Passive Safety Systems Research Articles

Analysis of the application of natural circulation of cooling media in shipboard reactor installations

In projects of floating facilities with a nuclear power plant, great importance is attached to the formation of a set of properties of the internal self-protection of the reactor, i. e. the development of properties that ensure its safety based on natural feedbacks, processes and characteristics. To do this, the RU project is based on physical patterns and technical solutions that prevent or minimize possible consequences from emergency situations. In particular, the natural circulation of the coolant in case of loss of power supply by the circulation pumps of the first round allows cooling the core after its transfer to a subcritical state. However, in order for a natural circulation to occur sufficient to ensure nuclear and radiation safety, certain conditions must be met in a particular case. The paper notes that the most important property of reactor installations of nuclear vessels is their ability to ensure natural circulation of the coolant in emergency situations, as well as when operating at capacity. This makes it possible to reduce the dependence of reactor plant safety on the technical condition of power supplies. Currently, natural circulation of auxiliary cooling media in passive safety systems is also widely used in reactor installations. These include emergency reactor cooling systems using an emergency cooling system, including using intermediate heat exchangers; reducing emergency pressure in the protective shell in case of an accident with a large coolant leak; cooling the reactor vessel in an out-of-design accident with the formation of corium. The paper proposes a method for preliminary analysis of the reactor plant’s ability to ensure natural circulation of the coolant, which allows us to compare various design solutions that contribute to an increase in coolant consumption during natural circulation. It is shown that for the operation of the reactor plant in stand-down mode and at energy power levels with natural circulation of the coolant, it is advisable to switch to boiling reactors. Passive safety systems using natural circulation have received special development on new nuclear vessels of projects 20870, 22220, 10510. Further development of safety systems using natural circulation to move cooling media is predicted.

Frontal Impact Energy Absorbers for Passenger Cars.

Road accidents cause considerable losses to road users and to society. The steady increase in the number of vehicles leads to increased traffic volume. Therefore, there is a real need to improve passenger safety by developing passive safety systems. This article presents the results of experimental tests of structures absorbing kinetic energy, which could be used in the front section of a vehicle in order to reduce the consequences of passenger car head-on collisions. A number of crash tests of selected structures were conducted under various load conditions. An analysis was carried out of parameters enabling the authors to assess the level of energy absorption by the absorbers made, and compare these to absorbers available on the market. The tests carried out made it possible to determine energy absorption capability of the crash boxes prepared and to identify a structure exhibiting the most advantageous properties from the point of view of its prospective use. Of all of the absorbers analysed, in the context of energy absorption, it was the absorber made of glass-fibre-reinforced polyphenylene sulphide that produced the most advantageous results. Nonetheless, favourable results were obtained for all of the structures tested.

Potential of heavy goods vehicle countermeasures to reduce the number of fatalities in crashes with vulnerable road users in Sweden

Heavy Goods Vehicles (HGVs) are involved in a large share of all serious and fatal collisions. Among these, about 30% are collisions involving Vulnerable Road Users (VRUs). The aim of the present study was to evaluate the potential of Heavy Goods Vehicle countermeasures to prevent fatalities with vulnerable road users in Sweden. Both the General Safety Regulation (GSR) and coming Euro NCAP test program were taken into account. Furthermore, elaboration on existing passive HGV safety systems were used to investigate any additive benefit. The Swedish Transport Administration carry out in-depth studies of all road fatalities. All in-depth studies for the period 2015–2020 were analysed retrospectively by a consensus group of three analysts, to assess the effectiveness of 22 active and passive safety systems. For each technology, target populations and boundary conditions were defined in order to facilitate the assessment. In total, 63 fatal crashes were found, compiled of 28 pedestrians, 13 bicyclists and 22 Powered Two Wheelers (PTWs, i.e. motorcyclists and moped riders). Overall, it was found that active and passive safety technologies could prevent up to 59% (37/63) of the included fatalities. For pedestrians, the potential of improved HGV driver vision, both with a surround view system and an improved direct vision, would have the larger potential to save lives. For bicyclists where the turn-right scenario is overrepresented, the implementation of Advanced Emergency Braking in junctions and Blind Spot Information Systems had the highest potential to save lives. For passive safety systems, HGV wheel protection had a potential to save many bicyclists by preventing them from being run over. Crash scenarios involving a PTW are the most challenging to address with HGV safety systems, mostly due to high PTW speed. Nevertheless, wheel protection on the HGV could save the lives of PTW drivers, by preventing them from being overrun. The present study showed that the included active and passive safety technologies for Heavy Goods Vehicles could prevent 59% of fatalities among vulnerable road users in Sweden. The fatalities not targeted by the HGV safety technologies included in the study would need other countermeasures such as connected safety technology (e.g. V2V or V2I), infrastructure, or education.

