For a real cycle the heat sink is limited by the fact that the "earth" is our final heat sink. And therefore, is fixed at about 60 F (520 R). The heat source is limited to the combustion temperatures of the fuel to be burned or the maximum limits placed on nuclear fuels by their structural components (pellets, cladding etc.). In the case of fossil fuel cycles the upper limit is ~3040 F (3500 R). But even this temperature is not attainable due to the metallurgical restraints of the boilers, and therefore they are limited to about 1500 F (1960 R) for a maximum heat source temperature.