Leakage models in Apollo 2 and Eco

Assembly homogenized cross sections are computed in a reference situation where approximated boundary conditions are introduced to emulate the effect of the assembly environment in the reactor core, and leakage is used to achieve criticality. The leakage model is an essential part in this approach to assembly homogenization for reactor core calculations. This note discusses leakage models that are presently available in the transport codes APOLLO2 and ECCO. In both codes, the concept of Laplacian has been used to develop homogeneous as well as heterogeneous leakage models. In the homogeneous approach, critically is achieved via an absorption term proportional to the scalar flux, and the corresponding leakage coefficients are computed in an equivalent homogeneous medium. On the other hand, in a heterogeneous model critically is obtained with a leakage term produced by a macroscopic current that is defined per region and per group. In the heterogeneous models the scalar flux and the macroscopic current obey a system of coupled integral equations, and the corresponding numerical values must be simultaneously determined by external iterations. (author)