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.

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.

Steam injection through blowdown pipes and spargers into a large water pool, also known as Pressure Suppression Pool (PSP), is employed in Boiling Water Reactors (BWRs) to prevent containment overpressure. Thermal stratification in the pool results in an increased pool surface temperature compared to a mixed pool condition, leading to higher 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 the quantification of uncertainties. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been developed to represent the impact of steam injection on the pool while avoiding the detailed simulation of steam-water interface dynamics, which is a computational challenge in itself. These models are compatible with any Computational Fluid Dynamics (CFD) code using 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 of the global pool behavior and local flow characteristics demonstrates that the proposed models can provide an adequate prediction of the relevant phenomena.

In advancing the study of the In-Vessel Retention (IVR) strategy, a three-layer corium pool configuration has been proposed. Owing to density inversion and the thinning of the top metallic layer, the thermal focusing effect on the vessel sidewall becomes more pronounced, posing a higher risk to Reactor Pressure Vessel (RPV) integrity. To investigate the thermal behavior of this configuration, Computational Fluid Dynamics (CFD) simulations were conducted based on the TROSE experiment, which features a three-dimensional hemispherical vessel (diameter = 2.4 m) and employs mineral oil, water, and Cerrobend alloy as simulants. The results demonstrate that the Wall-Modeled Large Eddy Simulation (WMLES) turbulence model effectively captures the corium pool behavior under a Rayleigh number of 10¹⁵. Additionally, assuming only energy exchange (without mass transfer) across the interfacial boundaries is shown to be a reasonable simplification that improves computational efficiency. In Test-T3 of the TROSE experiment, strong natural convection in the light metallic layer produces a nearly uniform temperature field, whereas limited convection in the oxidic layer leads to significant thermal stratification. In the heavy metallic layer, heat transfer is dominated by conduction, accompanied by crust formation at the boundaries. These findings confirm the applicability of the CFD method for simulating three-layer molten pools and highlight its potential for future safety analyses in large-scale passive nuclear power plants.

Boiling Water Reactors (BWRs) employ the Pressure Suppression Pool (PSP) to prevent containment overpressure. Steam released from the primary coolant system is injected into the pool and condensed rapidly upon direct contact with subcooled water. However, steam injection can lead to the development of thermal stratification in the pool. The increased surface temperature of the stratified pool also increases containment pressure compared to a mixed pool. This process can become of safety significance, as it was observed in the Fukushima Daichi accident. 

The primary objective of PSP safety analysis is to verify that pool temperature remains within acceptable limits. The state-of-the-art approach has to rely on assumptions about the fraction of the pool acting as a heat sink, i.e. elevation of the thermocline. There is a need for more mechanistic approaches that can adequately resolve the interactions among various phenomena, safety systems, and operational procedures that can clarify the degree of conservatism in the currently employed approaches to safety analysis. For this purpose, a computationally efficient tool known as the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been proposed. The models are used to simulate the integral effect of steam injection on the large-scale pool without explicit modeling of the dynamics of the interface between steam and water.

This thesis aims to make a significant step toward the development and validation of the predictive capabilities of the EHS/EMS models for the assessment of the PSP performance during steam injection through spargers. To achieve this goal, a synergic framework that integrates both experimental and numerical campaigns has been developed.

Conceptual design and conditions for a series of Integral Effect Tests (IETs) of steam injection through a sparger into a large-scale pool in the PANDA facility have been proposed. A set of Separate Effect Tests (SETs) of steam injection in the SEF-POOL facility is proposed to develop new correlations for the EMS. A Bubble-based Particle Tracking Velocimetry (Bub-PTV) technique is developed and implemented in SEF-POOL to measure the streamwise velocity profiles induced by steam injection.

Modeling guidelines for CFD simulations using EHS/EMS models are developed for the prediction of the thermal behavior of the pool. A turbulence source to represent the effects of steam condensation is proposed. The applicability and validity of the modeling approaches are assessed by comparing them with the measurements obtained in IETs. Also, an approach of scaling based models is proposed to predict the erosion velocity of the thermocline.

The developed modeling approaches are applied to analyze the PSP performance of a Nordic BWR during various realistic scenarios. The possibility of thermal stratification and the effects of activation of different systems on the pool behavior are investigated.

