Feedwater System Research Articles

VVER 1000 Severe Accident Analyses Using MELCOR Code

This work describes the computer model development of the water–water energetic reactor (VVER) 1000 nuclear power plant (NPP) in the methods for estimation of leakages and consequences of releases (MELCOR) 1.8.6 code and its subsequent use for the accident scenarios analysis leading to the core melting. The baseline accident scenario was a stress test case—the station blackout (SBO, the complete loss of alternating current electric power in a nuclear power plant). In addition to this, four other scenarios were analyzed in which the SBO was combined with other technological failures—the loss of steam generator feedwater system and small, medium, and large break coolant accidents (LOCA). The results provided detailed information on the time course of accident scenarios, their temperature and pressure parameters, hydrogen production, and the mass inventory released from the molten corium and debris into the containment of the NPP.

Research on multi-scale simulation model for single-phase water pipe networks

Abstract Fluid transients occur mainly during fluid transfer in the networked pipe systems in nuclear power plants. Research on pipe network simulation is presented in literature. However, there are still some open technical gaps. In this paper, to satisfy the different time step requirements in different stages of nuclear power plant operation, and to improve the accuracy of simulation results, a multi-scale simulation model of single-phase water pipe networks is proposed. Therefore, the pipe networks are divided into control nodes and flow lines in a complete simulation map. The multi-scale simulation is obtained through the combination of simplified equations, time step selection and refined nodes. The accuracy and stability of the simulator are verified by modeling the feed water system of the Chinese experimental fast reactor. In addition, the output is compared with output of the commercial software, Jtopmeret. As a conclusion, the new model calculates results for this feed water system with a higher accuracy under dynamic operations which consistent with theoretical analysis.

Development of Anti-windup PI Control and Bumpless Control Transfer Methodology for Feedwater Control System

The windup phenomenon happens when the proportional-integral (PI) algorithm output is saturated. This phenomenon results in deteriorated performance, even instability, in a control system. Additionally, because control systems such as the feedwater control system (FWCS), are dynamic, it is vital to ensure that the state of the system is correct when switching the controller between manual mode and automatic mode. Therefore, the objective of this paper is to present an anti-windup scheme, and a bumpless transfer technique for the PI controller of the FWCS. The anti-windup scheme is developed using the back-calculation method, and the bumpless control transfer methodology is illustrated using a tracking mode of the PI controller at 100% reactor power level. In order to verify these designs, four scenarios are proposed: (1) performance without using an anti-windup scheme, (2) configuration of the block for anti-windup by the back-calculation approach, (3) control transfer with bumps, (4) bumpless control transfer through the use of tracking mode. Then, these scenarios are simulated in MATLAB Simulink. Throughout this simulation, the improved speed responses are obtained when the proposed methods are applied.

Systems engineering based advanced optimization for sustainable water management in refineries

Refineries are increasingly compelled to play an imperative role in achieving sustainable management of water as mandated by sustainable development goals (SDGs). Existing optimization models lack the ability to addresses this challenge of obtaining optimal refinery water network configuration for problems of industrial scale due to difficulty in application of complex solution algorithms of mixed integer non-linear problem (MINLP) formulations to large dimensions. Therefore, the optimization formulation in this work exploits process engineering derived constraints to build an integrated superstructure, which results in a simplified non-linear problem (NLP), and thus reduces computational complexity. Further, to substantially minimize fresh water consumption, the proposed model incorporates novel features of a modern refinery complex viz. distributed effluent treatment plant based on total dissolved solids (TDS) levels in waste water, segregation of sour water treatment based on phenolics content in condensate stream, and minimizing cooling tower blowdown rates by optimizing the cycles of concentration in cooling tower operation. The formulation is demonstrated for optimal water routing and economic benefits of the proposed approach through a representative case study of a real refinery complex. Towards the goal of zero liquid discharge, an efficient new scheme of recycling treated water to the boiler feed water system and cooling tower make up water requirements in optimal ratios, has been suggested from the results of the proposed model. Sensitivity of water reallocation in optimal refinery water network with respect to crude quality and desalter performance is also presented through application of proposed optimization framework in the case studies.

