Acceleration techniques for the characteristic method in unstructured meshes

Acceleration techniques are essential for the efficient iterative solution of the transport equation in reactor applications. We present two new acceleration techniques for the method of characteristics in unstructured meshes. The DPN scheme generalizes a previous asymptotic acceleration by incorporating a general DPN angular approximation for intercell angular fluxes. In a radically different approach we show how to construct a scalar product such that the transport operator becomes symmetric, and apply Lanczosʹ iterations. Both, the DPN and Lanczosʹ accelerations, are able to accelerate higher angular flux moments. Numerical tests for realistic assembly geometries are used to compare the efficiency of the acceleration methods.