Heat pipe type double-containment vessel as passive safety system for small modular reactors

This paper proposes a new concept of a passive safety system for small modular reactors as a heat pipe type double-containment vessel. The proposed system configures an additional vessel outside the existing one and functions as a heat pipe that utilizes phase change heat transfer to reduce the requirement of a large pool design. The double-containment vessel features a design consisting solely of the evaporation and condensation sections for heat pipe function with compactness. The ultimate heat sink only serves as a condenser part for the double-containment vessel, therefore significantly reducing the volume of stored fluid. The study numerically evaluates several postulated accidental cases to compare the performance of the double-containment vessel using the thermal hydraulic system code, MARS-KS. Even for serious conditions such as multiple failure accidents without safety systems operation, the double-containment can secure extra golden time to core damage by more than 2,090 sec compared to the original design. This study highlights the feasibility and potential of the heat pipe-based double-containment vessel as a passive safety system, enhancing the safety and reliability of small modular reactors. This kind of design can provide the advantages of safety and economics in the construction of small modular reactors.

Theoretical and experimental considerations regarding the airbag system operation

Abstract The concept of Passive Safety aims to reduce the harmful effects on vehicle occupants and pedestrians in the event of an accident. Intelligent car bodies and restraint systems have been designed. Thus, side, front, knee and head airbag systems have been introduced, as well as pre-strained seat belts and force limiters. The objectives of car manufacturers are not only to reduce injuries that can cause death, but also a significant reduction of those that can result in permanent damage such as a whiplash. Advanced airbag systems are currently introduced, which offer protection to the occupants at the knees, torso, hips and abdomen. The topic proposed in the paper concerns both theoretical research on the operation of passive security systems, respectively the airbag system, and experimental research. Regarding the theoretical research, this paper proposes a virtual prototyping method by means of the LS Dyna software package of the airbag system functioning in order to protect the driver in the event of a frontal collision. For the analysis of the airbag system functioning, a complex assembly which contains of a dummy, seat, steering wheel, passive safety systems was used. For the experimental research a stand was designed for the analysis and testing of the airbag system, which has the role to protect the driver in the event of a frontal collision of the vehicle. In order to carry out the experimental tests, it was proposed to create a sledge type stand, which simulates frontal collisions.

Comparison of Energy Absorptive Capacities of Different Aluminum Alloy Foams Placed Inside the Crash Box

Increasing population worldwide and the resulting increasing number of automobiles increase the risk of traffic accidents. Due to this increasing risk, automobile manufacturers take various safety measures to protect drivers and passengers in case of possible accidents. Crash boxes are one of the passive safety system elements that are the first to absorb the impact in the event of a front or rear impact accident, absorbing the resulting deformation energy and ensuring that it is transmitted into the car at the least possible level. Therefore, increasing the energy absorption ability of crash boxes is an extremely important issue. In this study, it was aimed to increase the energy absorption capabilities by placing aluminum foam based materials produced by using the powder metallurgy method using three different aluminum alloys (Al2024, Al5083, and Al6061) inside the crash boxes, which are normally manufactured as hollow. In addition, the produced aluminum foams were compared in terms of pore sizes with SEM images. It can be said that Al6061 is the most ideal material among the alloys used in terms of pore structure and homogeneity. On the other hand, Al6061 alloys produced the greatest damped energy value within the parameters of the investigation, 221.711 J. This value was 169.556 J for Al2024 alloy and 214.101 J for Al5083 alloy. As a result, it was concluded that the amount of energy absorption can be increased by about 4-5 times by using metallic foams produced using aluminum materials compared to the empty crash box.

