TRACG-CFD Analysis of ESBWR Reactor Water Cleanup Shutdown Cooling System Mixing Coefficient

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1 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 TRACG-CFD Analysis of ESBWR Reactor Water Cleanup Shutdown Cooling System Mixing Coefficient José Gallardo Universidad Nacional Autónoma de México Av. Universidad 3000, Ciudad Universitaria, D. F., euqrop@hotmail.com Wayne Marquino, Adrian Mistreanu, Jake Yang General Electric Hitachi Nuclear Energy 3901 Castle Hayne Road, M/C A-70, Wilmington, North Carolina 28401, USA wayne.marquino@ge.com Abstract The ESBWR is a 1520 nominal [MWe] Generation III+ natural circulation boiling water reactor designed to high levels of safety utilizing features that have been successfully used before in operating BWRs, as well as standard features common to ABWR. In September of 2014, the US NRC has certified the ESBWR design for use in the USA. The RWCU/SDC is an auxiliary system for the ESBWR nuclear island. Basic functions it performs include purifying the reactor coolant during normal operation and shutdown and providing shutdown cooling and cooldown to cold shutdown conditions. The performance of the RWCU system during shutdown cooling is directly related to the temperature of the water removed through the outlets, which is coupled with the vessel and FW temperatures through a thermal mixing coefficient. The complex 3D geometry of the BWR downcomer and lower plenum has a great impact on the flow mixing. Only a fine mesh technique like CFD can predict the 3D temperature distribution in the RPV during shutdown and provide the RWCU/SDC system inlet temperature. Plant shutdown is an unsteady event by nature and was modeled as a succession of CFD steady-state simulations. It is required to establish the mixing coefficient (which is a function of the heat balance and the coreflow) during the operation of the RWCU system in the multiple shutdown cooling modes, and therefore a range of coreflows needs to be estimated using quasi steady states obtained with TRACG. The lower end of that range is obtained from a system with minimal power decay heat and coreflow; while the higher end corresponds to the power at the beginning of RWCU/SDC operation when the cooldown is transferred to the RWCU/SDC after the initial depressurization via the turbine bypass valves. Because the ESBWR RWCU/SDC return and suction designs provide good mixing, the uniform mixing energy balance was found to be an adequate alternative for deriving the mixing coefficient. The CFD mass flow weighted average outlet temperature is above the uniformly mixed temperature, hence the mixing coefficient derived from the uniformly mixed temperature is conservative from the standpoint of shutdown cooling heat removal. 1/14 Memorias Puerto Vallarta 2015 en CDROM

2 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient 1. INTRODUCTION The Economic Simplified Boiling Water Reactor (ESBWR) [1] is a 1520 [MWe] Generation III+ natural circulation boiling water reactor designed to high levels of safety utilizing features that have been successfully used before in operating BWRs, as well as standard features common to ABWR (Advanced BWR). In September of 2014 [2], the US Nuclear Regulatory Commission (NRC) has approved a rule to certify the ESBWR design for use in the USA. The Reactor Water Cleanup/Shutdown Cooling System (RWCU/SDC) is an auxiliary system for the ESBWR nuclear island. The basic functions it performs: Purify the reactor coolant during normal operation and shutdown. Provide shutdown cooling and cooldown to cold shutdown conditions. Overboard reactor coolant from the midvessel suction and bottom head to allow addition of heated water via the feedwater (FW) system, to reduce thermal stratification during startup. Provide heated primary coolant for reactor startup, among others. The RWCU/SDC [3] is constituted by two redundant trains, each with a capacity of 1 [%] of the rated FW flow rate. Figure 1 shows the general configuration of a typical train: A midvessel suction and its relation with the FW trains, two bottom drains, two heat exchangers and one demineralizer. Figure 1. RWCU/SDC system schematic 2/14 Memorias Puerto Vallarta 2015 en CDROM

