Generalization of the tibere B1 leakage model to the method of characteristics

Abstract

Along the years, several leakage models have been developed in deterministic transport codes to ensure that full-core neutron leakage effects are accounted for at the lattice calculation level. Most of these models are based on a homogenization of the assembly and can only compute direction independent diffusion coefficients. Two main models differ from this strategy, namely the heterogeneous TIBERE model developed by Petrovic and Benoist in 1994 for the collision probability technique (CP) in 2D and later implemented in the code DRAGON and more recently the so-called exact model developed by van Rooijen and Chiba and implemented in 2D for the method of characteristics (MoC) solver of the code CBG. The main differences between these two models are that TIBERE relies on a small buckling approximation and the exact method is based on a decomposition of the buckling dependent flux into real and complex components. In this work, we generalize the TIBERE heterogeneous leakage model to the MoC solver of DRAGON for 2D and 3D geometry.To verify our MoC implementation of the TIBERE model, several reference benchmarks are analyzed including a 2-group PWR pin cell, a 69-group CANDU cell and a 69-group 3D PWR pin cell assembly with reflectors. We also compare the TIBERE model with the van Rooijen and Chiba model for a hexagonal sodium cooled fast reactor pin cell and show that they provide heterogeneous directional diffusion coefficients that generally differ by less than 1%.