A vapor generator and heat exchanger, particularly suited for use in a substantially cylindrical cavity in a pressure vessel includes high and low temperature sections of the vapor generator positioned coaxially with the annular outer section terminating a distance from the end of the central or axial section to form at the one end an annulus arranged between the central or axial section and the pressure vessel wall. A plurality of reheater or heat exchanger sections and various inlet and outlet conduits are positioned in the annulus. A heating fluid is directed first transversely over the tubes of the reheater sections, then along the tubes of the central or axial high temperature section after passing over the outer annular low temperature section.