3 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 The main RWCU/SDC operating modes are categorized as follows: (A) Reactor External Heating (B) Reactor Startup (C) Reactor Water Cleanup (D) Shutdown Cooling and (E) Post-Loss of Coolant Accident (LOCA) Shutdown Cooling During a planned outage and just after the insertion of control rods, the main condenser (preferred) or the isolation condenser(s) provide shutdown cooling. For a rapid shutdown, both RWCU/SDC system trains are placed in operation early to supplement cooling by the main condenser by drawing water from the Reactor Pressure Vessel (RPV) bottom drains and midvessel suction nozzle, directing it to the non-regenerative heat exchanger, and returning it to the RPV through the FW line. As the decay heat decreases, support from the main condenser and/or the isolation condenser(s) is discontinued and both trains of the RWCU/SDC system continue to perform shutdown cooling. The shutdown cooling operation begins at RPV rated pressure and temperature conditions and continues during the entire shutdown period. The definitions of four Shutdown Cooling Modes are given in Table I. Table I. Shutdown cooling mode D Initiation time after Mode Pressure Control Rod Drive (CRD) insertion [h] D1 High 0.5 D2 Low 8 D3 Low 24 D4 Low 40 The performance of the RWCU system during shutdown cooling is directly related to the temperature of the water removed through the outlets, which is coupled with the vessel and FW temperatures through a thermal mixing coefficient (σ). Given the complex configuration of the ESBWR RPV internals, it is necessary to investigate the impact of the geometry on σ using a fine-mesh technique like Computational Fluid Dynamics (CFD), and to provide a method to calculate it so it can be used to estimate the RWCU/SDC system outlet temperature. σ is estimated using the temperatures reported by CFD specifically during a steady state at the beginning of mode D2 for a case with low coreflow as well as a case with high coreflow, and the 3/14 Memorias Puerto Vallarta 2015 en CDROM

4 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient results are compared with those from an energy balance considering a homogenous mixture temperature. 2. ANALYSIS ASSUMPTIONS During cooldown, two-train operation was assumed as the most limiting configuration, since one train operation returns cooled water on the opposite side of the RPV from the suction location, and therefore two-train operation increases the possibility of removing a larger amount of poorly mixed water ( short circuiting ). Separator 2nd, 3rd stage flows and dryer drain flow are assumed to be zero, since the flows are low enough that flow pass 1st stage is not significant. The FW return temperature in mode D2 was assumed to be [ C]. Except where specified, all walls were considered adiabatic. The heat transfer coefficient at the chimney inner diameter is greatly reduced from power operation (Table IV) and the temperature differences during cooldown to the drywell are also reduced relative to power operation. A reduction in reactor power or a vessel pressure transient are assumed to occur as a series of quasi-steady states, since the focus is on the conditions at the beginning of mode D2 and not on the process used to reach such mode. Due to the high number of mesh elements contained in the CFD model a transient simulation is not feasible and the cooldown will be studied assuming steady state snapshots, since the cooldown can be considered a quasi-steady state process and snapshots of the mixing at a point in the process are appropriate when considering a range of coreflows. 3. METHODOLOGY For this analysis it is required to establish the relationship between σ, which is a function of the heat balance, and the coreflow during the operation of the RWCU system in the shutdown cooling modes; therefore, a range of coreflows needs to be estimated using quasi steady states obtained with the Transient Reactor Analysis Code for BWRs GEH proprietary version (TRACG) [4,5] The lower end of that range is obtained from a system with minimal power and decay heat, even below the power and coreflow at the completion of mode D4; while the higher end corresponds to the power at the beginning of mode D2 when the cooldown is transferred to the RWCU/SDC after the depressurization via the turbine bypass valves. This analysis documents mixing in mode D2 because the cooldown can progress faster at higher pressures when the turbine bypass valves can be used (From D1 to D2). 4/14 Memorias Puerto Vallarta 2015 en CDROM

5 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 The FW mass flow is adjusted automatically to control water level and the coreflow was calculated with TRACG to be [kg/s] for the D2 conditions (Table II). Table II. TRACG RWCU/SDC mode D2 parameters Parameter Value Total Power [MW] FW Temperature [K] FW Mass Flow [kg/s] RWCU Mass Flow [kg/s] Dome Pressure [MPa] Vessel Temperature [K] Water Level [m] 20.7 Coreflow [kg/s] σ is defined as the equivalent fraction of cold water that does not return to the heat exchangers immediately. At σ = 100 [%] the water that returns to the heat exchangers has the same average temperature as the reactor vessel separator exit temperature (Optimal for heat exchanger cooling). At σ = 0 [%] the water returning has the same temperature as the FW (Full short circuit): The usefulness of this coefficient lies in the fact that it refines the estimate of the time required to cool down the vessel from operational values to ambient conditions. Assuming a fixed heat exchanger capacity the time to cooldown can be calculated, or on the other hand, the heat exchanger capacity needed to satisfy a time requirement associated with shutdown cooling can be estimated. Local σ can be estimated for all the RWCU/SDC suction locations, left and right mid-vessel suctions and 4 bottom drains. To improve convergence and better assess its response, the system was initialized considering the temperature of all outlets equal to the temperature of the mixture, reducing the energy balance from: (1) To: (2) (3) 5/14 Memorias Puerto Vallarta 2015 en CDROM

