1. A computer implemented method for modelizing a nuclear reactor core, comprising the steps of: partitioning the core in cubes to constitute nodes of a grid for computer implemented calculation, calculating a neutron flux for the core by using an iterative solving procedure of at least one eigensystem, components of an iterant of the eigensystem corresponding either to a neutron flux, to a neutron outcurrent or to a neutron incurrent, for a respective cube to be calculated, wherein neutrons are sorted in a plurality of neutron energy groups, and wherein the iterative solving procedure includes a substep of conditioning the eigensystem into a restricted eigensystem where said neutron energy groups are merged into a smaller number of coarse neutron energy groups, wherein an iteration of the restricted eigensystem solving procedure is: A Gmus ( out ) ⁢ d Gmus ( out ) ︸ `` ⁢ outflow ″ = μ [ ∑ G ′ = 1 NGC ⁢ ⁢ ∑ u ′ = 1 3 ⁢ ⁢ ∑ s ′ = 1 , r ⁢ ⁢ Q m ⁢ ⁢ G G ′ ⁢ u ′ ⁢ s ′ ⁢ d G ′ ⁢ η ⁡ ( mu ′ ⁢ s ′ ) ⁢ u ′ ⁢ s ′ ( out ) ︸ `` ⁢ isotropic ⁢ ⁢ production ″ + ∑ s ′ = 1 , r ⁢ C Gmus s ′ ⁢ d Gmus ′ ( out ) ︸ throughflow ″ `` ] wherein d (out) designates the components of the iterant corresponding to an individual cube m; u, u′ are Cartesian axis indexes; s, s′ are orientations equal to 1 or r and corresponding respectively to a left or a right surface of the individual cube along the respective Cartesian axes u, u′; G, G′ being neutron energy groups, A and C being mono-energetic factors, and Q designating a spectral factor, NGC being a number of neutron energy groups in the selection of neutron energy groups, and μ is a neutron eigenvalue, in which an isotropic term ∑ G ′ = 1 NGC ⁢ ⁢ ∑ u ′ = 1 3 ⁢ ⁢ ∑ s ′ = 1 , r ⁢ ⁢ Q m ⁢ ⁢ G G ′ ⁢ u ′ ⁢ s ′ ⁢ d G ′ ⁢ η ⁡ ( mu ′ ⁢ s ′ ) ⁢ u ′ ⁢ s ′ ( out ) is independent of an outgoing current direction specified by the Cartesian axis u and orientation s, and a throughflow term ∑ s ′ = 1 , r ⁢ ⁢ C mGus s ′ ⁢ d Gmus ′ ( out ) being defined within the same neutron energy group G along the same Cartesian axis u.