Pipe Break Research Articles

Velocimetry during depressurized conduction cooldown events in the HTTF

Abstract The High Temperature Test Facility (HTTF) is a quarter-scale integral-effect facility designed to study pressurized and depressurized conduction cooldowns (PCC and DCC, respectively) in high temperature, gas-cooled reactors (HTGR). This study focuses on DCC and aims to characterize the gas exchange between the reactor core and the reactor cavity to assess the risk for air ingress and subsequent potential for natural circulation core cooldowns. High-resolution velocity profile measurements are performed at the coolant pipe break location over a long time-scale using molecular tagging velocimetry. This non-intrusive, laser-based technique is applied for the first time in the field of nuclear thermal hydraulics, providing new insights for understanding the underlying phenomena at play during a DCC event. Such data are also valuable for validation of numerical simulations such as Reactor Excursion and Leak Analysis Program (RELAP). Results are reported here for various gas mixtures of helium (the coolant) and nitrogen (surrogate for air) to study the effect of density ratio. The short transient (30 s) lock-exchange phase is well captured with helium flow velocity between 0.8 and 2 m/s. The flow is shown to persist over much longer time-scales (hours) at a velocity of about 0.2 m/s, a likely consequence of gas mixing in the hot leg and in the lower plenum of the reactor. Flow visualization at the exit of the hot leg shows shear instabilities, which supports the hypothesis of significant mixing.

Water-jacket reactor cavity cooling system concept to mitigate severe accident consequence of high temperature gas-cooled reactor

Abstract The High Temperature Gas-Cooled Reactor (HTGR), as a Gen-IV reactor, adopts a Reactor Cavity Cooling System (RCCS) to remove core decay heat after a reactor trip. During a postulated accident such as a pipe break in the reactor coolant system, the core decay heat can be properly removed by a RCCS to ensure the integrity of the fuel, reactor pressure vessel (RPV), and concrete silo. In this study we are more interested in the impact of a failure beyond that of the design basis of the RCCS functions, particularly the case in which natural convection of external air is not established. Such a case could happen when a chimney is collapsed by an external event. Against a severe disaster in which air convection is completely lost, a mixed air-water RCCS concept having a water-jacket system is suggested in order to mitigate the significant overheating of the RPV. In order to quantify the effectiveness of the water-jacket RCCS functions, the Depressurized Conduction Cooldown event (DCC) of a HTGR system with a thermal power of 350 MW is analyzed using the GAMMA+ system transient analysis code. As the RCCS air-cooling function is impaired, the water-jacket system increases its role to take part in the heat removal. From the analysis results of a DCC event with complete failure of the RCCS air-cooling, it is confirmed that, compared to the case without the water-jacket system, the maximum temperature of the reactor vessel can be reduced by about 400 °C thus ensuring its integrity. Therefore, the water-jacket system can be utilized as a backup system to mitigate the consequences of a severe accident such as the collapse of the chimney of the RCCS.

A Comprehensive Framework to Evaluate Hydraulic and Water Quality Impacts of Pipe Breaks on Water Distribution Systems

AbstractPipe breaks have significant impacts on the hydraulic and water quality performance of water distribution systems (WDSs). Therefore, it is important to evaluate these impacts for developing effective strategies to ultimately minimize the consequences of these events. However, there has been surprisingly limited research focusing on impact evaluation for pipe breaks so far. To address this gap, this paper proposes a framework to comprehensively evaluate hydraulic and water quality impacts of pipe breaks on a WDS using six quantitative metrics. These metrics primarily focus on identifying (i) break outflow volume, (ii) water shortage, (iii) nodes with reduced service quality, (iv) pipes with affected pressures, (v) pipes with reversed flow directions, and (vi) pipes with significantly increased velocities, for each breaking event within a WDS. Statistical behaviors, spatial properties, and pipe rankings of metric results are analyzed to reveal the underlying characteristics of impacts induced by pipe breaks. We illustrate the proposed framework using three WDSs with different properties. Results show that impacts of pipe breaks not only vary with pipe diameters but are also significantly influenced by pipe locations, when the break occurs, and the specific metric considered. The proposed framework greatly enhances the fundamental understanding of the underlying properties of breaking impacts on the hydraulic and water quality of WDSs, as well as the ranking of pipes based on the consequences of breaks. Such understanding offers important guidance to develop effective pipe management, resource planning, and break restoration strategies to minimize the impacts of breaking events on WDSs.