6 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient This energy balance can be solved to calculate T Mix, the uniformly mixed RWCU suction temperature: (4) And for a fully adiabatic case: (5) 4. CFD ESBWR MODEL The RPV components included in the CFD model include the Core plate, Ring and skirt assembly, the Orificed fuel support, Steam separator exterior and standpipes, Core shroud, FW injection nozzles, Core shroud head bolts, shroud support and Chimney barrel. The reactor dimensions for these components verified against mechanical drawings. Flow boundary conditions were established at the fuel support inlet orifice, and the separator discharge, and calculated with TRACG. This simplified the problem by limiting the CFD model, to single phase portions of the natural circulation loop, and avoiding modeling of the nuclear fuel. Similarly the separator flow is applied as a boundary condition. Executing the reactor cooldown as a continuous CFD transient would be prohibitive in the terms of the required run time. Therefore the problem was divided into quasi-steady state snapshots. In this way complete 3-D distribution of the temperature during the chosen steady segments of the event can be obtained. The 3-D computational domain was meshed using tetrahedral, and prism elements. 16 layers of inflation were applied to the wall. The automatic wall functions coupled with SST turbulence model to ensure turbulence closure. The automatic wall functions allowed a flexible approach to the height of the prism elements near the wall. Figure 2 presents the schematic with the boundary condition locations in the computational model. Table VI, VII, VIII and IX show the boundary conditions and the assumption s made to apply these boundary conditions to the model. Two cases (Table III) were selected to reproduce σ because they span a significant range of mass flows so the relationship between σ and the coreflow can be analyzed. 6/14 Memorias Puerto Vallarta 2015 en CDROM

7 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 Table III. Mixing coefficient CFD cases Cooldown Case Core Flow [kg/s] Tmix [K] Low Flow ~ High Flow ~ The low and high core flow values were obtained from TRACG at 0.01 [%] power which would be the limiting low natural circulation and worst mixing. The CFD ESBWR model used only includes the regions which are predominantly single phase liquid (namely, bulkwater 1, downcomer and lower plenum), considering a steady state snapshot related to beginning of RWCU/SDC mode D2. This is a simplification relative to including two phase regions in the CFD model, but it requires providing the core flow as an input boundary condition. Except where specified, all walls were considered adiabatic. The boundary conditions associated with the RWCU/SDC top and bottom outlets were split in two to implement the two train operation as shown in Figure 2. The pressure above water level was set to 1 [atm] over the reference pressure (Saturation) to enforce all regions contain single phase liquid. The heat coming from chimney wall can be estimated from: (6) Table IV. Chimney data Item Description Dimension H Height ~8.6 [m] K 304SS Conductivity 17 [ ] D in Inner Diameter ~6.1 [m] D out Outer Diameter ~6.2 [m] Wall Heat Transfer ~1.4 [MW] 1 Pool surrounding the separators where water drains back after separated and is mixed with FW to be returned to the lower plenum via the downcomer. 7/14 Memorias Puerto Vallarta 2015 en CDROM

8 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient Figure 2. Boundary conditions schematic The adiabatic assumption is considered a good approximation given that the decay heat at the beginning of RWCU/SDC cooldown mode D2 is around 36.6 [MW]. Convergence was assessed using two convergence criteria. The first requires that the mass, momentum, energy and turbulence residuals drop three orders of magnitude. The second convergence criterion requires the mass, momentum, energy and turbulence imbalances to be at 10-2 The relevant quantities monitored during this analysis are the RWCU/SDC outlets temperatures: One Midvessel suction and two bottom drains per train. 8/14 Memorias Puerto Vallarta 2015 en CDROM

9 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 The temperature distribution is displayed in Figures 3 and 5, where essentially a homogeneous temperature can be observed at and below the midvessel suctions. In the first case a clear distinction in the temperature distribution can be made between the region surrounding the separators above the standpipes and the rest of the model due to the low coreflow. Figures 4 and 6 depict the corresponding temperature colored streamlines generated from the FW inlet and separator return. This shows the impact the coreflow can have in the flow trajectories. Figure 3. Low flow case temperature distribution 9/14 Memorias Puerto Vallarta 2015 en CDROM

10 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient Figure 4. Low flow case streamlines Figure 5. High flow case temperature distribution 10/14 Memorias Puerto Vallarta 2015 en CDROM

