Pressure Suppression Pool (PSP) is a passive safety feature in Boiling Water Reactors (BWR) and Advanced Pressurized (AP) reactors. Steam released from the primary coolant system is condensed in a large water pool to prevent containment overpressure. Injected steam induces sources of heat (buoyancy force) and momentum (inertia). The competition between the sources might result in the development of thermal stratification or mixing of the pool. Increased temperature of the top pool layer leads to higher partial pressure of steam in the containment and thus reduces pressure suppression capacity of the pool. Models with predictive capabilities are needed for the analysis of the reactor pool transients. Development and validation of the models require adequate experimental data. In this work we discuss results of the pre-test analysis that was carried out to select conditions for the tests with steam injection through sparger head and Load Reduction Ring (LRR) in a large scale PANDA facility. The aim of the tests was to obtain data on pool thermal stratification and mixing under different regimes of steam injection. Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were implemented in a computational fluid dynamics (CFD) code in order to carry out the analysis. Evolution of the pool temperature and velocity characteristics were analyzed in the scoping analysis to provide suggestions for selection of (i) pool depth, (ii) elevations of the sparger head and LRR, (iii) number of open LRR holes, (iv) layout of instrumentation, and (v) steam injection procedure for each test.

Steam injection through spargers into a Pressure Suppression Pool (PSP) is used to prevent containment overpressure during primary system depressurization in normal operation and accident conditions. Direct Contact Condensation (DCC) of steam induces mass, momentum, and heat sources and thermal stratification can develop in the pool if the momentum source is not sufficient to overcome the buoyancy created by the heat source. The thermal stratification reduces the heat storage capacity of the pool, increasing the pool surface temperature and thus containment pressure compared to mixed pool configuration. In this work we analyze the results of the large scale pool experiments in PANDA and PPOOLEX facilities to develop better understanding of the phenomena and regimes that govern the multi-scale system. Specifically we compare the results of the tests performed with the steam injection through the sparger head (horizontal injection) and the load reduction ring (downwards injection). We demonstrate that the response of the pool in terms of stable position of the thermocline and velocity of the thermocline motion can be described with proper selection of the non-dimensional scaling parameters for both facilities and directions of the steam injection. We also summarize and interpret other observations from the tests that are important for understanding of the pool thermohydraulic phenomena and development of respective predictive capabilities.

Advanced Pressurized Water Reactors (APWRs) and Boiling Water Reactors (BWRs) employ a suppression pool as a heat sink to prevent containment overpressure. Steam can be discharged into the pool through multi-hole spargers or blowdown pipes in both normal and accident conditions. Direct Contact Condensation (DCC) creates sources of momentum and heat. The competition between these two sources determines the development of thermal stratification or mixing of the pool. Thermal stratification is of safety concern as it reduces the cooling capability compared to a completely mixed pool condition. In this work we develop a scaling approach to prediction of the thermal stratification in a water pool induced by steam injection through spargers. Experimental data obtained from large-scale pool tests conducted in the PPOOLEX and PANDA facilities, as well as simulation results obtained using validated codes are used to develop the scaling. Two injection orientations, namely radial injection through multi-hole Sparger Head (SH) and vertical injection through Load Reduction Ring (LRR), are considered. We show that the erosion rate of the cold layer can be estimated using the Richardson number. In this work, scaling laws are proposed to estimate both the (i) transient erosion velocity and (ii) the stable position of the thermocline. These scaling laws are then implemented into a 1D model to simulate the thermal behavior of the pool during steam injection through the sparger.

