Passive Safety Systems Research Articles

Safety analysis of an advanced passively-cooled small modular reactor during station blackout scenarios and normal operation with RELAP5/SCDAP

NuScale reactor design utilizes natural circulation for heat removal in the primary system, and it is maximized the usage of passive safety systems. Elimination of pumps from the primary system leads to specific phenomenology in this reactor. In this research, NuScale design is selected as a reference case study based on its design certification application (DCA). The main objective of this study is to evaluate the capability of the reactor to withstand against station blackout and to represent phenomenology involved in this specific integral PWR design. In this research, RELAP5/SCDAP is used to model the thermal-hydraulics performance of the reactor during steady-state and station blackout conditions. Furthermore, a new loading pattern is proposed for the reactor core, and its neutronic parameters are calculated using MCNP5, and three-dimensional visualization of the neutron flux in the core is depicted. Critical heat fluxes along the reactor core height are calculated based on Groenveld look-up table and Bowring correlation in the steady-state operation. Also, minimum departure from nucleate boiling ratio is computed for the proposed core based on hot rod heat flux. The results of this research are validated based on design certification application data. Then, the reactor behavior during two station blackout scenarios is getting focused. In the first scenario, no additional failure is assumed, while in the second scenario, failure of the emergency core cooling system is taken into account, and the performance of the reactor is studied for three days after the initiation of these accidents. The dominant phenomenon in each phase of these accidents has been analyzed. Results show significant improvements in decay heat removal in comparison with typical current PWRs during the station blackout accident. However, in some stages of accidents, remarkable fluctuations in the fluid flow of the primary system have been observed. In the case of long term exposure of the reactor to these flow oscillations, it may lead to structural damages and its further consequences.

Crashworthiness analysis and collaborative optimization design for a novel crash-box with re-entrant auxetic core

Crash-box is a significant part of automotive passive safety system and serves as a main energy absorption device in frontal impact scenario. In this paper, a novel crash-box integrating an outer thin-walled tube and an inner auxetic cellular core has been designed and comprehensively investigated under axial load. The impact simulation of the auxetic crash-box has been carried out, and the results show that introduction of the auxetic core can improve its energy absorption capacity without increasing too much peak impact force. Based on the sensitivity analysis, the effects of its geometric parameters on the crashworthiness performance have been studied. Finally, collaborative optimization of the auxetic crash-box has been performed to simultaneously improve its crashworthiness under the low-speed (15 km/h) and high-speed (40 km/h) impact cases. In the optimization procedure, the least square support vector regression (LS-SVR) method and an improved particle swarm optimization (IPSO) algorithm with time-varying coefficients have been utilized. The results demonstrate that the optimized crash-box can comprehensively improve the energy absorption and impact force characteristics effectively. The auxetic crash-box and the collaborative optimization approach provide extensive references for the application of auxetic structure in vehicle crashworthiness design.

Drivers alert system in the conditions of information shortage

The article presents the results of an accident analysis in the Russian Federation. Trends in reducing the number of accidents have been identified, but over the past 5 years the severity of their consequences has remained at a high level, despite the development of vehicles active and passive safety systems. A new accident prevention system is proposed by informing drivers about the occurrence of an emergency in conditions of insufficient and limited visibility, based on measuring the parameters of the vehicle movement in real time using GLONASS / GPS technologies.

Passive Safety System for Side Impact in Cars

The ultimate aim of this work is to provide the safest ride for the peoples. The following contribution is to protect the precious life of the driver and the co-passengers during the accidents by increasing the distance between the impact zone and the passengers, peculiarly during the crash on the sides. This can be achieved by incorporating a pneumatic cylinder under the seats, which is then actuated by the solenoid valve triggered by the deformation caused by the crashing vehicle. This makes the seats tilted at the time of the accident away from the near side of the door, which saves the life. The high-pressure energy in the container gives ultimate kinetic energy using high velocity gives a sudden actuation of seats in a fraction of seconds clearly say 0.3 seconds. We have re-designed the structure and mechanism of car seats to make it possible to save precious lives. Our design idea does not stop with single possible way of saving the life, it extends to lot of customization and adaptability based on the car structure and available space specific to various brands and models.

