what device in a nuclear power plant creates nuclear energy?
The origin of the energy is the reactor core. Control rods are pulled out of the core to start the reactor, and a controlled chain reaction begins and is maintained. Neutrons released in fissions create other fissions to keep the chain alive. The fissions release energy, much of which is heat energy. The heat is transferred out of the core into the reactor coolant. The coolant transfers the heat into steam generators, and the heat creates steam which is then piped to steam turbines. The turbines drive generators to create electricity, which is then put on the power grid.
The device in a nuclear power plant that creates nuclear energy is called a nuclear reactor. It is the core component responsible for the nuclear fission process. The fission of uranium or plutonium atoms in the reactor releases a tremendous amount of heat, which is then utilized to produce steam that drives turbines connected to electrical generators, ultimately generating electricity.
To get this answer, you could simply understand the basic components of a nuclear power plant and how they function. Alternatively, you can look up "components of a nuclear power plant" or "nuclear reactor" in a search engine to find information about the specific device responsible for generating nuclear energy.