Hybrid microscopic depletion model in nodal code DYN3D

The paper presents a general hybrid method that combines the micro-depletion method with correction of micro- and macro- diffusion parameters to better account for the spectral history effects. The fuel in a core is subjected to time- and space-dependent operational conditions (e.g. coolant density), which cannot be predicted in advance. However, lattice codes assume some average conditions to generate cross sections (XS) for nodal diffusion codes such as DYN3D. Deviation of local operational history from average conditions leads to accumulation of errors in XS, which is referred as spectral history effects. This paper demonstrates gaps that exist in the current methods used to account for the spectral history effects and presents an alternative, more generalized method. The method, which in principal extends the micro-depletion methods, was implemented in DYN3D and verified on multiple test cases. The results obtained with DYN3D were compared to those obtained with Serpent, which was also used to generate the XS. The observed differences are within the statistical uncertainties.