the absence of cooling, a maximum temperature of 1600 °C can be reached for a few tens of hours. The temperature is limited by radiation cooling. This is efficient because not only the total power but also the specific power of the reactor are kept small. The specific power is limited to 6kW/l, to be compared to the 100kW/l for PWRs.

The probability of significant radiation release has been estimated to be 10~8 per year, i. e. three orders of magnitude less than for PWRs.

The main safety concern for the HTGR is that air intrusion in the vessel would cause the graphite to burn.