The invention relates to a simulation method which simulates the flow of a fluid inside a vessel of a nuclear reactor. The reactor includes the vessel and a core arranged within said vessel, the core comprising nuclear fuel assemblies, each assembly extending in an axial direction and comprising nuclear fuel rods and at least one grid for supporting the rods, each assembly being spaced from the other assembly by a clearance between the grids in a transverse direction perpendicular to the axial direction, the fluid being suitable for circulation inside the core. The method includes determining (200, 210, 220, 230) head loss coefficients in the core, and calculating (240) the pressure (P) of the fluid and the component(s) of the velocity (V) of the fluid in the core, using the following equation: [Nabla]P = - K x V, where P is the pressure component of the fluid, K is a matrix comprising the determined head loss coefficients, and V is a vector comprising the component(s) of the velocity of the fluid. During the determination step, a transverse head loss coefficient in the assemblies is determined on the basis of a transverse Reynolds number in the transverse direction, and an axial head loss coefficient in the clearance is determined on the basis of the size of the clearance in the transverse direction.