atw - International Journal for Nuclear Power | 05.2021

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atw Vol. 66 (2021) | Issue 5 ı September 50 RESEARCH AND INNOVATION Improving Henry-Fauske Critical Flow Model in SPACE Code and Analysis of LOFT L9-3 BumSoo Youn Introduction The SPACE code offers several options for critical flow model. One of the option is Henry/Fauske – Moody model. When using this model, Henry-Fauske critical flow model is used for single phase liquid and Moody model is used for 2-phase flow. For Henry-Fauske model, SPACE code assumes non-equilibrium(NE) factor of 0.14. In previous OPR1000 SBLOCA analysis methodology based on RELAP5 code, non-equilibrium factor of 1.0 was used to get more conservative break flow. To develop SBLOCA analysis methodology for OPR1000 using SPACE code, it was necessary to use different non-equilibrium factor from SPACE default values for Henry-Fauske model. The SPACE code was improved by adding additional option for Henry/Moody – Moody model, which uses user input non-equilibrium factor. To accept user input equilibrium factor, the SPACE code is improved by expanding lookup table used in Henry/ Fauske – Moody model. To verify the new model, we perform verification calculations on LOFT L9-3 which is a representative integral effect test(IET). Experimental evaluation Henry-Fauske critical model applied to existing SPACE codes(NE=0.14) will predict lower critical flow rates over the period of transition from subcooled liquid to two-phase fluid compared to RELAP5 critical flow models assuming a nonequilibrium(NE) factor of 1.0. Overview of LOFT L9-3 Experiments The LOFT L9-3[1] experimental purpose is to provide experimental data to developers of analysis codes for ATWS analysis, evaluate alternative methods of reaching long-term shutdown without inserting control rods after ATWS, and verify the applicability of point kinetics model or transients. In addition, the experimental data provided is used to determine the behavior characteristics of the primary system due to loss of main feedwater flow rate on the secondary side of the steam generator and to determine the two-phase and overcooling flow characteristics released through PORV and SRV at high pressure. The LOFT L9-3 experimental device is a 50MWt pressurized light water reactor designed at a scale of 1/60 based on a 4-Loop Westinghousetype nuclear power plant. As shown in Figure 1 it consists of five main components and subsystems, including the reactor core, primary coolant system, blowdown mitigation system, emergency core cooling system, and secondary cooling system. The blowdown mitigation system consists of a pump and steam generator simulation device, a Quick-opening discharge valve that | Fig. 1. LOFT Facility Schematic [Ref.1]. Research and Innovation Improving Henry-Fauske Critical Flow Model in SPACE Code and Analysis of LOFT L9-3 ı BumSoo Youn
atw Vol. 66 (2021) | Issue 5 ı September simulates a break, and a tank that collects coolant released through a break simulation valve. The reactor core of the LOFT L9-3 experimental device is approximately 1/2 length(1.68m) of the commercial reactor core, and the square core group contains 21 guide tubes out of 225 rod positions(15×15). Experimental Conditions The LOFT L9-3 experimental conditions begin at normal conditions, such as core power of 48.7MWt, cold leg temperature of 557.0K, hot leg temperature of 576.4K, pressurizer pressure of 14.98MP, and steam generator pressure of 5.61MPa. The key steady-state initial conditions of the experiment are shown in Table 1. LOFT L9-3 SPACE Modeling SPACE modeling is shown in Figure 2. SPACE code input from LOFT L9-3 experiments was written with reference to NUREG/IA-0192[2]. The reactor pressure vessel was divided to simulate the upper and lower cavities of the core bypass flow, and the core was modeled as a non-fuel part in the upper and lower part and a fuel part represented by three vertical nodes. The steam generator was modeled by dividing it into 12 volumes for U-tube and 19 volumes for secondary. The main feedwater model was modeled using TFBC Component, especially the main feedwater and steam flow parts, using control logic to maintain constant pressure on the secondary side. The pressurizer was modeled with nine volumes using the SPACE code pressurizer model, and the surge line under the pressurizer is connected to cell C110 and the pressurizer spray system is modeled with TFBC Component C407 and C406. In particular, Parameter Experiment SPACE Mass flow rate(kg/s) 467.6 467.63 Hot leg pressure(MPa) 14.98 14.95 Cold leg temperature(K) 557.0 555.04 Hot leg temperature(K) 576.4 574.56 Power level(MWt) 48.7 48.7 PZR Liquid temperature(K) 615.2 614.78 PZR Pressure(MPa) 14.98 14.98 SG Liquid level(m) 3.15 3.19 SG Pressure(MPa) 5.61 5.55 SG Mass flow rate(kg/s) 25.7 25.6 | Tab. 1. Initial value of experiment. in the case of spray system, the spray system is operated using the trip signal so that fluid from cold leg(C150) enters the pressurizer through TFBC Component C407. In the evaluation of LOFT L9-3 experiments using SPACE code, the normal state was verified using null transients, and then the transient state was simulated using the Restart file and the results were evaluated. LOFT L9-3 Evaluation Results Figure 3 shows the pressure change of the pressurizer. According to the experimental values in the figure, the pressure of the system gradually increases as the main feed water supplied to the steam generator is initially closed and the heat transfer from the primary to the secondary side of the steam generator decreases. The pressurizer pressure increases and reaches the setting of the pressurizer spray system. When the setting is reached, the pressurizer spray system is activated and the pressurizer pressure is reduced at 31 seconds. The continuous decrease in heat transfer to the steam generator causes the pressure to rise again. The pressure of the pressurizer is increased further as the steam generator MSCV closes at 67.3 seconds. The pressurizer pressure then reaches the opening setting of the PORV and decreases when the PORV is opened at 73.8 seconds. However, due to the absence of a replenishment of the main feedwater of the steam generator, the pressure rises again and the SRV setting of the RESEARCH AND INNOVATION 51 | Fig. 2. LOFT L9-3 Facility Nodalization for SPACE Code Verification. Research and Innovation Improving Henry-Fauske Critical Flow Model in SPACE Code and Analysis of LOFT L9-3 ı BumSoo Youn
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