A multiscale heat transfer model for nuclear reactor assemblies

[Display omitted] •A new multiscale approach for modeling nuclear assemblies is proposed.•Hybrid space dimensions are used for the coolant (3D) and the fuel (1D).•The fuel and the coolant numerical discretizations and meshes are independent.•The approach simplifies mesh generation constraints and reduces the cost of simulations.•The approach is validated with respect to well established reference models. We develop a new multiscale approach to investigate the heat transfer in a nuclear assembly. Fuel rods are represented as mono-dimensional (1D) inclusions into a three-dimensional (3D) bulk domain representing the coolant. More precisely, the interaction between the fuel rod grid and the coolant is modeled by means of projection operators that couple the 1D and 3D problem taking into account the actual dimension of the rods together with the cladding layers. Due to the dimensional reduction, the inclusions do not need to conform with the bulk computational grid. As a result, the computational challenges of the numerical discretization, due to the high geometrical complexity of the problem, are significantly reduced, as confirmed by the numerical simulations presented in this work.