Kokvattenreaktorer (BWR) använder tryckavlastningsbassängen (PSP) för att förhindra övertryck i inneslutningen. Ånga som frigörs från det primära kylsystemet injiceras i bassängen och kondenseras snabbt vid direkt kontakt med underkylt vatten. Ånginjektion kan dock leda till utveckling av termisk skiktning i bassängen. Den ökade yttemperaturen hos en skiktad bassäng ökar även trycket i inneslutningen jämfört med en blandad bassäng. Denna process kan bli av betydelse för säkerheten, något som observerades vid olyckan i Fukushima Daichi.

Det primära målet med PSP-säkerhetsanalys är att verifiera att bassängens temperatur förblir inom acceptabla gränser. Toppmoderna metoder bygger på antaganden om vilken del av bassängen som fungerar som en värmesänka, dvs. termoklinens höjd. Det finns ett behov av mer mekanistiska tillvägagångssätt som på ett tillfredsställande sätt kan lösa samspelet mellan olika fenomen, säkerhetssystem och operativa procedurer, för att klargöra graden av konservatism i nuvarande säkerhetsanalyser. För detta ändamål har en beräkningsmässigt effektiv metod, känd som modellerna för Effektiv Värmekälla (EHS) och Effektiv Rörelsemängdskälla (EMS), föreslagits. Modellerna används för att simulera ånginjektionens integrerade effekt på bassängen i stor skala utan att explicit modellera dynamiken i gränssnittet mellan ånga och vatten.

Denna avhandling syftar till att ta ett betydande steg i utvecklingen och valideringen av EHS/EMS-modellernas prediktiva förmåga för bedömning av PSP-prestanda vid ånginjektion genom spridare. För att uppnå detta mål har en synergetisk ram utvecklats som integrerar både experimentella och numeriska kampanjer.

Konceptuell design och förhållanden för en serie av integrerade effektförsök (IET) av ånginjektion genom en spridare i en stor bassäng vid PANDA-anläggningen har föreslagits. En uppsättning separata effektförsök (SET) av ånginjektion i SEF-POOL-anläggningen föreslås för att utveckla nya korrelationer för EMS. En bubbelbaserad partikelspårnings velocimetri (Bub-PTV)-teknik har utvecklats och implementerats i SEF-POOL för att mäta strömningshastighetsprofiler som induceras av ånginjektion.

Riktlinjer för modellering för CFD-simuleringar med hjälp av EHS/EMS-modeller har utvecklats för att förutsäga bassängens termiska beteende. En turbulenskälla föreslås för att representera inverkan av ångkondensation. Modellernas giltighet och tillämpbarhet bedöms genom att jämföra dem med de mätningar som erhållits i IET. Dessutom föreslås en skalningsbaserad metod för att förutsäga termoklinens erosionshastighet. 

Modelleringsmetoderna som utvecklats används för att analysera PSP-prestandan för en nordisk BWR under olika realistiska scenarier. Möjligheten till termisk skiktning och effekterna av aktivering av olika system på bassängens beteende undersöks.

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.

Transportation of fresh fuel assemblies requires the utilization of transport casks to ensure the internal radioactive material does not affect the environment. In this work, we introduce an innovative design of a transport cask for the fresh fuel assembly of a Small Module Reactor (SMR). The cask is constituted by two containers, supports and various energy-absorbing structures welded at the outer container. The structural integrity of the transport cask under variant scenarios determined by the International Atomic Energy Agency (IAEA) and domestic regulations was numerically evaluated. The detailed 3D model of the cask was created, and the simulations were performed via the Finite Element Method (FEM) using commercial software ABAQUS. Simulation results obtained from static and dynamic analyses indicated that the integrity of the designed transport cask can be assured during normal and accident conditions. The most severe damage during the 9 m drop tests was caused by a horizontal oblique condition and this configuration is recommended to be tested in future experiments.

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.

Measurement of the velocity field in thermal-hydraulic experiments is of great importance for phenomena interpretation and code validation. Direct measurement by means of Particle Image Velocimetry (PIV) is challenging in some multiphase's tests where the measurement system would be strongly affected by the phase interaction. A typical example can refer to the test with steam injection into a water pool where the rapid collapse of bubbles and significant temperature gradient makes it impossible to obtain main flow information in a relatively large steam flux. The goal of this work is to investigate the capability of the use of machine learning for the flow reconstruction of the jet induced by steam condensation from sparse temperature measurement with ThermoCouples (TCs). Two frameworks of (i) 'FDD' using pure data-driven modeling and (ii) 'FPINN' combining data-driven and Physics-Informed Neural Networks (PINN) are proposed and investigated. The frameworks are applied to a single-phase turbulent planar jet with data generated by CFD simulations.