CFD Simulation on the Main Steam Line Break Accident with the ATLAS (SLB-GB-02T) in Terms of Thermal Mixing and Asymmetry Effects

The MSLB accident was initiated by a double-ended guillotine break at one of Steam Generator (SG). The MSLB accident is characterized as an increase in heat removal by the secondary system. This causes excessive heat removal from reactor coolant system and decrease in reactor coolant temperatures. As a result, the core reactivity is increased by the negative moderator and Doppler reactivity coefficients. The viewpoint of MSLB analysis is to know whether the thermal asymmetry effect is sustained on the core or not when the cold water flows from the affected SG to the affected RCS loop and the hot water flows from the intact SG to the intact RCS loop. KHNP CRI, as a participant of the group B, performs the CFD analysis to analyze the thermal mixing and asymmetry effect on the downcomer and the core using the ANSYS CFX version 14.57 code. In general, CFD code still has a limitation on the application of two-phase phenomena, but the applicability and accuracy in the single phase flow condition is validated by many researchers in recent years. Based on the experiment, the primary fluid is maintained under subcooled liquid phase without the vapor phase, and thereupon CFD code can be applicable in the simulation of ATLAS vessel flow. In this paper, the CFD analysis model is bounded by the ATLAS vessel, and the RCS loop flow rate and temperature is treated as a boundary condition using the experiment data. Firstly, a steady-state analysis is conducted and this result is analyzed as below. - Major fluid behavior such as pressure, temperature, flow rate is compared with experiment data - a axial pressure distribution in the downcomer and the core Based on the steady-state results, a transient calculation is performed and the boundary conditions are determined by the experimental data. 2 cases are calculated as below - Case1: Period from the break initiation time to the reactor trip time by low SG pressure (LSGP) signal. This period represents that the temperature difference between the affected loop and the intact loop is relatively lower than case 2 and the flow rate is relatively faster than case 2. - Case2: time at a maximum temperature difference between the affected loop and the intact loop before the start of safety injection pumps. This case represents a most conservative situation in the viewpoint of thermal asymmetry. These results are discussed about the thermal mixing and asymmetry in the downcomer and the core using a statistical approach, standard deviation in the spatial planes. CFD analysis is performed to analyze the flow characteristics of the ATLAS facility in the steady state at full power and the thermal mixing and asymmetry in the MSLB accident. The results of CFD analysis for steady-state give us the understanding of detailed flow characteristics in the steady-state such as the recirculation flow near the cold-leg, the swirling flow near the flow skirt and detailed pressure distribution. The results of the transient condition shows that the flow mixing is well produced in the downcomer and the lower plenum region. This study is the first attempt inside the nuclear industry circles to analyze the 3D effect on the ATLAS test facility. However, the reinforcement of the measurement instrumentations in the ATLAS facility might be necessary to analyze the 3D effects