Feasibility study to increase the reactor power at natural convection mode in Tehran Research Reactor (TRR) through a hybrid thermal-hydraulic simulation and analysis using the RELAP5 code and Computational Fluid Dynamic (CFD) modeling by ANSYS-FLUENT

Abstract

Some irradiating applications can be used economically at a low power range and energy consumption. Moreover, removing the decay heat by the natural convection plays a very crucial role in Design Basis Accidents (DBAs) such as Loss of Flow Accident (LOFA) or Station Black Out (SBO). In such cases, the reactor can be operated by natural convection instead of forced convection. Such an operation needs careful thermal-hydraulic analyses to confirm nuclear safety. In this paper, Tehran Research Reactor (TRR) is chosen as the case study. A careful hybrid methodology is taken into account for safety analyses and visualizing the fluid streamlines and temperature profiles assuming both short-term and long-term cooling terms and conditions. Firstly, the RELAP5 code is used for the relevant simulation and safety analysis. Moreover, it is also used to trace any perturbation or oscillation and for the hot spot responses due to the flow regime changes and the reactor dynamic. Then the ANSYS-FLUENT is used for further Computational Fluid Dynamic (CFD) simulation of natural convection flow in detail. A modified 3-D porous media is modeled to simulate and observe the overall behavior of the reactor core in the reactor pool for long-term operations. The Boussinesq approximation is assumed for the buoyancy force. The pressure drop along the porous media is replaced with an accurate model of the Standard Fuel Element (SFE). Moreover, the k- ω turbulence model is also included in the Navier-Stokes equations to fix approximations. The relevant correlations have been also chosen for the heat transfer from the reactor pool to ambient air by convection, radiation, and surface evaporation. Finally, an accurate 2-D model of the hot-channel is also used for the accurate single-channel analysis regarding short-term operating conditions. Presented velocity vectors and temperature profiles depict the domain and behavior of the natural convection in both fuel channels and the reactor pool as well. Concerning both short-term and long-term issues, results are quite promising to increase the maximum operating limit of the natural convection mode from 100 kW to 200 kW. Moreover, regarding short-term operations or emergency conditions, this limit can be also increased to 400 kW due to the safety limit of 378 K for the maximum permissible fuel cladding temperature to protect the fuel from long-term corrosions. Keeping this limit also prohibits any perturbation or oscillation as well.