Application of the passive turbocharger to PRHRS in integral PWR type SMR with an sCO2 power cycle

One of the potential design concepts for small modular reactor(SMR) is coupling an integral pressurized water reactor(IPWR) with a supercritical CO2(sCO2) power cycle to reduce the plant footprint. However, the passive safety systems of this SMR concept have not been well established compared to steam power cycle based SMRs. In this study, a passive turbocharger utilizing the unique characteristics of sCO2 is proposed as a way to improve the passive safety of sCO2 power cycle. This is the concept of installing a turbine-compressor set at the connection of passive residual heat removal system(PRHRS) to effectively remove the high level of residual heat in the early stages of an accident rather than relying only on natural circulation cooling. Based on the findings, it can be concluded that proposed sCO2 turbocharger concept has the potential to enhance performance of PRHRS and reduce the height of natural circulation loop leading to smaller volume.

On multi-stage deformation and gradual energy absorption of 3D printed multi-cell tubes with varying cross-section

Thin-walled structures are widely used in passive safety systems due to their excellent lightweight and controllable progressive deformation performance. While designing to improve the energy absorption performance of thin-walled structures, it is also crucial to consider reducing the damage received by protected objects during accidental collisions. In this work, stepwise and continuous graded multi-cell tubes with varying cross-sections were proposed based on the fused deposition modeling technique to achieve a gradual energy absorption process. Compression responses and energy absorption characteristics of the graded multi-cell tubes were investigated by quasi-static compression tests. Experimental results showed that the stepwise graded multi-cell tubes produced obvious multi-stage deformation and energy absorption during axial compression. The specific energy absorption presented an upward trend with the increased number of stepwise segments. For the same ranges of cross-section variations, continuous graded multi-cell tubes exhibited more plastic hinges and better energy absorption performance due to the absence of discontinuous interfaces. In addition, continuous graded multi-cell tubes showed continuous folding deformation with the gradually decreasing folding wavelength. Thus, it exhibited a steadily increasing trend in crushing force and realized the gradual energy absorption mode.

Numerical study on the effects of tube inclination angle and heat conductive cover plate on freeze plug performance in molten salt reactors

The freeze plug is an important passive safety system used in the molten salt reactors (MSRs). However, although the inclination angle is one of the fundamental design parameters, its influence on the freeze plug performance has not been explored systematically. Multidimensional numerical simulations were hence carried out to analyze the effects of tube inclination on the solidification and melting processes of salt encountered in the freeze plugs of the MSRs in this work. The effects of placing a high thermal conductivity cover plate on the outer wall of the freeze plug were also investigated. In the solidification process, the tube inclination had a significant impact on the equilibrium shape of the frozen salt. A decrease in the inclination angle from the horizontal resulted in a reduction in the plug volume, and closing of the freeze plug became more difficult when the inclination angle was small and the tube diameter was large. In the melting process, natural convection induced in the liquid salt was intensified with a decrease in the inclination angle. This accelerated the melting of solid salt to shorten the opening time significantly. Optimizing the tube inclination angle was hence shown to be of great importance to improve the safety of MSRs. When a copper plate was placed on the freeze plug outer wall to facilitate the heat transfer to the frozen salt, its influence on the solidification process was not noticeable, while melting was accelerated and the opening time was reduced by about 20%.

