A domain decomposition methodology for pin by pin coupled neutronic and thermal–hydraulic analyses in COBAYA3

Nowadays, coupled 3D neutron-kinetics and thermal–hydraulic core calculations are performed by applying a radial average channel approach using a meshing of one quarter of assembly in the best case. This approach does not take into account the subchannels effects due to the averaging of the physical fields and the loose of heterogeneity in the thermal–hydraulic modelization. Therefore the models do not have enough resolution to predict those subchannels effects which are important for the fuel design safety margins, because it is in the local scale, where we can search the hottest pellet or the maximum heat flux. The aim of this paper is to present a domain decomposition methodology as our choice to asses this multi-scale issue in order to correct the results at the core scale with the ones from the subchannel scale.The UPM advanced multi-scale neutron-kinetics and thermal–hydraulics methodologies being implemented in COBAYA3 include domain decomposition by alternate core dissections for the local 3D fine-mesh scale problems (pin cells/subchannels) and an analytical nodal diffusion solver for the coarse mesh scale coupled with the thermal–hydraulic using a modelization of one channel per assembly or per quarter of assembly.The multi-scale domain decomposition is optimal for the thermal–hydraulic calculations, where the neutronic nodes (assemblies or quarters) can be mapped one-to-one to average channels and fuel rods and the pin cells to the detailed fuel pins and subchannels. For both levels we use the same channel code and, in order to facilitate the multi-scale mesh definition for the TH modules, the development of an input pre-processor has been a relevant part of this work.