1. A method for at least one of operating and building a nuclear reactor core comprising: modelizing a nuclear reactor core by performing the computer implemented steps of: partitioning, by a computer processor, the core in cubes to constitute nodes of a grid for computer implemented calculation, splitting, by the computer processor, the cubes into a first category and a second category, each cube of the first category being adjacent only to cubes from the second category so that the first category and second category of cubes are oriented in a checkerboard pattern with the first category of cubes and the second category of cubes alternating in a two-dimensional representation, ordering, by the computer processor, the cubes of the first category and then the cubes of the second category, and calculating, by the computer processor, neutron flux and/or thermohydraulics parameters by using an iterative solving procedure of at least one linear system, components of an iterant of the linear system constituting the neutron flux and/or thermohydraulics parameters to be calculated, wherein, during the iterative solving procedure, calculations are conducted on the cubes of the first category then on the cubes of the second category; and at least one of operating and building the nuclear reactor core on the basis of the calculated neutron flux and/or the calculated thermohydraulics parameters, wherein the iterative solving procedure is conducted only on the cubes of the first category thus calculating the components of the iterant corresponding to the cubes of the first category, and then the components of the iterant corresponding to the cubes of the second category are calculated on the basis of the components of the iterant corresponding to cubes of the first category, wherein the linear system to be solved for the first category of cubes is preconditioned to amount to: A _ red ⁢ x _ red = b _ _ red ⁢ ⁢ with ⁢ ⁢ { A _ red = 1 - A ND 2 b _ _ red = b _ red - A ND ⁢ b _ black , wherein x red designates the components of the iterant corresponding to the cubes of the first category, A ND designates a non-diagonal matrix coupling cubes of the first category with cubes of the second category, and b red and b black designates source factors.