A reactor vessel is a type of pressure vessel used in nuclear power plants to contain the reactor core and its coolant. The vessel and its primary components are designed to contain the fuel and sustain the extreme temperatures and pressures of a nuclear reactor. The vessel's primary purpose is to provide a sealed environment for the reactor core, which contains the nuclear fuel assemblies, control rods, and coolant. The reactor vessel also provides a secure containment for the radioactive materials, shielding from radiation and other hazards.