Recent methods for calculating anisotropic neutron diffusion in fast reactor lattice cells

Different ways of defining (anisotropic) homogenized diffusion coefficients for lattice cells are discussed. It is demonstrated that - contrary to statements made in the literature - in the case of an infinite lattice, a unique definition is possible, which preserves the single contributions to the neutron balance. However, the partial currents at the cell edges are not preserved. Usually simplified definitions are used for practical calculations; they are accurate enough for fast reactor lattices. The calculation of diffusion coefficients for voided fast reactor lattices, including the buckling dependence, is an important issue. The methods developed in recent years are briefly presented. At KfK, sufficiently accurate methods are available for idealized lattices (without structural parts). These methods are discussed in some detail. (orig.)