Comparison Between Experimental Data and Numerical Modeling for the Natural Circulation Phenomenon
The study of natural circulation phenomenon has been object of crescent interest in scientific community in recent years. The new generation of compact nuclear reactors uses natural circulation of the fluid as a residual heat removal cooling system in case of accident or shutdown. The objective of this paper is to present a comparison between experimental data and numerical simulation results for the natural circulation phenomenon in single and two-phase flow regime. Experimental data were obtained from a circuit built with glass tubes, through which thermal hydraulic phenomenon visualization was possible. The circuit is composed of an electric heater as the heat source, a heat exchanger as the heat sink and an expansion tank to accommodate fluid density changes. Instrumentation data acquisition is performed through thermocouples and pressure meters, and controlled by a computer interface developed using LABVIEW. Numerical modeling and simulations were done with the thermal hydraulic code RELAP5, which is widely used for this purpose. The cyclic reverse flow observed in the circuit was well represented by the numerical model. The comparison demonstrated that the numerical simulations are very close to the experimental data.
Assessment of RELAP5 matrix solvers for a two-phase natural circulation loop
Abstract The heat transfer mechanism of natural convection has been extensively studied as a passive heat removal system of new nuclear power plants. Considering this aspect, the main objective of this study is to present an assessment of RELAP5 linear-equation solver under a transient two-fluid model for a two-phase natural circulation loop (NCL). For this assessment, three different approaches of linear-equation solvers for the hydrodynamic model are presented: the sparse matrix solver based on the Lower-Upper (LU) decomposition, the Border-Profile Lower Upper (BPLU) solver and the iterative method named Generalized Minimal Residual Method (GMRES). For comparison purposes, an experimental natural circulation loop made of glass tubes and using water as working fluid is analyzed. The onset of nucleate boiling observed during the experiment was predicted by all RELAP5 solvers as well as the representation of flow oscillations along the loop. Furthermore, it was noticed that the choice of the solver algorithm has a strong influence on the prediction of the two-phase natural circulation phenomena, since different wavelengths and amplitudes of flow instabilities were obtained for each approach.
Analysis of Transient Events Without Scram in a Research Reactor Using the RELAP5/MOD3.2 Computer Code
Simulations of two different events without scram were conducted for a hypothetical research reactor, based on the High-Flux Australian Reactor (HIFAR) moderated and cooled by heavy water circulating under atmospheric pressure. The simulations were performed with the RELAP5/MOD3.2 computer program. Although the simulations neglected reactivity feedback effects, the focus on the thermal-hydraulic aspects represents a step toward full analyses of hypothetical events in HIFAR. Two simulations focused on events associated with the failure of the primary coolant circulation pumps, and three simulations focus on the events associated with the reduced heat removal via the nonavailability of heat exchangers. The critical heat flux subroutine of the RELAP program was modified to account for the concentric annular fuel element geometry of HIFAR fuel elements.