An Integral Multidomain DP_N Operator as Acceleration Tool for the Method of Characteristics in Unstructured Meshes

This paper presents recent developments of the acceleration techniques for the method of characteristics (MOC) in the code APOLLO-2. The main contribution concerns the introduction of a multidomain DP_N technique where all regions belonging to the same macrodomain are coupled by an integral operator that is strictly equivalent to the MOC. Different macrodomains are coupled via currents that are defined with the DP_N formalism. This new integral DP_N(IDP_N) operator is built by using transmission and escape probability factors that respect symmetries/antisymmetries and complementary properties that are enforced to preserve the physics of the problem and to save computational effort. These factors are computed using the numerical tracking of the MOC operator. This paper presents results on realistic assembly calculations that demonstrate the effectiveness of the IDP_N operator as a synthetic acceleration tool.