Experimental research on characteristics of condensation induced water hammer in natural circulation systems

Abstract Condensation induced water hammer (CIWH) is a common phenomenon possibly appearing in industrial pipelines, which may cause pipeline oscillation and even serious damage to the components. Previous research on the CIWH mostly focuses on the steam-water counterflow under forced flow conditions. For new floating nuclear power plants proposed recently, the CIWH may occur in some passive natural circulation safety systems. Nevertheless, the CIWH in natural circulation systems (NCS) has never been reported publicly yet. In this paper, a series of experiments are performed to investigate the flow characteristics during the CIWH in a NCS. The basic flow and temperature characteristics of the CIWH caused by counterflow of saturated steam-water mixtures and subcooled water in the NCS are studied in detail. Thereinto, the steam condensation induced flashing (SCIF) along with CIWH is firstly observed and analyzed. Moreover, the influences of CIWH on the flow behavior of the system are discussed under natural circulation and forced circulation conditions. The experimental results indicate that the occurrence of CIWH can result in further reverse flow of the subcooled water, which causes a sudden decrease in the flow rate of NCS. Moreover, the flow rate is more unstable under natural circulation than under forced flow.

Dynamic reliability framework for a Nuclear Power Plant using dynamic flowgraph methodology

Passive safety systems are being considered in advanced reactor designs to provide inherent stability for the operation of the nuclear reactor. Passive shutdown is provided for guaranteed removal of decay heat under emergency conditions. The reliability of such systems should be very high. The static reliability assessment of such systems has been considered using various techniques such as fault tree analysis, failure mode effect analysis, and reliability block diagrams. Dynamic reliability methods are powerful mathematical frameworks capable of handling interactions among components and process variables explicitly. In principle, they constitute a more realistic modelling of systems for the purposes of reliability, risk and safety analysis. Although there is a growing recognition in the risk community of the potentially greater correctness of these methods, no serious effort has been undertaken to utilize them in industrial applications. The dynamic flowgraph methodology is an integrated methodological approach to modelling and analyzing the behavior of software-driven embedded systems for the purpose of reliability/safety assessment and verification. In the present work, dynamic flowgraph methodology has been used to analyze the Station Blackout Scenario for a Nuclear Power Plant. The benefits of the proposed method are brought out with respect to the traditional methods like fault tree analysis, which deteriorates in applicability with increasing system size and complexity and fails to accommodate the dynamics of the system. The proposed method has been validated on the passive residual heat removal system of pressurized heavy water reactor.

Experimental validation of simulating natural circulation of liquid metal using water

Liquid metal-cooled reactors use various passive safety systems driven by natural circulation. Investigating these safety systems experimentally is more advantageous by using a simulant. Although numerous experimental approaches have been applied to natural circulation-driven passive safety systems using simulants, there has been no clear validation of the similarity law. To validate the similarity law experimentally, SINCRO-V experiment was conducted using Wood's metal and water for simulant of the Wood's metal. A pair of SINCRO-V facilities with length-scale ratio of 14.1:1 for identical Bo’ was investigated, which was the main similarity parameter in temperature field simulation. In the experimental range of 0.2–1.0% of decay heat, the temperature distribution characteristics of the small water facility were very similar to that of the large Wood's metal facility. The temperature of the Wood's metal predicted by the water experiment showed good agreement with the actual Wood's metal temperature. Despite some error factors like discordance of Gr’ and property change along the temperature, the water experiment predicted the Wood's metal temperature with an error of 27%. The validity of the similarity law was confirmed by the SINCRO-V experiments.

Finite Element Validation of an Energy Attenuator for the Design of a Formula Student Car

Passive safety systems of cars include parts on the structure that, in the event of an impact, can absorb a large amount of the kinetic energy by deforming and crushing in a design-controlled way. One such energy absorber part, located in the front structure of a Formula Student car, was measured under impact in a test bench. The test is modeled within the Finite Element (FE) framework including the weld characteristics and weld failure description. The continuous welding feature is almost always disregarded in parts included in impact test models. In this work, the FE model is fully defined to reproduce the observed results. The test is used for the qualitative and quantitative validation of the crushing model. On the one hand, the acceleration against time curve is reproduced, and on the other hand, the plying shapes and welding failure observed in the test are also correctly described. Finally, a model that includes additional elements of the car structure is also simulated to verify that the energy absorption system is adequate according to the safety regulations.

Numerical study on transient response characteristics of natural circulation in coupled loops under ocean condition

The application of passive safety systems (PSSs) can greatly improve the safety level of the Offshore Floating Nuclear Power Plants. As one of the important component of PSSs, the secondary passive residual heat removal system can be seen as the multiple natural circulation (NC) loops coupled system. The transient response characteristics between coupled natural circulation loops may affect the core decay heat removal progress under accident conditions. Besides, the ship motions caused by ocean waves can induce oscillations of the system parameters and variations of the reactor thermal hydraulic characteristics. The superposition effects of the transient response characteristics of the multiple NC loops coupled system and the system parameters fluctuation caused by ocean conditions can generate more complex system behaviors. In this paper, effects of the transient response of key parameters during process of increasing power in coupled natural circulation loops under ocean condition was analyzed by the modified RELAP5 code. The simulation results show that the response time of each parameter of the system changes greatly under inclination and rolling condition, but the effect of pitching and heaving motion on the response time of multi-loop natural circulation system is basically negligible.

