THERMALHYDRAULIC ANALYSIS OF PASSIVE RESIDUAL HEAT REMOVAL SYSTEM IN MOLTEN SALT REACTOR USING FLUENT

• Main Author: Nurhasanah, Siti
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69417
• Institution: Institut Teknologi Bandung

Reactor safety should be one of the things that must be considered properly when designing a reactor system. Among all the issues related to safety, one of the keys is to ensure that residual heat can be removed efficiently. Simulation was developed to study the heat transfer that occurs in the Molten Salt Reactor (MSR) Residual Heat Removal System designed by Oak Ridge National Laboratory (ORNL), especially in the bayonet cooling thimble. As well as looking at the effect of variations in the diameter of the steam riser and gas gap on the heat transfer performance that occurs in the thimble. This study looks at the geometry for the bayonet cooling thimble which includes 3 elements, feed tube, feed tube wall, steam riser, steam riser wall, gas gap, thimble wall (6 Geometry) in 2D using Workbench 18.2, which includes Computational Fluid Dynamics analysis (CFD) using FLUENT. In the bayonet cooling thimble, the cooling water/feedwater enters through the feed tube (downward flow), then reverses direction (upward flow) through the steam riser. The water in the cooling thimble absorbs heat and produces steam/vapor. Geometry is created by the design modeler. The total area of the geometry is 57,873 m2 . The operating setting in inlet water temperature and molten salt temperature is 300 K and 977 K, respectively. The meshing size for the geometry that has been made is 1.25 mm with the average value of the mesh quality parameters for skewness 0.07, orthogonality 0.98, aspect ratio 1. Monitor residuals are checked using 7 criteria with a default value 10-3 and 10-6 for energy. The number of iterations until it reaches convergence, for each geometry is different. In the study, the diameter of the steam riser which was varied was 3.6 mm – 8.1 mm while the gas gap was 2 mm – 8.31 mm. Judging from the domain value in the steam riser, it appears that temperatures approaching the left or right walls have higher temperatures, this is due to the close distance to the gas gap. The temperature observed in the output feed tube, the part that is in contact with the steam riser. The temperature distribution along the feed tube and steam riser, there is an increase in temperature in line with the flow direction. However, in the gas gap, the temperature is more unstable in the feed tube and steam riser. The highest temperature observed in the gas gap is about 700 K. In this case, the thimble wall has a very important role. The walls of the thimble and the gas-filled air gap create high temperature difference between the molten salt outside the thimble and the feedwater entering the thimble. It can also prevent corrosion when molten salt touches the feed water directly and provide great thermal resistance. From the simulation, it is found that in the presence of a large temperature difference, radiant heat transfer plays an important role between the thimble and the bayonet tube. The decay heat generated from the molten salt in the drain tank is lowered by the flow of water in the bayonet. When viewed from the comparison of temperatures at the outlet, it can be seen that on average the temperature produced is getting smaller along with the increase in the diameter of the steam riser. From the calculation results, the density of the fluid mixture at the outlet is greater than the inlet, this is due to an increase in the density of steam/vapor that enters the steam riser. When viewed from the width of the gas gap made from 2 mm - 8.31 mm it can be seen that the heat transfer rate is getting smaller with the addition of the gap width.