Abstract: The apparatus for simulation of the operation of a pressurized water reactor comprises a reactor vessel with a heater, a steam generating vessel divided into a primary portion and a secondary portion. The reactor vessel and the primary portion of the steam generating vessel are connected in a primary feed loop equipped with a coolant pump, while the secondary portion of the steam generating vessel and the condenser unit are joined together in a secondary feed loop provided with a feed pump. With such an apparatus a steam generating vessel heating pipe break or fracture can be simulated when the reactor vessel and a primary part of an additional steam generating unit are joined in another primary feed loop parallel to the primary feed loop having the cooling pump and a secondary part of the additional steam generating unit is connected with the aid of at least two branch pipes to the secondary feed loop.