원자력 발전용 증기발생기 내부 습분분리기 및 액적 거동에 대한 수치 해석적 연구

Abstract

Nuclear power plants are classified into two categories: pressurized water reactors (PWR) and boiling-water reactors (BWR). The primary difference between the two is that in the BWR, the coolant in the reactor is heated directly without a separate steam generator. However, in the PWR, the primary and secondary systems are separated and connected by a steam generator. In the PWR, when the thermal energy generated from nuclear fission in the reactor is transferred to pressurized water, the heated, high-pressure primary system coolant flows to the steam generator. The primary system coolant that flows in the steam generator flows through the heat transfer tube, and heat exchange occurs in the secondary system located outside the steam generator tube through the heat transfer tube. Owing to the heat exchange, the coolant in the secondary system is heated. The coolant then starts to boil and is converted into steam which turns a turbine to generate electricity, which steam has a high moisture content. This moisture must be removed because it can corrode or damage the power generation system, which in turn can damage the turbine blade. A swirl vane is mounted on the cylindrical structure of the primary steam separator for eliminating moisture. When perpendicularly ascending moist steam passes the swirl vane, the centrifugal force generated through rotation is used to remove the moisture. Using a steam separator (dryer), the secondary steam separator eliminates the moisture by allowing the previously flowing steam through the stacked structure of the bent chevron vane to collide on its surface. Following the primary and secondary moisture separation processes, the steam with a quality fraction of ~99.75% or above enter the turbine. Steam generators in south korea utilize steam separators designed either by Combustion Engineering (CE) or Westinghouse (WH). Although South Korea has yet to develop independent design techniques for WH-type steam separatoe, the technology is owned entirely by foreign companies (Westinghouse, Areva, Japan, etc.). This means that promptly responding to domestic nuclear power plant issues is difficult due to complete dependence on foreign experts and their technical support. At present, it is necessary to collect steam separator performance, thermal-hydraulic, and technical data to assess the safety and thermal-hydraulic impact of WH-type steam generators to overcome this dependency. Thus, this study performed a numerical analysis of the WH-type steam separator, identified the flow characteristics within the steam separator, and evaluated the main performance indicators of moisture carry-over and pressure drop characteristics. The results of this study were compared with those of the experiment. The Eulerian-Lagrangian particle tracking model and the homogeneous flow model based on the mixture model were utilized to simulate the complex two-phase phenomenon inside the steam separator. Regarding the steam separator pressure drop, the numerical analysis results obtained using the particle tracking model (two-way) and homogeneous flow model were found to be in agreement with the experimental results and correlation predictions. Numerical analysis results obtained using the particle tracking model and experimental results of moisture carry-over at the steam separator outlet showed that the MCO was within 5%. Moreover, several conditions have been assumed for the numerical analysis due to the lack of visual information on droplet breakup, drop entrainment at the liquid film, and droplet distribution at the steam separator inlet. Therefore, MCO determination through numerical analysis is insufficient at this stage. In addition, this study discussed the direction of improvement through the shape deformation of the swirl vane inside the WH-type steam separator. To improve the moisture elimination performance of the WH-type steam separator, the bending angle of the swirl vane was reduced, the number of wings was increased, and the swirl vane was installed far from the outlet.