[en] The fuel in a High Temperature Reactor (HTR) is in the form of myriads of particles (microspheres) of about 1 mm overall diameter (Figure 1). In each, a ''kernel'' of UO₂ is surrounded by successive layers of pyrocarbon and silicon carbide. The particles are embedded in a matrix of carbon to form fuel elements over which high pressure helium flows as a heat transport medium (coolant). Such ''dispersed'' fuel transmits its energy efficiently to the coolant so that fuel maximum temperatures are only a little above the coolant exit temperature. The coolant exit temperature can, therefore, be relatively high.