Process of Air Ingress during a Depressurization Accident of GTHTR300

A depressurization accident is the design-basis accidents of a gas turbine high temperature reactor, GTHTR300, which is JAEA’s design and one of the Very-High-Temperature Reactors (VHTR). When a primary pipe rupture accident occurs, air is expected to enter the reactor core from the breach and oxidize in-core graphite structures. Therefore, it is important to know a mixing process of different kinds of gases in the stable and unstable density stratified fluid layer. In order to predict or analyze the air ingress phenomena during the depressurization accident, we have conducted an experiment to obtain the mixing process of two component gases and the characteristics of natural circulation. The experimental apparatus consists of a storage tank and a reverse U-shaped vertical rectangular passage. One side wall of the high temperature side vertical passage is heated and the other side wall is cooled. The other experimental apparatus consists of a cylindrical double coaxial vessel and a horizontal double coaxial pipe. The outside of the double coaxial vessel is cooled and the inside is heated. The results obtained in this study are as follows. When the primary pipe is connected at the bottom of the reactor pressure vessel, onset time of natural circulation of air is affected by not only molecular diffusion but also localized natural convection. When the wall temperature difference is large, onset time of natural circulation of air is strongly affected by natural convection rather than molecular diffusion. When the primary pipe is connected at the side of the reactor pressure vessel, air will enter the bottom space in the reactor pressure vessel by counter-current flow at the coaxial double pipe break part immediately. Afterward, air will enter the reactor core by localized natural convection and molecular diffusion.

Cylindrical breeding blankets for fusion reactors

Blankets with a cylindrical structure have been developed for a water-cooled ceramic breeder concept considering a pipe break inside the blanket container. Most existing studies have examined a box structure for the blanket container. Structural change from a box to a cylinder can decrease in first wall thickness while maintaining the pressure resistance of the container. Two blankets with radial and toroidal/poloidal cylindrical axes have been developed. Their first walls are hemispherical and half cylindrical. The tritium breeding capabilities of the developed blankets were studied using Monte Carlo code MCNP5.14. Different combinations of breeder and neutron multiplier materials were considered. With the combination of lithium oxide and beryllium, the tritium breeding ratio of the blankets including surrounding gaps was evaluated as higher than 1.20, leading to a net tritium breeding ratio higher than 1.05.

A PC-based high temperature gas reactor simulator for Indonesian conceptual HTR reactor basic training

In planning for nuclear power plant construction in Indonesia, helium cooled high temperature reactor (HTR) is favorable for not relying upon water supply that might be interrupted by earthquake. In order to train its personnel, BATAN has cooperated with Micro-Simulation Technology of USA to develop a 200 MWt PC-based simulation model PCTRAN/HTR. It operates in Win10 environment with graphic user interface (GUI). Normal operation of startup, power maneuvering, shutdown and accidents including pipe breaks and complete loss of AC power have been conducted. A sample case of safety analysis simulation to demonstrate the inherent safety features of HTR was done for helium pipe break malfunction scenario. The analysis was done for the variation of primary coolant pipe break i.e. from 0,1% - 0,5 % and 1% - 10 % helium gas leakages, while the reactor was operated at the maximum constant power of 10 MWt. The result shows that the highest temperature of HTR fuel centerline and coolant were 1150 °C and 1296 °C respectively. With 10 kg/s of helium flow in the reactor core, the thermal power will back to the startup position after 1287 s of helium pipe break malfunction.

Probability of network disconnection of water distribution system for maintenance prioritization

AbstractUnexpected pipe breaks in municipal water distribution systems may cause isolation of parts of the network or reduction of redundancy, leading to reduced system reliability. While a network with less redundancy implies less tolerance to further breaks, the isolation of nodes explicitly indicates unavailability of the system to service the nodes. This paper presents a method of measuring these topological changes using algebraic connectivity (AC). AC is a parameter that can be used to assess robustness and redundancy of a network. The changes in AC associated with pipe breaks are compared with the AC of intact networks to assess whether the removal of the pipe causes reduction of redundancy or isolation in the network. An AC of 1.5625 × 10−5 is calculated for an intake of a medium-sized water distribution network (WDN). The fluctuation in AC is used to assess the criticality of each pipe segment to the overall structure of the network. This study also evaluates the failure probability of the WDN, assuming that the network failure probability is equivalent to the probability of isolation of parts of the system due to pipe breaks. The breaks leading to the failure are identified using the method of the minimum cut-sets.