Boiling Water Reactors (BWR) and Advanced Pressurized Water Reactors (APWR) often use spargers to release steam from the primary coolant system into a pool with subcooled water to prevent containment overpressure. Direct contact Condensation (DCC) creates sources of mass, heat, and momentum determined by the condensation regimes in a subcooled pool. Thermal stratification can develop in the pool if buoyancy forces created by the heat source dominate the momentum source. Only part of the stratified pool volume can be used as the heat sink which is a safety concern. Modeling of steam injection and its effect on a large pool is computationally expensive due to the considerable spatial and temporal scale differences between the steam-water interface dynamics and global pool circulation. To enable the prediction of realistic plant transients, Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were proposed. These models aim to calculate the timeaveraged integral effects of the steam injection on the pool without resolving the dynamics of the interface and DCC phenomena. This work aims to develop the empirical correlations that predict the time-averaged effective momentum induced by steam injection into a subcooled pool. The experimental data were collected in a Separate Effect Facility (SEF-POOL) at LUT, Finland. The force acting on the injection pipe was measured in SEF-POOL to estimate the effective momentum rate created by steam injection. The condensation regime coefficient C, defined as the ratio of effective momentum rate to the theoretical momentum rate of injected steam, is presented as a function of Jakob and Mach numbers. We found that the pitch-to-diameter ratios (P/D) had a significant effect on both the effective (time averaged) momentum rate and instantaneous forces measured in the low Jakob number region, which might be attributed to the interactions between neighboring nozzles.

A Separate Effect Facility (SEF-POOL) was designed to measure the time-averaged momentum induced by steam injection into a subcooled water pool. Recent analysis of large-scale pool data has shown that the turbulence generated by the steam injection affects not only velocity field in the vicinity of the steam injection point but also integral pool behavior (thermal mixing and stratification). Unfortunately, the application of existing techniques for the velocity field measurements (such as Particle Image Velocimetry) is difficult due to presence of small gas bubbles and significant temperature gradients in the liquid. In this paper we introduce an experimental approach to quantification of the velocity field using Bubble based Particle Tracking Velocimetry (Bub-PTV) in which the streamwise velocity is inferred by stereoscopic tracking of air bubbles entrained by the flow. This paper presents the development of in-house code for bubble tracking and preliminary results obtained from the tests using water injection into a water pool. These water injection tests are intended to verify the setup of the experiment (e.g. air generating system, stereo cameras) and provide databases for code development and validation. The results are also compared with Computational Fluid Dynamics (CFD) simulations performed in ANSYS Fluent, and good agreement was achieved. The experimental measurements suggest that the proposed approach can provide a 3D velocity field measurement of the jet. Moreover, it indicates the potential of Bub-PTV as a reliable technique for measuring downstream axial velocity fields induced by steam injection.

The time-averaged Effective Momentum Source (EMS) induced by steam injection into a subcooled water pool was measured in a Separate Effect Facility (SEF-POOL) under a wide range of injection conditions. Post-test simulations of large-scale pool experiments conducted in PANDA and PPOOLEX facilities indicate that diffusion of the momentum is another important factor that determines the downstream momentum transport and dynamics of the stratified layer. Thereby, an experimental quantification approach was introduced to measure the streamwise velocity profiles induced by steam injection. It is achieved by using Bubble based Particle Tracking Velocimetry (Bub-PTV) where the velocity is inferred by stereoscopic tracking of the injected air bubbles. In the previous work, we validated the approach using tests with water injection. In this paper, we discuss Bub-PTV application to steam injection tests. The Bub-PTV code was further developed to improve the performance of bubble detection under the steam injection conditions. The momentum induced by steam injection diffuses much faster compared to the single-phase liquid jet injection. We demonstrate that, in the far field where steam has condensed completely, the jet can be simulated using a single-phase solver with the Effective Heat and Momentum sources (EHS/EMS) models along with an additional turbulence source term to account for the turbulence generated in the process of steam condensation. Good agreement can be achieved on the downstream velocity profiles if the added turbulence source is properly calibrated.