Numerical Investigation of Air Natural Convection in the AP1000 Passive Containment Cooling System Following LBLOCA Using ANSYS FLUENT

The innovative design of the AP1000 power plant has various layers of passive safety systems aiming to enhance reactor safety during normal and transient conditions. The passive containment cooling system (PCCS) is a safety-related system capable of removing heat from the steel containment vessel (SCV) to the atmosphere and preventing the containment from exceeding the design pressure and temperature following a postulated design-basis accident. The PCCS heat removal mechanisms include condensation on the internal SCV surface, heat conduction, natural convection, evaporation of water film, and radiative heat transfer. In two basic postulated scenarios, the reactor decay heat can ultimately be removed from the SCV only by air natural convection. The first scenario occurs 72 h following a large-break loss-of-coolant accident (LBLOCA) when the passive containment cooling water storage tank becomes unavailable. The second scenario occurs following a postulated loss of shutdown decay heat removal event. Hence, investigating the thermal-hydraulic behavior of the containment under transient conditions is essential to ensure its safety and integrity. In this study, a simplified three-dimensional model using ANSYS FLUENT is developed to investigate the cooling capability of air natural convection outside the SCV during a LBLOCA event. Because of the lack of experimental data, code-to-code validation was performed using the actual results of AP1000 alongside other research findings. The results show good agreement with available data, which can be used for future research.

An active learning approach for reliability assessment of passive systems combining polynomial chaos expansion and adaptive sampling

Passive safety system is extensively employed in newly designed reactors to increase their inherent safety. However, the driving force is smaller, and the uncertainty of the thermal–hydraulic (T-H) process may potentially result in the inability of system to perform its intended function, which is often known as functional failure. Therefore, the passive systems reliability has received increasing attention. Unfortunately, the assessment of system failure probability utilizing Monte Carlo simulation (MCS) methods necessitates a significant quantity of repeated calculations employing the thermal–hydraulic code, which may be impractical in terms of computational cost. To enhance the assessment calculation efficiency, an adaptive surrogate model is proposed. The bootstrap method was employed to introduce an error estimate into the polynomial chaos expansion (PCE) model, which solves the obstacle of the PCE model in reliability evaluation applications due to the lack of error estimation. The method constructs an active learning approach through error estimation, strategically identifies and selects the best candidate samples, and iteratively updates the initial experimental design. The effectiveness of the method was verified through three benchmark cases, and it was employed for assessing the passive residual heat removal system (PRHRS) reliability within integral-type pressurized water reactor (IPWR200). The results demonstrated that the proposed method significantly diminishes the frequency of invocations to the T-H code and improves computational efficiency to a large extent. Furthermore, it can serve as a valuable guide for the design, operation, and reliability evaluation of the passive system.

Modeling and validation of a water-cooled Reactor Cavity Cooling System (RCCS)

High Temperature Gas-cooled Reactors (HTGRs) have garnered interest for use in commercial energy production and for providing high temperature process heat for industrial applications. One of the passive safety systems suggested for decay heat removal in accident conditions is known as the Reactor Cavity Cooling System (RCCS). Two steady-state experimental data sets with varying Reynolds numbers (Re = 2490, 10279) conducted with the TAMU RCCS experimental facility were modeled and simulated using the computational fluid dynamics code STAR-CCM + and the TRACE system thermal hydraulics code for code validation. It was found that both the CFD and system code models showed good agreement with respect to the global conditions of the internal flow within the RCCS. Comparisons of the local temperature measurements showed that the TRACE code tended to overpredict temperatures toward the bottom of the RCCS cooling panel but showed good agreement above the midpoint of the RCCS risers. The CFD model found significant thermal stratification and recirculation in the Low Reynolds case due to the asymmetric heating profile applied to the riser panel. The bottom and lower manifold geometries of the RCCS design were also found to produce asymmetrical flow conditions which led to differences in local riser temperatures.

