The picture illustrates how a gas-cooled nuclear reactor generates the steam. It shows the process of the system in a reactor.
Overall, a reactor is covered by a concrete radiation shielding to protect a pressure vessel that includes the main factors to make the energy. In the reactor, the water that is put into the circulator is changed to steam by hot gas in a heat exchanger, and a reactor makes the circulation possible with generating the hot gas.
Looking at the details, the main process of the facility is that gas circulates in a reactor and is transited a state from cool one to hot one. Firstly, the cool gas gets through the gas blower to a pressure vessel and passes by graphite moderators. Secondly, the fuel elements get into the moderators through the charge tubes, and it makes a chemical reaction with boron and uranium fuel. In the phase, generated heat warms the cool gas; also, the heated gas finally arrives in the heat exchanger. As a result, it gets water to transform to steam; in contrast, the gas that is cooled down gets through the circulation.