Direct contact condensation (DCC) of steam in the pressure suppression pool (PSP) is used to control containment pressure in Boiling Water Reactors (BWRs) and Advanced Pressurized Water Reactors (APWRs). The competition between momentum and heat sources induced by steam injection through multi-hole spargers and blowdown pipes determines whether the pool is thermally stratified or mixed. Development of thermal stratification affects capacity of the PSP to condense steam. To enable computationally efficient modeling of the PSP transients, the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been proposed previously. The EHS/EMS models enable simulation of the large scale pool behavior without explicit modeling of the steam water interface and DCC phenomena. One of the problems for an optimal implementation of the EHS/EMS models is the definition of the boundary conditions that impose distribution of the momentum and heat sources. In this work, EHS/EMS models are implemented using “Unit Cell” approach in ANSYS Fluent to provide detailed numerical analysis of the individual turbulent jets induced by steam injection through the sparger holes. The model is validated against data from Particle Image Velocimetry (PIV) and temperature measurements for a range of steam injection conditions in the PANDA HP5 tests. Good agreement between the test data and simulations suggests that the proposed model can provide sufficiently accurate prediction of both local and large scale phenomena induced by steam injection into the pool.

Steam injection through spargers and blowdown pipes into the Pressure Suppression Pool (PSP) is employed in Boiling Water Reactors (BWRs) to prevent containment overpressure. The development of thermal stratification in the pool leads to a higher (compared to a mixed pool) pool surface temperature and thus increased containment pressure. Therefore, adequately validated predictive capabilities for modeling of the pool behavior are essential for the safety analysis of containment performance. The thermal behavior of the pool (e.g. thermal stratification or mixing transient) depends on the interplay between the heat and momentum sources induced by direct contact condensation of steam. Computational efficiency of the models for the simulation of the long transients in the large-scale pools is critical, especially for quantification of uncertainties. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been developed to simulate the effect of steam injection on the pool without resolving dynamics of the steam-water interface, which is a computational challenge in itself. These models can be applied with any Computational Fluid Dynamics (CFD) code with a single-phase solver. In this work we further develop the EHS/EMS models using (i) new EMS model correlation based on the latest results from Separate Effect Test (SEF-POOL) facility; and (ii) Condensation-Induced Turbulence (CIT) model calibrated against integral pool experiments conducted at PANDA, PPOOLEX, SJTU, and HEU facilities under a wide range of steam injection conditions. The good agreement between predictions and measurements of the global pool behavior and local flow characteristics suggests that the proposed modeling approach can provide an adequate prediction of the relevant phenomena.

Boiling Water Reactor (BWR) employs the Pressure Suppression Pool (PSP) as a heat sink to prevent overpressure of the reactor vessel and containment. Steam can be injected into the PSP through spargers in normal and accident conditions and through blowdown pipes in case of a loss of coolant accident (LOCA). There is a safety limit on the maximum PSP temperature at which such steam injection might cause dynamic loads on the containment structures. The performance of the pool can be affected if thermal stratification is developed when temperature of the hot layer grows rapidly while cold layer remains inactive. Simulation of pool behavior during realistic accident scenarios requires validated models that can sufficiently address the interaction between phenomena, safety systems and operational procedures. Direct modeling of steam injection into a water pool in long-term transients is computationally expensive due to the need to resolve simultaneously the smallest space and time scales of individual steam bubbles and the scales of the whole PSP. To enable PSP analysis for practical purposes, Effective Heat source and Effective Momentum source (EHS/EMS) models have been proposed that avoid the need to resolve steam-water interface. This paper aims to implement mechanistic approaches previously developed by authors for the simulation of transient thermal stratification and mixing phenomena induced by steam injection through spargers in a Nordic BWR PSP. The latest version of the EHS/EMS models using the 'Unit cell' approach has been validated against integral effect pool tests and applied to plant simulations. Several scenarios with boundary conditions corresponding to postulated accident sequences were simulated to investigate the possibility of stratification development and the effects of activation of different systems (e.g., blowdown pipes, high momentum nozzle) on the pool behavior.