The invention discloses a heat exchange system and a nuclear reactor system. The heat exchange system comprises a heating system, a heat consumption device and steam, wherein the heat consumption device is connected with the heating device through a pipeline to form a loop; and the steam is located in a wet steam state before a heat source is supplied, and becomes dry steam through heat exchange with the heating device or is supplied to the heat consumption device after being overheated. With water vapor as a heat exchange medium, the heat exchange efficiency is improved; and the safety of the nuclear reactor system is improved.