An improved steam generator for use in a nuclear power plant of the pressurized water type in which a turbine generator is driven by the steam output of the steam generator to provide electrical power therefrom. The improvement comprises providing vertically assemblable modules which are removably mounted together in sealing relationship, with the modules comprising a base module, a tube bundle module removably mountable on the base module in sealing relationship therewith and an uppermost dryer module removably mountable on the tube bundle module in sealing relationship therewith whereby ready access to and removal of the tube bundle module in situ from the nuclear power plant steam generator is facilitated. The dryer module contains moisture separator means within the interior thereof in communication with a steam outlet for drying the generated steam provided to the steam outlet to the turbine generator. The base module, upon which the associated weight of the vertically assembled dryer module and tube bundle module are supported, contains the inlet and outlet for the heat exchange fluid. The tube bundle module contains the tube bundle through which the heat exchange fluid flows as well as an inlet for feedwater. The tube sheet in which the tube bundle is supported also serves as a closure flange for the tube bundle module, with the associated weight of the vertically assembled dryer module and tube bundle module on the tube sheet closure flange effectuating the sealing relationship between the base module and the tube bundle module for facilitating closure.