Mixed and mixed-hybrid elements for the diffusion equation

Among the classical methods used for solving the neutron diffusion equation, the Lagrange finite element method can be efficiently implemented to provide a fast numerical treatment. Mixed elements are used because they allow simultaneous approximations for the flux and its gradient of the same order. Although the linear systems produced are not positive definite, a solution ca be achieved after eliminating some of the unknowns. Numerical results include core calculations of two types of reactors