Determination of the physicochemical properties of boric acid for VVER emergency modes

The results of experimental research of physicochemical properties of water solutions of H3BO3 are presented in this article. The literature review about the influence of hydrogen ion exponent of H3BO3 solutions on its heat and mass transfer, and corrosiveness of an acid in the VVER reactor core in is given. It is found that the existing data does not cover the entire range of parameters (acid concentration, temperature and pressure) characteristic of a possible Loss-of-Coolant Accident (LOCA) at nuclear power plant with a VVER. The problem of the accumulation and crystallization of boric acid in case of LOCA and the operation of passive safety systems (such as hydro accumulators of the first, second and third stages, and a passive heat removal system) is formulated. Test equipment and techniques of research are described. The generalizing equations for the pH and acidity constant of hydrous solutions of H3BO3 in range of the concentration (5-100 gkg−1 H2O) and temperature (25-50 °C) has been obtained.

An efficient method of key parameter screening for PCCS under SLB accident in AP1000

Variance decomposition is an effective sensitivity analysis method to screen the key parameters influencing passive safety system operation based on the uncertainties of thermal-hydraulic (T-H) model inputs. However, such method needs a large number of samples gained from T-H model running with the inputs sampled randomly from their probabilistic distributions, and the T-H model always takes quite long time to run once, then it will be a heavy calculation burden to do the analysis. In this paper, we propose a method to improve the analyzing efficient: based on the system T-H characteristics, the system behavior in a short time after an accident happening can represent the system T-H performance and be used to do the sensitivity analysis.Passive Containment Cooling System (PCCS) in AP1000 is used as a case study in our analysis, by which the heat produced in the containment can be transferred to the atmosphere through natural circulations. After steam line break (SLB) accident, the peak value of pressure in the containment appears within 1000s, we do the sensitivity analysis to screen key parameters in two ways: firstly, we screen the key inputs with variance decomposition method directly, 100 samples are gained from T-H model simulating 1000s, the inputs are sampled based on their probabilistic distributions, and the results show that air pressure is the most important parameter and the others don't have enough differentiation degrees. Then we get more 100 samples under the condition that air pressure is supposed as 0.1 MPa, here air temperature and steam mass flow are important ones besides air pressure. In another way, we analyze the correlation between pressure in the containment in a short time after SLB and the peak value according to the system T-H behavior, and get 600 samples from T-H model simulating 50s, the results are in accordance with that from T-H model simulating 1000s, air pressure, air temperature and steam mass flow are important parameters, and it just needs 1.55 h to calculate the important factor for one input.

Numerical child restraint system analysis in 6 years old infant during a dolly rollover test

This research analyzes the head and chest injuries with a finite element model of Hybrid III six years old during the vehicle rollover, using a passive safety Child restraint system (CRS). Vehicle seats and passive safety systems were modeled in CAD software. Subsequently, all elements were analyzed using the finite element method in the LS-DYNA® software. The border conditions were established for each study, in accordance with the Federal Motor Vehicle Safety Standard (FMVSS), following the FMVSS 213 standard for the mounting and fastening of the infant, the FMVSS 208 for the dolly rollover test methodology with which the vehicle rollover was performed. The vehicle Toyota Yaris 2010 was considered in all the analyzes. The numerical simulations were performed during an interval of 1 s, and there were obtained values every 2 ms to analyze the efficiency of the system and the restriction performance of the CRS system. It was compared the behavior when the chair uses a 5-point seatbelt, with respect to using the 3-point seatbelt, and what happens when holding the chair with the single seatbelt and with the ISOFIX anchorage. Three cases of studies were analyzed: Study I; CRS Study II; CRS with ISOFIX anchorage; and Study III; CRS with a seatbelt of 5-points. The outcome shows that the best restriction efficiency is that of the CRS 5 points seatbelt because it presents the lowest level of chest injury. In conclusion, the main factor to reduce injuries during a rollover accident is the correct anchoring of CRS, this is achieved with ISOFIX system, on the other hand, a good quality of material and a correct thickness used for the headrest could reduce the risk of a head injury.