Structural assessment of jet impingement loads under postulated main steam line break conditions

When a sudden rupture occurs in high energy lines, the ejection of inner fluid with high temperature and pressure causes jet impingement loads on adjacent components and structures, as well as on the ruptured pipe itself. This study examines the jet impingement phenomenon under postulated HELB (High Energy Line Break) conditions. In this context, typical numerical models were generated considering the SG (Steam Generator), MSL (Main Steam Line) piping, and containment. Subsequently, two sets of numerical analyses were carried out by changing the break locations. One set is computational fluid dynamics to assess the ejected fluid characteristics and define pressure histories. The other uses finite element analyses to calculate the stresses and displacements of the SG, piping, and containment building caused by the jet impingement loads. As a result, detailed analyses provided more realistic and conservative data than those in the ANSI/ANS 58.2 standard used in the current HELB design while the jet loads did not threat secondary structural integrity after the MSL piping break.

Flow regime transition criteria for co-current downward two-phase flow

Downward two-phase flow is observed in light water reactor accident scenarios such as loss of coolant accident (LOCA) and loss of heat sink accident (LOHS) due to loss of feed water or a secondary pipe break, and so, it is vital to have a thorough understanding of the flow mechanisms and regimes. With this point of view, flow regime transition criteria for vertical downward flow for a range of pipe diameters of 24–101.6 mm has been developed. Several models looked at the radial distribution of the bubbles and the wake effect of leading bubbles while others looked into the Kelvin-Helmholtz instability seen at the gas-liquid interface. The newly developed criteria have been compared to flow regime maps obtained via subjective and objective means, consisting of air-water data at atmospheric conditions as well as at an elevated pressure of 0.2 MPa. The new model is also compared to flow regime maps developed with different inlet conditions. Overall, the present model showed good agreements with the available data, with the exception of several 50.8 mm ID flow regime maps of different inlet conditions as well as a self-organizing neural network. This study also highlights the need for a more objective and consistent flow regime map data for large diameter pipes, the identification of cap-bubbly and churn-turbulent flows in these maps, and the deviations observed between a supervised and self-organizing neural network (SONN).

Numerical Simulation of Supercritical Carbon Dioxide Critical Flow in the Nozzle Tube

The supercritical carbon dioxide (S-CO2) Brayton gas turbine cycle has been studied as an efficient and cost-effective option for advanced power systems. One major safety issue for any power cycle is a pipe break and the associated discharge of the working fluid and subsequent decrease in system pressure. In this paper, an S-CO2 critical flow in the nozzle tube is analyzed numerically with fluent 15.0. The Redlich–Kwong real gas equation is selected to calculate carbon dioxide density and the standard k-epsilon turbulence model is selected. Experimental data are used as a benchmark to examine the capability of the current approach. Compared with experimental data, the simulation results overestimate the critical mass flux; the error range is between 15% and 25%. The simulation results show that as L/D increases, critical mass flow decreases. As stagnation temperature increases, critical mass flow decreases. The complex thermal hydraulic behavior in the nozzle tubes is analyzed. Three flow patterns in the nozzle tube during transient critical flow are obtained and discussed. From inlet to outlet of the tube, CO2 may undergo the following phases in turn: (1) supercritical phase; (2) supercritical phase—gas phase; (3) supercritical phase—gas phase—liquid phase. The simulation results are also helpful for further experimental and theoretical research.

Two-phase CFD simulation of research reactor siphon breakers: A verification, validation and applicability study

During a nuclear reactors design phase many possible accidents are postulated. The main accident that could occur in an open pool reactor, such as the Brazilian Multipurpose Reactor (RMB) is the Loss-Of-Coolant Accident (LOCA). This type of accident occurs when the primary cooling piping breaks and the pressure gradient induces the refrigerant (water) to flow through this pipe by siphon effect, draining the pool. This accident can be avoided with a siphon breaking mechanism, which could consist of an orifice in the coolant pipe at a level above the minimum required water level in the pool. If the pipe breaks, air will enter through this orifice, interrupting the siphon flow. However, tests have shown that the mechanism in most cases does not stop the water flow immediately, causing a reduced water level. The siphon break phenomenon is a complex two-phase transient flow. Experimental studies of the siphon break are very expensive and time-consuming tasks. In this case, it would be appropriate to make a Computational Fluid Dynamics (CFD) analysis of this phenomenon, allowing a detailed evaluation of the flow characteristics in a simpler and cheaper way than an experiment. However, the complexity of this phenomenon imposes difficulties to its computer modeling and its accuracy must be assessed. In this study, a methodology is proposed for the simulation of orifice type siphon breaker for research reactors. In order to properly evaluate the numerical methodology, an experimental model available in the literature was simulated and a Verification and Validation (V&V) procedure was performed based on the Grid Convergence Index (GCI) method. Numerical uncertainties due to mesh were evaluated and estimated for other simulations with different siphon breaker orifice diameters to assess the applicability of the proposed methodology. Results showed good agreement with experimental results with reasonable uncertainty range. The proposed simple numerical uncertainty estimation procedure aims to encourage the discussion towards an uncertainty quantification culture in the CFD community.

