Thermal hydraulic investigations of ALLEGRO ceramic fuel assemblies

Abstract

Gas-cooled Fast Reactor (GFR) is one of the generation IV reactor concepts. Fast neutron spectrum and high outlet temperature of the coolant make this reactor type a contributor to sustainability of nuclear technology. One of the main challenges of the concept is the development of innovative fuel assemblies, which can withstand the high temperature. Because of the high temperature, the thermal conditions in the core have to be known in detail to design and operate the reactor safely.Main issue of this paper is to investigate the thermal hydraulics of inner and corner regions of the ceramic assemblies of the GFR demonstrator, ALLEGRO. First, models have been built for bare rod bundle regions to determine the inlet boundary conditions of the complex models. Based on these simulations models have been developed, which include active length ceramic rods with four spacer grids in the regions. These models describe heat convection in the active part of the rod bundles and heat conduction in the clads and assembly shroud. Using these models temperature fields in typical regions are determined and effects of the heat conduction in the clads and thermal radiation are also investigated. Considering the computational effort and accuracy, modelling of the rod cladding and shroud material is required, while modelling of the radiative heat transport is less important.