Numerical Low-Back Booster Analysis in a 6-Year-Old Infant during a Dolly Rollover Test

This paper analyzes the possible head and chest injuries, produced in a Hybrid III dummy model of a six-year-old child during a rollover test, while the child uses a passive safety system low-back booster (LBB). Vehicle seats and passive safety systems were modeled with a CAD (Computer Aided Design) software; later, all elements were analyzed using the finite element method (FEM) with LS-DYNA® software. The border conditions were established for each study, in accordance with the regulations of Federal Motor Vehicle Safety Standards (FMVSS), and following the FMVSS 213 standard for the mounting and fastening of the infant, the FMVSS 208 for the dolly methodology test with the vehicle rollover was performed, implementing such analysis under the same conditions for a vehicle Toyota Yaris 2010. The numerical simulations were performed during an interval of 1 second, obtaining data values for periods of 2 milliseconds. This paper examines the efficiency of the system; three case studies were carried out: Study I: vehicle seat belt (VSB); Study II: the LBB system was secured by the seat belt; Study III: the LBB system with ISOFIX anchorage. The values of decelerations for the head and thorax of the infant were obtained, as well as neck flexion and thoracic deflection. The main factor to reduce injuries during a rollover accident is the correct anchorage of the LBB, and this is achieved with the ISOFIX system, since it prevents the independent movement of the LBB, unlike when it is fastened with the seat belt of the vehicle. The results show low levels of head and chest injury when ISOFIX is used because of reduced thoracic deflection during infant retention.

Multi-objective crashworthiness optimization of lattice structure filled thin-walled tubes

Abstract Thin-walled tubes have been mostly used in passive vehicle safety systems as crash energy absorber. With the use of additive manufacturing technology, it is possible to produce novel filler materials to further enhance the crashworthiness performance of thin-walled tubes. In this study, optimal designs of novel lattice structure filled square thin-walled tubes are investigated under axial impact loading by using a compromise programming based multi-objective crashworthiness optimization procedure. Types of filler lattice structures (i.e., body-centered cubic, BCC and body-centered cubic with vertical strut, BCC-Z), diameter of lattice member, number of lattice unit cells and tube thickness are considered as design parameters, and the optimum values of these design parameters are sought for minimizing the peak crash force (PCF) and maximizing the specific energy absorption (SEA) values. The validated finite element models are utilized in order to construct the sample design space and carrying out results verification; an artificial neural network is employed for predicting values of the objective functions; the weighted superposition attraction algorithm is used to generate design alternatives and searching for their optimal combination. The compromise programming approach is used to combine multi-objectives and to produce various optimal design alternatives. The optimization results showed that the proposed approach is able to provide good solutions with high accuracy and proper selection of design parameters can effectively enhance the crashworthiness performance of the lattice structure filled thin-walled tubes. The optimum results revealed that BCC hybrid designs have generally superior crashworthiness performance compared to that of their BCC-Z counterparts for the same compromise solutions. In particular, the PCF value of the optimized BCC-Z hybrid structures is up to 44% higher than that of BCC hybrid structures while these structures have similar energy absorption performances. The compromise solutions also show that the SEA of BCC and BCC-Z hybrid structures increases respectively by 29% and 51% depending on the selected weight factors for the design objectives.

An efficient method for passive safety systems reliability assessment

Safety by passive systems is a key design feature for new generation Nuclear Power Plants (NPPs). The Passive Containment Cooling System (PCCS) of the AP1000 NPP is a typical passive safety system, by which the heat produced in the containment is transferred to the environment through natural circulation and atmosphere is used as ultimate heat sink. Then, the climate conditions at the plant location influence the system performance. Monte Carlo (MC) simulation of random scenarios of embedding Thermal-Hydraulic (T-H) response of the passive system is commonly used for passive safety systems reliability assessment. However, T-H codes are usually computationally burdensome, in this paper, an effective way to overcome this issue and estimate passive safety system reliability is proposed and applied to the PCCS of the AP1000 NPP, showing that the T-H model runs can be reduced for efficient reliability assessment.

Advanced Natural Circulation Reduced-Order Model With Inclined Channel for Low Pressure Conditions

Abstract Natural circulation with two-phase flow is a nonlinear dynamical systems, which can show a very complex and strange behavior under specific conditions. The application of stability analysis requires a large computational effort and is cumbersome in case of prediction the dynamical behavior by system codes alone. Therefore, model-order reduction techniques are used to compensate this disadvantage by coupling with a bifurcation code such as MatCont. A reduced-order model is derived by employing the proper orthogonal decomposition (POD) to analyze the stability landscape of a low pressure natural circulation system representing passive safety systems such as the containment cooling condenser. The required full-order model contains a classical natural circulation loop with a heated section and a riser. The two-phase region is modeled by a drift–flux mixture model. The reliability of the full-order model is investigated by comparison with a reference model by the validated system code ATHLET.