Lead Fast Reactor Thermal-Hydraulic Testing Facilities in Support of the UK Advanced Modular Reactor Program

The Westinghouse lead-cooled fast reactor (LFR) is a next-generation nuclear power plant whose primary mission is to reduce front-end capital costs and generate flexible and cost-competitive electricity for global markets, while offering mission versatility and satisfying the highest safety and sustainability standards. The LFR is an economical choice for carbon-free power generation to meet the ambitious goals of reducing carbon emissions. The plant utilizes passive safety systems for high reliability and public safety. The LFR testing program aims to fill the technology gaps in the key materials, components, and systems of the LFR plant. It provides demonstrations of LFR engineering that reduce the uncertainty of the LFR design and experimental data for the verification and validation of the modeling and simulation computer codes. With the support of the LFR phenomena identification and ranking table, the phenomena important to plant safety with insufficient states of knowledge have been identified and a Westinghouse testing plan developed to address them. The United Kingdom Advanced Modular Reactor Program Phase 2 was purposed to advance the level of design maturity toward eventual regulatory approval and deployment. Westinghouse and its partners have developed eight state-of-the-art test facilities within the program. Among them, the Passive Heat Removal Facility, Versatile Lead Facility, Lead Water Interaction Facility, and the Lead Freezing Facility are thermal-hydraulic testing facilities, with the other four facilities dedicated to testing lead corrosion and material mechanical behavior in lead. The design, development, and testing of the four thermal-hydraulic facilities are presented in this paper. A variety of computer codes ranging from system-level analysis, component-level analysis, and high-resolution computational fluid dynamics analysis, are utilized to support the development of the facilities. The analyses provide operational and accidental conditions in the LFR to identify major testing conditions and inform the facility design and testing matrix. With a significant amount of experimental data being generated with these highly instrumented test facilities, the computer codes and models will be benchmarked against the experimental results to improve their validation and verification status.

Analysis of Double-Ended Guillotine Break Accident of Surge Line in ACP100 Based on Coupling Method: Development and Validation of the Coupling Method

The ACP100 is a small modular reactor (SMR) designed and built in China, featuring an integrated primary loop and passive safety systems. In SMRs, the interaction between the reactor coolant system, containment vessel, and other systems is highly interdependent. Therefore, it is necessary to use high-precision analysis codes, which can be achieved by coupling multiple codes. This paper is the first part of a study which investigates the LOCA in the ACP100 using a coupled platform. In this paper, a coupling platform was developed for transient analysis of SMR accidents, based on the system code NUSOL-SYS and the Integrated Severe Accident Analysis (ISAA) code. An inter-process communication module was developed adopting shared memory and event objects to exchange data between the two codes. A coupling interface defining the data to be exchanged was proposed. Both NUSOL-SYS and ISAA were modified to perform synchronous time step control. The coupling platform were validated through a hypothetical scenario and Edwards’ pipe blowdown experiment, demonstrating exact consistency and high accuracy. This coupling platform offers a new method for SMR accident analysis, providing a foundation for future work.

High-Fidelity Velocity and Concentration Measurements of Turbulent Buoyant Jets

Accurate models of turbulent buoyant flows are essential for the design of the cooling circuit of nuclear reactors and passive safety systems. However, available models fail to fully capture the physics of turbulent mixing when buoyancy becomes predominant with respect to momentum. Therefore, high-fidelity experiments of well-controlled fundamental flows are needed to develop and validate more accurate models. We analyze experiments of positive and negative turbulent buoyant jets, both in uniform and stratified environments, with the aim of understanding the thermal hydraulics of turbulent mixing with variable density and providing high-fidelity data for the development and validation of turbulence models. Non-intrusive, simultaneous particle image velocimetry and laser-induced fluorescence measurements were carried out to acquire instantaneous velocity and concentration fields on a vertical section parallel to the axis of a jet in the self-similar region. The refractive index matching method was applied to measure high-resolution buoyant jets with up to 8.6% density difference. These data are free of the typical errors that characterize optical measurements of buoyancy-driven flows (e.g. natural and mixed convection) where the refractive index of the fluid is inhomogeneous throughout the measurement domain. Turbulent statistics and entrainment of buoyant jets in uniform and stratified environments are presented. These data are compared with non-buoyant jets in a uniform environment, as a reference to investigate the effects of buoyancy and stratification on turbulent mixing. The results will be used for the assessment of current turbulence models and as a basis for the development of a new model that captures turbulent mixing.

