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.

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.

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.

As a newly designed 200 MW nuclear heating reactor (NHR200-II), flow-induced vibration (FIV) of the fuel rod has attracted extensive attention due to its slender shape and the hydrodynamic loads arose from the turbulent flow of the surrounding fluid. Fretting wear and/or damage of fuel rod induced by FIV would highly affect system operation and nuclear safety. In this article, a particle finite element method (PFEM) based partitioned paradigm (i.e., implicit finite element method for structure dynamics, PFEM for fluid flow, and unsteady Reynolds averaged Navier–Stokes for turbulence modelling) toward FIV problems was proposed, implemented, and validated. Axial FIV of a single NHR200-II fuel rod was then analyzed through this finite-element based framework. Vibration characteristics of the fuel rod against varied turbulent inflow velocities with a constant turbulence intensity Tv of 5% were discussed in detail. The results showed that horizonal displacement is larger than vertical displacement but both within the same order of magnitude. The effect of inflow velocity of 1.0–2.0 m/s on the dominant frequency is also captured. Besides, fluctuating horizonal pressure is identified as the main source of forced vibration. Therefore, reinforcements on the horizontal constraint are recommended to better eliminate the vibration and enhance the reactor safety.

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 discharging through spargers and blowdown pipes into the Pressure Suppression Pool (PSP) is employed in Boiling Water Reactor (BWR) to prevent overpressure of the reactor vessel and containment. The capability of suppression can be reduced during the operation when the thermal stratification is developed. Direct modeling of steam injection into a water pool with long-term transient is computationally expensive due to the large-scale difference in space and time. To enable such prediction, Effective Heat source and Effective Momentum source (EHS/EMS) models are proposed. In previous work, we demonstrated the implantation of EHS/EMS models in the Computational Fluid Dynamics (CFD) tool and its application to plant simulation. In this work, we use the developed model to further investigate the thermal stratification and mixing in the PSP of a Nordic BWR. The event to be analyzed is initiated by spurious activation of one valve in the safety injection system. The focus of the simulations is to investigate the possibility of stratification development and understand the effects of the activation of different systems on pool behavior. Pool transient is simulated by CFD code (ANSYS Fluent) with EHS/EMS models and the injection conditions of the steam are derived from the simulation results performed by the system-level codes (GOTHIC).