An Experimental and Numerical Study of Wall Effect on Freeze Valve Performance in a Molten Salt Reactor

Abstract Freeze valve is an important passive safety system used in the molten salt reactor (MSR). In the freeze valve, salt is frozen by external cooling in the pipe connecting the reactor core and drain tanks. On the occasion of station blackout, the cooling system stops and the frozen salt melts automatically to open the freeze valve. Then, the molten salt containing fuel is drained out from the reactor core to the drain tanks and the nuclear reactions stop passively. Obviously, the melting time of frozen salt is important from the viewpoint of MSR safety. In this study, experiments and numerical simulations were conducted to investigate the effects of important parameters on the opening time of freeze valve. A particular attention was paid to the effect of pipe wall since its thermal conductivity is much higher than that of salt. The calculated opening time agreed with the experimental results fairly well, indicating that the present numerical model is useful to investigate the opening time of freeze valve in accidental condition of MSR.

Accident analysis of supercritical water reactors during startup

To analyze the typical characteristics of accidents occurring during the startup of a supercritical water reactor (SCWR), a comprehensive SCWR system model was established with the SCTRAN analysis code, based on models of the high-performance light-water reactor (HPLWR) steam cycle, SCWR circulation startup loop, passive safety systems, and CSR1000 core parameters. According to the sequence of the loss of coolant flow accident (LOFA), without a safety system and trigger signal of the safety system under the rated condition, a new trigger signal during the startup was designed and then used to assess “LOFA,” “uncontrolled CR withdrawal accident,” and “loss of coolant accident (LOCA)” during the startup process. The results show that the new trigger signal can ensure the effective and timely response of the safety system, as well as reactor safety during the startup process. Moreover, the maximum cladding surface temperature (MCST) of the reactor peaks (850 °C, well below the safety criterion of 1260 °C) at the end of the fourth stage of the startup process in the case of LOCA.

Preliminary experimental validation of multi-loop natural circulation model based on RELAP5/SCDAPSIM/MOD 4.0

Passive safety systems mostly working on the principles of natural circulation are given much priority in increasing the reliability of reactors’ inherent safety features. In view of this, many researchers carry out numerical and experimental studies in single-loop natural circulation model to study the natural circulation behaviors. In the present study, the experimental loop consists of three loops; two of which operate on natural circulation while the third on forced circulation. The experimental loop has successfully established natural circulation under different pressure conditions and the experimental results obtained are compared with those of the RELAP5 code simulation for validating the multi-loop natural circulation RELAP5 model. Single-phase natural circulation flow rates and temperatures at various locations give reasonable agreements between the code and the experiment. The results show that height difference between the heat source and the heat sink is more influential than system pressure in increasing natural circulation flow rate. The system pressure maintains the stability of the system, thus keeping it within single-phase region with increase in heat. In addition, temperature difference in the first loop being higher than that of the second loop at all pressures, did not influence the first loop flow rate to be higher due to the advantage of height difference in the second loop. However, extending the conditions beyond those of the experiment, that is higher power and system pressure, the first loop flow rate increased more than that of the second loop as predicted by the RELAP5 code.

Best estimate plus uncertainty analysis of the China advanced large-scale PWR during LBLOCA scenarios

China advanced large-scale Pressurized Water Reactor (PWR) implements series of passive safety systems to provide core cooling and decay heat removal in case of some unexpected accidents. The development of reactor best estimate system analysis codes has realized the transition from conservative safety analysis to Best Estimate plus Uncertainty (BEPU) analysis for the purpose of achieving more realistic results and digging larger safety margins. The BEPU analysis during Large Break Loss of Coolant Accident (LBLOCA) has been widely used for the licensing application of Nuclear Power Plants (NPPs). In this paper, the BEPU analysis of China advanced large-scale PWR was performed under the conditions of LBLOCA scenarios employing RELAP5 code. The models of reactor coolant system with all the passive safety systems were established and the influence of uncertainties of important parameters on the Peak Cladding Temperature (PCT) was evaluated. The results show the obtained PTC (1314 K) through the BEPU analysis is far below the 10CFR50.46 PCT acceptance criteria (1477 K) in case of LBLOCA and is also lower than the conservative calculation value (1348 K), indicating a larger safety margin. This work provides a meaningful evaluation method for China advanced large-scale PWR safety assessment.