Study on Natural Circulation Heat Transfer and Flow Characteristics of High-Pr Oil Simulant of Molten Salts

Abstract

After the Fukushima accident, the application of the passive safety system was emphasized to prevent the expanded or secondary disaster of the accidents. This trend brought the spotlight back onto the molten salt application in nuclear system which had inherent safety. Among the concepts of the six advanced reactors, molten salt reactor which uses molten salt as fuels and/or coolants was not a good choice due to the problem of the nuclear proliferation and corrosion. However, with the new concept of the reactors such as FHR or MCR, molten salt became to have advantages without the problems. The strengths of molten salts in nuclear systems come from the thermo-physical properties they hold. Molten salts have high boiling point and operating temperature compared to water. And it is already in liquid phase so that it gives the superiority of core safety. Especially, molten salts have high Prandtl number (Pr) and high volumetric heat capacity compared to other coolants such as He and sodium. Therefore, it can perform the passive heat transfer through the natural convection well without electric power. According to the strong point, passive safety systems of high-Pr molten salts are applied to the various nuclear systems. For the application of molten salts, the study on the understanding of heat transfer capability and characteristics of high-Pr molten salts based on the natural convection is important. However, lab-scale molten salt heat transfer experiment possesses some difficulties due to its characteristics including high operating temperature and high temperature corrosion and toxicity with materials. Fortunately, the similarity techniques can be used to simulate specific thermal-hydraulic phenomena. Using simulant materials for the experiment enables the research to reproduce the thermal behavior and fluid dynamics of molten salt at reduced temperature, pressure, dimension, and power scale. However, the actual application of the similarity technique is not enough for molten salts. Also, the limitation of the similarity was not proved exactly. Thus, in this paper, similarity study on high-Pr molten salt heat transfer using a new simulant fluid, DOWTHERM RP, was performed on the natural circulation loops. Through the performance, the feasibility of a new simulant fluid was evaluated based on the similarity technique using scaling laws. In addition, the quantitative and qualitative analysis of DOWTHERM RP natural circulation was conducted in experiment and code simulation using MARS code and CFX code. From the experiment, heat transfer correlation of single-phase natural circulation on the high-Pr range was developed. In MARS code application, heat transfer simulation and sensitivity study on the thermo-physical properties on natural circulation were conducted. Using CFX code, flow characteristics of high-Pr natural circulation were observed. By visualization of the natural circulation in experiment and CFX simulation, flow patterns and boundary layers were analyzed following different Pr range using temperature and velocity distribution. This study on the natural circulation of high-Pr simulant fluid, DOWTHERM RP, will contribute to the evaluation of the similarity and its limitation in terms of both quantitative and qualitative similarity.