A prioritization method for replacement of water mains using rank aggregation

Pipe breaks in municipal water distribution networks may cause serious damage economically and socially. Existing methods for replacement scheduling of pipes do not provide practical indicators for replacing an individual deteriorated pipe. This work formulates the selection problem as the decision of preference ordering or ranking and proposes a bipartite ranking-based approach. The suggested approach also considers loss from broken pipes in terms of the costs associated with broken water main and its repair. We use rank aggregation method to integrate multiple ranks into replacement order of water mains. The suggested framework prioritizes current pipe sections for replacement based on the aggregated ranks. Multiple ranks given by the reliability of water pipe sections are aggregated and a cost effective policy for pipe replacement is derived.

Two-phase discharge from a stratified region through two horizontal branches with centerlines falling on an inclined plane

The main objective of this study was to obtain new experimental data for conditions not previously tested for discharging two-phase flow through two 6.35 mm diameter branches with centerlines falling in an inclined plane. The present results are relevant to many industrial applications including headers and manifolds, multichannel heat exchangers and small breaks in horizontal pipes. In the experimental investigation, the critical heights for the onsets of liquid and gas entrainment (OLE and OGE, respectively) were obtained, analyzed and correlated for two different branch spacings and two different angles between the branches. For each combination of branch spacing and angle between the branches, a wide range of Froude numbers was used. Two-phase mass flow rate and quality results were also obtained and analyzed for a range of interface heights for 16 different combinations of branch spacing, inclination angle, test section pressure and pressure drop across each branch. New empirical correlations were developed to predict the dimensionless mass flow rate and quality. The new correlations show good agreement with the present data and with previous correlations.

Preliminary safety assessment on two LOCAs under ITER-like condition for the optimized CFETR helium cooled solid breeder blanket

A conceptual design of helium cooled solid breeder (HCSB) blanket, one of three blanket candidates for Chinese Fusion Engineering Test Reactor (CFETR), has been performed recently. The transient analysis for different possible accidents should be carried out to assess its safety performance. In this paper, the ITER-like conditions are adopted since the associated system for CFETR is missing, such as helium cooling system, plasma shutdown condition, pump behavior, etc. The complete model is recreated including the optimized typical outboard HCSB blanket (NO.12) and its ancillary helium cooling loop, and accident analyses of two loss of coolant accident (LOCA) cases are investigated using RELAP5. The influences of different break areas under in-vessel LOCA are compared, and the accident consequence after small area break is chosen to be investigated. Regarding the ex-vessel LOCA, the influences of different break locations are thoroughly analyzed. Since the plasma cannot terminate passively after ex-vessel pipe break, the plasma termination behaviors are investigated with different shutdown time. The computational results show that with the safety criteria for ITER the HCSB blanket can be cooled down effectively by the helium cooling system (HCS) and the integrity of pressure barriers can be guaranteed for both accidents.

A metric approach to measure resilience in water supply systems

Water utilities face challenges and should ensure overall resilience. A comprehensive calculation of resilience for unusual problems and frequent events is proposed. The resilience definition includes the capacity of a system to delimit impacts in the aftermath of a disruptive event. Different types of threats are classified in consideration of water service disruption: (A) water scarcity, (B) water supply discontinuity, (C) discontinuity of hydraulic conditions and (D) discontinuity of drinking water quality conditions. Failure thresholds distinguish: (1) a standard level, (2) a normative level, (3) an accepted level and (4) a critical level. The resilience model considers service level and time and is modified by protocols, resources and technologies. The methodology is applied to the water supply system managed by Canal de Isabel II (Spain). The case studies are a drought, pipe breaks and water quality failures. Real data help managers contrast the effectiveness of their protocols and plan investments.

A Utility Dodges Pipe Breaks With Solid Planning

Under a sweeping asset management program, a municipal water utility targeted distribution system vulnerabilities and planned cost‐effective pipe replacements.

