High Temperature Gas Reactors[28]

The largest coolant temperature are limited to 350 ${}^{\circ}C$ by pressure in the case of water cooled reactors and to 600 ${}^{\circ}C$ 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 only be reached with gas coolant, especially helium. These considerations were at the origin of the studies on High Temperature Gas Reactors (HTGR). These reactors have, also, potentially, interesting safety properties, although they use graphite as neutron moderator like the British Windscale or the Tchernobyl RMBK reactors. The high working temperature would prevent the Wigner effect which le$\nabla$d to the accident of the Windscale reactor. Use of helium rather than water as coolant would ensure strong negative coefficients, in contrast to the case of the water cooled RMBK reactors. The strong negative temperature coefficient insures a stop of the chain reaction in case of loss of cooling. After the reactor shut down, the fuel elements temperatures would rise until radiation cooling takes over. This is made possible by the specificities of the fuel elements which can sustain very high temperatures. The fuel is made of microspheres (TRISO spheres) of fissile and fertile nuclei surrounded by several layers of carbon, which prevent fission products escape from the spheres. The micro-spheres are themselves imbedded in carbonaceous material making up 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 behavior of the fuel with temperature. The operating temperature is around 1000 ${}^{\circ}C$ The fuel was tested at 1600 ${}^{\circ}C$ for several hundred hours and very small fission products release was observed. For moderate power reactors with around 150 Mwe, calculations show that, in the absence of cooling, a maximum temperature of 1600 ${}^{\circ}C$ can be reached for a few tens hour. The temperature is limited by radiation cooling. This is efficient because, not only the total power, but, also, the specific power of the reactor is kept small. The specific power is limited to 6 Kw/l, to be compared to the 100 Kw/l for PWRs.

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

The main safety concern for HTGR is that intrusion of air in the vessel would lead to combustion of the graphite.