atw - International Journal for Nuclear Power | 05.2020

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atw Vol. 65 (2020) | Issue 5 ı May ENVIRONMENT AND SAFETY 282 Initial vacuum degree of the loop Initial vacuum degree of the loop Containment pressure pressure is maintained constant. When the temperature and flow rate of the loop reach a steady state, the natural circulation flow rate of the system is recorded. Initial conditions of the decay heat simulation conditions are shown in Table 4. The decay heat simulation conditions study on the steady-state characteristics of PCSHP under different input power. The decline of the core decay heat is simulated by the step change of heating power to verify whether the system can reach the steady state of natural circulation after the core decay heat decreases. In order to study the influence of heating power and initial vacuum degree on the steady-state characteristics, the heating power of the heater simulator and the initial vacuum degree of the loop are set as Table 4. Before the system starts, the condensing tank is at saturated temperature (about 100 °C), the containment is at environmental condition. Once the temperature and flow rate of the loop reach a steady state, the natural circulation flow rate of the system is recorded. Initial temperature of consensing tank 0.021 MPa 0.30 MPa ~ 0.52MPa Saturated temperature 0.045 MPa 0.30 MPa ~ 0.52MPa Saturated temperature 0.065 MPa 0.30 MPa ~ 0.52MPa Saturated temperature 0.100 MPa 0.30 MPa ~ 0.52MPa Saturated temperature | Tab. 3. Steady-state conditions. Initial containment pressure | Tab. 4. Decay heat simulation conditions. Initial containment temperature Initial temperature of the condensing tank Heating power 0.021 MPa 1 atm Ambient temperature Saturated temperature 10 kW ~ 80 kW 0.045 MPa 1 atm Ambient temperature Saturated temperature 10 kW ~ 80 kW 0.065 MPa 1 atm Ambient temperature Saturated temperature 10 kW ~ 80 kW 0.100 MPa 1 atm Ambient temperature Saturated temperature 10 kW ~ 80 kW well as the heat transfer inside and between solids and fluids [EPRI, 2014]. The GOTHIC version 8.1 code is used to model the principle experiment. The model diagram is shown in Figure 3. Wherein, the boundary condition 1P refers to the environment, the control volume 1 is the containment, the control volume 2 is the heat exchanger in the containment (evaporator), and the control volume 5 is the heat exchanger in the cooling water storage tank outside the containment (condenser), and the control volume 8 is the cooling water storage tank outside the containment, control volume 3, control volume 4, control volume 6 and control volume 7 indicate connected pipes [Hui-Un et al., 2013; Philipp et al., 2011]. In order to accurately simulate the thermal stratification effect and natural circulation in the containment, the evaporator and the condenser (control volume 1, control volume 2 and control volume 5) are subdivided. That is to say, a large control volume is divided into many small subdivided volumes. Conversely, other control volumes are simu lated using lumped parameters. The flow between the above control volumes is simulated by the flow path, wherein the cooling water storage tank outside containment is connected to the environment with flow path 7, which simulates the chimney structure of the storage tank so that the heated steam in the storage tank can be smoothly discharged. The heating power in the containment is simulated using a heater component. Valves are placed upstream and downstream of the evaporator to model system start-up and shutdown. The heat transfer between the condenser and the cooling water storage tank outside containment is simulated by the thermal conductor 1. The heat transfer between the condenser and the thermal conductor 1 uses a diffusion layer model (DLM) to simulate steam condensation. The temperature rise of the cooling water storage tank 3 GOTHIC modeling of the principle experiment GOTHIC (Generation of Thermal Hydraulic Information for Containment) is a general purpose code for thermalhydraulic calculation, mainly used for containment design, license application, safety analysis and operational analysis of nuclear power plants. GOTHIC is capable of modeling multiphase fluid flow involving steam, gases, pools, droplets, bubbles and ice and the interaction between phases, including sublimation, evaporation, condensation, boiling and flashing as | Fig. 3. GOTHIC model of Passive Containment Cooling System with Closed-Loop Heat Pipe Technology (PCSHP). Environment and Safety Experimental and Computational Analysis of a Passive Containment Cooling System with Closed-loop Heat Pipe Technology ı Lu Changdong, Ji Wenying, Yang Jiang, Cai Wei, Wang Ting, Cheng Cheng and Xiao Hon
atw Vol. 65 (2020) | Issue 5 ı May (a) Experimental results | Fig. 4. Containment pressure in start-up conditions. (b) GOTHIC simulation results ENVIRONMENT AND SAFETY 283 (a) Experiment results | Fig. 5. Flow rate of the loop in start-up conditions. (b) GOTHIC simulation results outside containment is simulated by the FILM heat transfer model that GOTHIC built in. At this point, the GOTHIC code automatically selects the single-phase liquid heat transfer relationship built in the FILM model. The heat transfer between the evaporator and the containment is simu lated by the thermal conductor 2, wherein the condensation of the containment simulated with the Uchida relation. And the boiling heat transfer of the evaporator is simulated with the FILM heat transfer model that GOTHIC built in. There may be multiple heat transfer modes such as single-phase liquid natural convection heat transfer, subcooled nucleate boiling heat transfer, saturated nucleate boiling heat transfer, film boiling heat transfer, and single- phase steam natural convection heat transfer in the evaporator. The GOTHIC code is able to recognize the above mentioned heat transfer modes and automatically switches relations. The heat transfer inside both 2 thermal conductors is heat conduction. Since the pipe materials are all made of stainless steel, the properties of the stainless steel are used such as density, thermal conductivity and specific heat. In the model, the ambient temperature is conservatively set at 30 °C, and atmospheric pressure is taken as 101 kPa. Conservatively, the heat absorption of the equipment, walls, floors or ceilings isn’t considered, neither does the heat loss of the experimental system to the environment. Other initial conditions and parameters are exactly the same as those in the experiments. 4 Simulation results and analysis The principle experiment is modeled using the GOTHIC code, and the startup conditions, steady-state conditions and decay thermal simulation conditions are simulated and analyzed. 4.1 Start-up conditions In the start-up conditions, under different initial containment pressures, the curves of containment pressure over time are shown in Figure 4, where (a) shows the experimental results and (b) shows GOTHIC simulation results (P1 indicates the initial containment pressure). When the heat transfer capacity of PCSHP is greater than the heating power, the containment pressure will gradually decrease. Conversely, when the heat transfer capacity of PCSHP is less than the heating power, the containment pressure will gradually increase. According to the experimental results, when the initial containment pressure is 0.35 MPa and 0.40 MPa, the containment pressure begins to rise after a brief decline. When the initial containment pressure is 0.45 MPa and 0.52 MPa, the containment pressure will gradually decrease after the brief decline. However the GOTHIC Environment and Safety Experimental and Computational Analysis of a Passive Containment Cooling System with Closed-loop Heat Pipe Technology ı Lu Changdong, Ji Wenying, Yang Jiang, Cai Wei, Wang Ting, Cheng Cheng and Xiao Hon
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atw - International Journal for Nuclear Power | 05.2020

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