[pfdietz]

Sure, but the center of a PWR fuel rod reaches high temperature in normal operation. Uranium dioxide is not a great conductor of heat.

Chips have the advantage that the workings are right at the surface, mere microns from it. So the heat can easily get out. The power density in that thin surface layer can be very high. Perhaps similarly, the power density of a PV cell can be very high, if one just looks at the active layer where light is being absorbed. In CdTe this layer is < 1 micron thick. The energy delivered over the life of the cell per atom in this layer can approach that of nuclear reactions.