Accelerated polynomial axial expansions for full 3D neutron transport MOC in the APOLLO3® code system as applied to the ASTRID fast breeder reactor

In recent years a solver based on the Method of Characteristics (MOC) allowing the treatment of 3D extruded geometries has been developed inside the TDT code of APOLLO3. The standard Step Characteristics (SC) approximation is used and results show an excellent agreement with Monte-Carlo simulations. However a fine mesh refinement is needed to converge axially the strong flux gradients customarily appearing in 3D reactor physics applications. In this paper an improvement of this method is proposed: the results of the previous work show that much of the flux variations are likely to be represented by a polynomial basis along the vertical direction. Since most of the geometrical and physical heterogeneities are radially located, the SC approach is preserved to represent the solution over the radial plane. As a matter of fact the strong irregularities in the geometrical meshes prevent from an efficient use of a polynomial expansion. On the contrary along the axial direction the computational meshes assume a Cartesian shape, well suited for a polynomial representation of sources and fluxes. A convenient polynomial development in this direction allows us to approximate the strong flux slopes without the help of a large number of axial meshes. Finally we will show how synthetic accelerated results for the fourth generation fast breeder reactor ASTRID perform with respect to computational time, memory efficiency and precision against Monte-Carlo calculations. We will consider assembly geometries that span the full-height of the reactor and that are characterized by the strong heterogeneity conceived to enhance the axial leakage which improves the safety features of this new concept, especially under void conditions.