An integral multi-domain DP$_N$ operator as acceleration tool for the method of characteristics in unstructured meshes

Abstract : The paper presents the recent developments of the acceleration techniques for the Method Of Characteristics (MOC) in the code APOLLO2. The main contribution concerns the introduction of a multidomain DP$_N$ technique where all regions belonging to the same macro-domain are coupled by an integral operator that is strictly equivalent to the MOC. Different macro-domains are coupled via currents that are defined with the DP$_N$ formalism. This new Integral DP$_N$(IDP$_N$) operator is build by using transmission and escape probabilities factors that respect symmetries/anti-symmetries 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. The paper presents results on realistic assembly calculations that demonstrate the effectiveness of the IDP$_N$ operator as a synthetic acceleration tool.