11 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 Figure 6. High flow case streamlines 5. COOLDOWN MIXING COEFFICIENT Table V gives the initial conditions for the TRACG and CFD simulations of RWCU/SDC mode D2. The pressure is used as the CFD boundary condition applied at the water surface, setting the separator exit temperature at the given value. Table V. RWCU/SDC initial conditions Initial Conditions Mode P [MPaA] T [ C] D Table VI summarizes the mass flows from the process flow calculation for RWCU/SDC considering two trains, where the FW return flow rate is taken as the combined (bottom and midvessel) suction flowrate. Table VI. RWCU/SDC mass flows Midvessel Suction Bottom Drain FW Return Mode Mass Flow [kg/s] Mass Flow [kg/s] Mass Flow [kg/s] D2 ~342 ~36 ~378 11/14 Memorias Puerto Vallarta 2015 en CDROM

12 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient Tables VII gives the FW parameters. The density was obtained with steam tables using the pressure and temperature from the initial reactor conditions, and it was used in TRACG for the RWCU mid-vessel suction and bottom drains. Table VII. RWCU/SDC FW return FW Return FW Return Density Mode Temperature [ C] Volumetric Flow [m3/h] [kg/m3] D2 ~131 ~1, Table VIII. Internal mass flows TRACG low flow Location [kg/s] Core (2 [%] of rated average) ~190 Separator 1st Discharge ~190 Separator 2nd & 3rd Discharge 0.0 Dryer Return 0.0 Table IX. Internal mass flows TRACG high flow Location [kg/s] Core ~3200 Separator 1st Discharge ~3200 Separator 2nd & 3rd Discharge 0.0 Dryer Return 0.0 σ was estimated using the area average of the temperature at the surfaces that define each outlet boundary: 2 Midvessel suctions (TopLeft and TopRight) and 4 Bottom Drains (Bottom L1, L2, R1, and R2). The mass flow boundary conditions are implemented distributing the flow imposed proportionally among the surfaces that constitute the boundary. The mass and energy balances match the boundary conditions, except for a small imbalance in the boundary condition imposed at water level. σ (Table X) for every outlet is estimated taking the average of the temperature from the last 10 iterations. The average σ corresponds to the mass flow weighted average considering the [%] mass flow distribution for the top and bottom RWCU/SDC outlets respectively. 12/14 Memorias Puerto Vallarta 2015 en CDROM

13 XXVI Congreso Anual de la Sociedad Nuclear Mexicana XIV Congreso Nacional de la Sociedad Mexicana de Seguridad Radiológica Puerto Vallarta, Jalisco, México, del 5 al 8 de Julio de 2015 Table X. Temperature and mixing coefficient Cooldown Case Low Flow High Flow Top Average T [C] Bottom Average T [C] Uniform Mix T [C] Top Average σ [%] Bottom Average σ [%] Average σ [%] Uniform Mix σ [%] CONCLUSIONS σ for low and high core flow is summarized in Table XI. Table XI. Summarized results Cooldown Case Average Uniform σ [%] Mix σ [%] Low Flow High Flow The mass flow weighted average for σ in the low flow case is 33.7 [%], a close match with the 33.6 [%] from the energy balance with uniform mixing. The mass flow weighted average for σ in the high flow case is 92.8 [%] which compares favorably to the value of 89.4 [%] from the energy balance with uniform mixing. Once the coreflow is established, the uniform mixing energy balance is an adequate alternative to derive σ for the other shutdown cooling modes. The calculated core flow is ~1200 [kg/s] (~2600 [lbm/s]) at 36.6 [MW] and D2 conditions. The CFD mass flow weighted average outlet temperature is above the uniformly mixed temperature, hence σ derived from the uniformly mixed temperature is conservative from the standpoint of shutdown cooling heat removal. 13/14 Memorias Puerto Vallarta 2015 en CDROM

14 José Gallardo et al, TRACG-CFD Analysis of ESBWR RWCU/SDC Mixing Coefficient ACKNOWLEDGEMENTS Valued guidance was provided by senior engineer Bobby Malone on the CFD modeling and analysis. This work was made possible through a collaboration initiated by Professor Jaime Morales from UNAM. REFERENCES 1. GE Hitachi Nuclear Energy, The ESBWR Plant General Description, USA (2007). 2. World Nuclear News, ESBWR html (2014). 3. GE Hitachi Nuclear Energy, ESBWR Design Control Document Tier 2, Chapter 5 Reactor Coolant System and Connected Systems, 26A6642AR R9, USA (2010, available in NRC Web based Adams). 4. GE Hitachi Nuclear Energy, TRACG Model Description, NEDO-32176, USA. 5. GE Hitachi Nuclear Energy, TRACG Application for ESBWR: Stability and Coreflow, NEDO , Supplement 1, USA. 14/14 Memorias Puerto Vallarta 2015 en CDROM