Verification of the Coupled Code PARCS/TWOPORFLOW with Rod Ejection Accident Calculations for Small Modular Reactors

Research on small modular reactors (SMRs) is gaining importance since they are key for addressing energy challenges in various sectors. These types of reactors integrate novel technologies that rely heavily on passive safety systems. Among the most developed light water reactors, SMR designs are SMART and NuScale. This work analyzes the academic boron-free Karlsruhe Small Modular Reactor (KSMR), which may fit in SMART, and a core design resembling NuScale. The research aims to explore the potential of new multiphysics tools under development to predict safety parameters of SMRs during normal operation and transients. For this purpose, the Purdue Advanced Reactor Core Simulator (PARCS) and TWOPORFLOW (TPF) codes have been coupled using the Interface for Code Coupling (ICoCo), orchestrating code execution through a C++ Supervisor program. In this work, a nodewise rod ejection accident (REA) has been analyzed for two different scenarios. The first is a hot-zero-power scenario for the KSMR core, and the second is initiated at 75% of nominal power for the NuScale core. Verification of results has been done though code-to-code comparison. Comparison of the PARCS/TPF results obtained for both KSMR and NuScale with reference cases shows acceptable differences. Key safety parameters predicted by the codes for the REA analysis of both cores have also been evaluated against the latest U.S. Nuclear Regulatory Commission regulation for reactivity initiated accidents showing that all parameters fulfill core coolability acceptance criteria and fuel rod cladding failure thresholds. The presented work highlights the transition from the coupling of PARCS with a mixture model code, such as SCF, to a coupling with a two-phase model code, such as TPF. These findings contribute to a better understanding of SMR phenomenology during accidental sequences and demonstrate the capabilities of the coupled codes.

Finite element analysis regarding folding behaviour of different crashbox versions

Low-speed collision is one of the most frequent road accident types nowadays. The consequences of this accident could be serious and long terminated. The passive safety system has an important role to reduce and prevent the amount of personal and technical injuries. Applying front and rear bumper beams is a good way to absorb more impact energy at the moment of collision reducing the load of the occupants. The background of the energy absorbing is the prescribed folding deformation of the bumper's support namely the crashbox. The main goal of the present study is to evaluate and compare differently shaped crashboxes as a thin-walled structure using finite element analysing to get information for more effective energy absorption taking into consideration of the weight optimization and spce requirements. To be comparable to the folding fluctuation of each specimen the compression test's curves are evaluated with relative standard density. Moreover, the specific energy absorption is also was determined. Based on the values determined by finite element analysis conclusions could be drawn.

On using machine learning algorithms for motorcycle collision detection

Globally, motorcycles attract vast and varied users. However, since the rate of severe injury and fatality in motorcycle accidents far exceeds that of passenger car accidents, efforts have been directed towards increasing passive safety systems. Impact simulations show that the risk of severe injury or death in the event of a motorcycle-to-car impact can be greatly reduced if the motorcycle is equipped with passive safety measures such as airbags and seat belts. For the passive safety systems to be activated, a collision must be detected within milliseconds for a wide variety of impact configurations, but under no circumstances may it be falsely triggered. For the challenge of reliably detecting impending collisions, this paper presents an investigation towards the applicability of machine learning algorithms. First, a series of simulations of accidents and driving operation is introduced to collect data to train machine learning classification models. Their performance is henceforth assessed and compared via multiple representative and application-oriented criteria.

