4. METHOD OF SOLUTION
The diffusion equation is solved using the finite differences method. Each group equation is discretized on a different mesh. The thermal flux is defined on a mesh that is three times smaller than that for the fast flux. The fission density is defined on the coarse fast flux mesh. SOR acceleration is used for the inner iterations. The SOR parameters may be evaluated in advance using the eigenvalue of the homogeneous part of the respective finite difference group equation. Alternatively the SOR parameters may be inserted externally. If an eigenvalue problem is solved, Chebyshev polynomials are used to accelerate the outer iterations.
Fuel elements are considered to be quadratic, and group constants (i.e. cross sections) assumed constant over each fuel element with discontinuities between different fuel elements.