The invention relates to a simulation method which simulates the flow of a fluid inside a nuclear reactor core. The reactor includes a core arranged within said vessel and provided with inlets and outlets for the fluid, the core comprising a lower plate, an upper plate, and fuel assemblies extending in an axial direction between the plates, the core having a space defined axially by the first and second interfaces corresponding to said plates. The method includes calculating (120, 130), for the core space, the pressure (P) and the velocity (V) of the fluid, on the basis of an initial value of the velocity or of the pressure at the first interface and at the second interface respectively, and, using mass balance equations, calculating the amount of movement and energy. The method includes the determination (100) of an additional space inside the vessel, the additional space being external to the core space and axially at an end thereof, the additional space being axially defined by two interfaces, one of which is the first or second interface, and calculating (110, 140), for the additional space, using said equations, the pressure and the velocity on the basis of an initial value of the velocity or of the pressure at each one of the interfaces of the additional space. The calculation of the pressure and of the velocity is first carried out for a first space among the additional spaces and that of the core, and at the interface shared by the additional spaces and the space of the core, then the calculation of the pressure and of the velocity is carried out for the second space among said spaces, the initial value of the velocity or pressure at said shared interface and for the second space being the value of the magnitude among the velocity and the pressure calculated at said interface for the first space.