Analysis of AP1000 Small-Break Loss-of-Coolant Accident Using Reactor Transient Simulator

The Westinghouse Electric Company’s Advanced Passive Reactor (AP1000) is characterized by the incorporation of passive safety systems (PSSs) designed to ensure core cooling during transient events. The assessment of PSSs requires evaluation of their performance through a combination of experiments and simulations employing various thermal-hydraulic codes. In addition, detailed evaluation of PSSs for a specific reactor system transient analysis such as loss-of-coolant-accident analysis supports understanding representative integral effects test facility development and the further evolution model development and assessment process. Developing a reactor system code is a complex and time-consuming process that requires significant engineering expertise and effort. It can take several months to even years to complete in the early stages of reactor system design and analysis. However, this process can be expedited through the use of transient simulator models for similar reactor systems, which can be used for lesson learning and training purposes. This study uses the Personal Computer Transient Analyzer (PCTRAN) code. The main advantage of PCTRAN is its ease of use and ability to run faster than real time. This study presents the results obtained for a small-break loss-of-coolant accident (SBLOCA) for two breaks using the full version (licensed) of PCTRAN. The purpose of this investigation is to evaluate the overall system behavior during the postulated SBLOCA event as well as assess the capability of the PCTRAN code to reproduce the system response during transient events. The obtained results were compared with the Westinghouse NOTRUMP system code. The PCTRAN code proved to be reliable in predicting the qualitative behavior of the system in both transient cases. As for the system response, it was found that it is contingent on the activation time of the PSSs. The differences in reactor coolant system pressure between the two codes were attributed to the critical flow model and simplification of mass and energy balance. Despite PCTRAN’s limitations, it can still provide a reasonable prediction of various reactor parameters such as pressure, mass flow rate, and void fraction during a SBLOCA scenario. It is worth noting that PCTRAN currently employs a bulk approach similar to that of the Modular Accident Analysis Program (MAAP) and MELCOR codes. However, the upcoming version of PCTRAN will include an artificial intelligence–based detection and accident prevention system, as well as different models for different reactor components. Consequently, PCTRAN has the potential to be upgraded to match the system thermal-hydraulic codes of the U.S. Nuclear Regulatory Commission and become more widely used in cybersecurity to safeguard nuclear power plants from cyberattacks.

Numerical and analytical investigation of thermal hydraulic behavior of large pools in passive heat exchangers

Passive residual heat removal safety systems are a modern design concept employed in Nuclear Power Plants (NPPs), and operate based on natural circulation. Several passive safety systems such as the emergency condensers (EC) use heat exchanger tube bundles immersed in large pools of coolant acting as a heat-sink (secondary side). The primary side of the heat exchanger is connected to the reactor core and, when activated, removes the decay heat in case of an accident. Recent research revealed the limitation of the state-of-the-art heat transfer models to capture the heat transfer rates achieved in these systems. The problems can be identified in both – primary and the secondary sides. The main focus of this study is to investigate the thermal hydraulic behavior of the secondary side of an EC. The data from the NOKO test facility has been considered as the validation base of this study. It has been observed that strong temperature stratification establishes in the pool due to the heat transfer process happening in a confined volume zone. Under this condition, the saturation temperature is reached at higher elevations of the tank while lower the liquid remains subcooled. This leads to different thermal behavior of the liquid at different elevations in the tank which is not properly captured by simple models. In addition, due to the complexity of the heat transfer process the required computational power is another challenge to study the thermal hydraulic behavior of the system. Therefore, the main aim of this study was to correctly predict the temperature stratification in the pool and identify boiling onset at different locations specially on the heated surfaces, while maintaining minimum numerical complexity. To achieve these targets, 2D multi-phase CFD simulations of secondary side using two different frameworks was conducted using ANSYS Fluent. Then, the results were validated against experimental data to assess their accuracy to achieve the best simulation approach.