A power system (10) is provided. System (10) generally comprises at least one generator module (100, 100'), and a heat source (200) encircling at least a portion of the generator module (100, 100'). Each generator module (100) includes at least one generator section (110) having a heat exchanger (120) extending axially therefrom. The heat source (200) is defined by a plurality of separable arcuate aeroshell segments (210, 210') extending angularly about the heat exchanger section (120, 120') of each generator module (100, 100') to collectively describe a substantially cylindrical outer contour coaxially oriented thereabout. Each arcuate aeroshell segment (210, 210') has formed therein at least one fuel compartment (212) extending axially inward from a front axial face (214) thereof in which a radioisotope fuel material (220) is stored and sealed therein by a covering member (230).