System Reliability Evaluation in Water Distribution Networks with the Impact of Valves Experiencing Cascading Failures

Water distribution networks (WDNs) represent a class of critical infrastructure networks. When a disaster occurs, component failures in a WDN may trigger system failures that result in larger-scale reactions. The aim of the paper is to evaluate the evolution of system reliability and failure propagation time for a WDN experiencing cascading failures, and find the critical pipes which may reduce system reliability dramatically. Multiple factors are considered in the method such as network topology, the balance of water supply and demand, demand multiplier, and pipe break isolation. The pipe-based attack with multiple failure scenarios is simulated in the paper. A case WDN is used to illustrate the method. The results show that the lowest capacity gets stronger when a WDN is short of supply, becoming the dominant factor that decides the evolution of system reliability and failure propagation time. The valve ratio (VR) and system reliability present a flattened S curve relationship, and there are two turning points in VR. The critical pipes can be identified. With the fixed 5% valves, a WDN can improve system reliability and resist cascading failures effectively. The findings provide insights into the system reliability and failure propagation time for WDNs experiencing cascading failures. It is proven to be useful in future studies focused on the operation and management of water services.

Study and demonstration of pressure wave-based leak detection in a district heating network

A district heating network (DHN) is one of the most important infrastructures in cities and towns of countries with colder weather. Citizens generally use DHN services for hot water supply all year round and also for space heating during winter. It is important that in case of an accident this service would be restored as soon as possible, causing minimal damage or inconveniences for the customers. In addition, it is important to minimise losses of the DHN operator. This study demonstrated a possibility of leak location in DHN using the data from the pressure sensors in the network, employing the negative pressure wave (NPW) method. The mass balance in the DHN is measured at the heating source, which acts as a confirmation of a leak in a closed system. The experiments in the real DHN were performed and the data was used to trace the leak location. Numerical modelling tools were used to model pressure transients during pipe break accident under various conditions in order to predict the effectiveness and limitations of the leak detection system.

Multiobjective Optimization of Pressure Dependent Dynamic Design for Water Distribution Networks

Optimal design and rehabilitation scheduling of water distribution networks (WDNs) have been often dealt separately over the past few decades. However initial design (which is named as static design in this paper) has indubitable influence on the operational condition and rehabilitation scheduling of network. This paper introduces an approach for simultaneous optimization of initial design and rehabilitation scheduling of WDNs during their life cycle. In this approach which is named as dynamic design, pipe diameters in the first year and their rehabilitation/replacement in the next years of the expected life of the network are determined considering the nodal demands growth and increase in pipes’ roughness. The proposed model consists of a multiobjective ant colony optimization engine linked to a pressure dependent analysis model and a pipe break prediction model. This paper introduces the following contributions: (1) it implements dynamic design based on the pressure dependent analysis with considering leakage; (2) a support vector machine based sub model is used for pipe break prediction. Then pipe breaks and their repair costs are considered in dynamic design process; (3) a new reliability index is used as one of the objective functions. Two networks are used to investigate the impact of static and dynamic designs on the reliability and total cost of design and rehabilitation of WDNs during their life cycle. The results show that the dynamic design produces more reliable and lower cost networks in comparison to the ones resulted from either static design or rehabilitation scheduling separately.

Statistical Dependence of Pipe Breaks on Explanatory Variables

Aging infrastructure is the main challenge currently faced by water suppliers. Estimation of assets lifetime requires reliable criteria to plan assets repair and renewal strategies. To do so, pipe break prediction is one of the most important inputs. This paper analyzes the statistical dependence of pipe breaks on explanatory variables, determining their optimal combination and quantifying their influence on failure prediction accuracy. A large set of registered data from Madrid water supply network, managed by Canal de Isabel II, has been filtered, classified and studied. Several statistical Bayesian models have been built and validated from the available information with a technique that combines reference periods of time as well as geographical location. Statistical models of increasing complexity are built from zero up to five explanatory variables following two approaches: a set of independent variables or a combination of two joint variables plus an additional number of independent variables. With the aim of finding the variable combination that provides the most accurate prediction, models are compared following an objective validation procedure based on the model skill to predict the number of pipe breaks in a large set of geographical locations. As expected, model performance improves as the number of explanatory variables increases. However, the rate of improvement is not constant. Performance metrics improve significantly up to three variables, but the tendency is softened for higher order models, especially in trunk mains where performance is reduced. Slight differences are found between trunk mains and distribution lines when selecting the most influent variables and models.