Assembly homogenization techniques for core calculations

We have applied the black-box paradigm to assembly homogenization and introduced current discontinuity factors (CDFs) for an arbitrary low-order operator in the presence of boundary leakage. The CDFs preserve average reaction rates and the assembly partial currents in a given reference situation as well for full assembly as for pin-by-pin homogenization. In the presence of surface leakage, the CDFs depend on the discretization of the low-order operator but can be determined from a few calculations with the low-order operator without scattering. For diffusion-like, low-order operators, the CDFs can be advantageously replaced by flux discontinuity factors (FDFs), which also preserve partial currents. However, the effect of the FDFs is not equivalent to that of the CDFs in the final core calculation. Unlike the CDFs, that are double-valued for homogenization with surface leakage, the FDFs are always single valued. The cases when the low-order operator is diffusion, SPN or quasidiffusion are discussed in detail. We also show that, for full-assembly homogenization without boundary leakage, the FDFs are identical to Smith's discontinuity coefficients (DCs) only if the reference calculation has also been done with diffusion. In the case of diffusion, preliminary test calculations for small PWR motifs show that the FDFs and Smith's DCs give close results, with a better precision for the FDFs when transport effects are predominant.