On the spectrum of the multigroup diffusion equations

► The spectrum of the eigenvalues of the multigroup diffusion operator is tackled for applications to nuclear reactor physics. ► A consistent analytical formulation of the problem and physical interpretation is given. ► The limits of the diffusion model and some problems associated to the energy grouping are highlighted. ► The set of the time eigenvalues is evaluated, verifying the possible appearance of complex eigenvalues. ► The influence of slowing down phenomena on the appearance of complex eigenvalues is highlighted.