The function of the steam generator is to convert heated water into steam for use by the turbine.
Water is heated in the reactor core and sent to the steam generator.
The steam generator is made up of tubes that are surrounded by a shell. The tubes contain the primary water and the shell contains the secondary side consisting of a two phase water steam mixture. As the water passes by the tube, it picks up heat and eventually gets converted to steam. The steam is sent to the turbine-generator where the steam is converted into mechanical energy in the form of shaft rotation so that the generator will turn and produce electrical energy. The purpose of the cleaning was to take preventative maintenance action against the metallic oxides on the secondary side of the steam generator.