[en] A 3D Finite Element Method (FEM) has been developed to compute a set of dominant modes of a nuclear power reactor with Z-Hex geometry. This method is based on a mesh made of triangular prisms and assumes that the flux can be approximated by means of a truncated polynomial expansion on each one of the elements in which the domain defining the problem is discretized. The polynomial basis used is defined by means of a tensor product of polynomials in the z variable per polynomials in the XY plane. The performance of the method is evaluated computing the dominant lambda modes of the 3D benchmark problems VVER 1000 and VVER 440. (authors)