Comparative analysis of auxiliary feedwater system and passive safety system under typical accident scenarios for integrated pressurized water reactor (IPWR)

The Integrated Pressurized Water Reactor (IPWR) designed with series of passive safety system has been regarded as a very promising reactor concept. In this paper, in order to assess the accident mitigation ability of passive safety systems in China 100 MW IPWR, the complete system models, including the reactor vessel, passive safety injection system and passive residual heat removal system, were established using RELAP5 software. The comprehensive and quantitative comparative analysis of auxiliary feed water system and passive safety system under typical accident scenarios, including loss of flow accident and Station Black-out (SBO), were investigated, respectively. Results show that the passive residual heat removal system has the similar accident mitigation capability compared with the auxiliary feed water system, which is adopted in most of traditional PWR design. And the IPWR could realize the reactor safe shutdown under typical accident scenarios with passive safety systems. Finally, the Small break loss of coolant accident (SBLOCA) induced by pressurizer surge line rupture was calculated to verify the function of passive safety systems for IPWR. Results demonstrate that the passive safety system could provide emergency coolant to the reactor and remove the reactor decay heat successfully. This work is meaningful for the quantitative evaluation of passive safety systems in IPWR.

Multi-objective optimization of maintenance program in multi-unit nuclear power plant sites

Maintenance program optimization of nuclear power plants (NPPs) has been a research focus over the past decades, and the existing works are mostly conducted with a one-reactor-at-a-time presumption. Multi-unit NPP site design (i.e., a single site houses multiple reactors), however, is a common case, in which the reactors are not independent from each other, rather, connected by complex intra- and inter-unit mechanisms. To bridge the research gap and generate practically useful results, a methodology of conducting multi-objective optimization for maintenance program in the context of multi-unit NPP sites is proposed in this research. The maintenance optimization is formulated as a tri-objective scheme aiming at minimizing multi-unit unavailability, site-wide risk and cost. Case studies are conducted on feedwater systems adapted from a real-world two-unit NPP site with and without uncertainties. It can be concluded that, for the case studies in this paper, (i) risk attitudes, in the form of weighting factors of risk types with radiological consequences of different severities, of NPP decision makers and regulators could notably affect the optimal maintenance scheduling and the projected objective values; (ii) the optimization model under uncertainties can be taken as a generalization of that without uncertainties and could have the chance of discovering new Pareto-optimal solutions leading to lower costs without compromising multi-unit unavailability or risk.

Risk-informed approach for design optimization during low power and shutdown operation

APR+ has been developed in Korea since 2007. APR+ adopts various advanced safety features including passive auxiliary feedwater system, four emergency diesel generators. Through the implementation of the advanced designs, APR+ increased the safety to the world best level of evolutionary reactors. The full power core damage frequency or containment failure frequency decreased significantly comparing to APR1400 that is base model of APR+. However, low-power shutdown risk has not been improved substantially. This paper suggests several design changes that optimize low-power shutdown risk. Based on the design alternatives, this paper discusses risk effectiveness of the proposed design including various factors, e.g. equipment reliability, human error, training, procedure and so on.

Pump cavitation in feed water systems - remedies

The Net Positive Suction Head incipient is a multiple of the Net Positive Suction Head Required in centrifugal pumps. With limited static heads of the feedwater systems, the suction impellers of the feed pumps work in cavitation. The article discusses the ways to deal with the problem of cavitation in such systems.

Bidirectional implementation of Markov/CCMT for dynamic reliability analysis with application to digital I&C systems

The paper presents a framework of Markov/CCMT search engine platform for dynamic system reliability modeling and analysis. The search engine platform implemented with a versatile Markov/CCMT algorithm that supports multiple search modes including forward-tracing, backtracking and bidirectional search. The scalability and practicality of the proposed bidirectional search algorithm is demonstrated with a Boiling Water Reactor feedwater control system. The comparisons of multiway search analysis indicate that the complete and optimal solutions can be assured in the implementation of Markov/CCMT algorithm using breadth first search. The bidirectional search algorithm implemented with forward-backward integration by allowing the use of pruning criterion is well supported for search efficiency improvement.

A Deductive Method for Diagnostic Analysis of Digital Instrumentation and Control Systems

Reliability and safety assurance are of supreme importance in the implementation of digital safety-critical control systems. A deductive method integrated with simulation-based fault injection and testing is presented for out-of-range permanent software fault localization. For fault modeling, an input–output mapping scheme is proposed to characterize the behavior of software modules and represent failure modes in an analogous manner to hardware state definitions. The Markov/cell-to-cell-mapping scheme is used for diagnostics. The diagnostic process is illustrated by several case studies for a boiling water reactor feedwater control system. The case study results show that the diagnostic algorithm is capable of software fault localization in the presence of both single and multiple faults.

Performance analysis of the passive safety features of iPOWER under Fukushima-like accident conditions

Abstract After the Fukushima Daiichi accident, there has been an increasing preference for passive safety features in the nuclear power industry. Some passive safety systems require limited active components to trigger subsequent passive operation. Under very serious accident conditions, passive safety features could be rendered inoperable or damaged. This study evaluates (i) the performance and effectiveness of the passive safety features of iPOWER (innovative Power Reactor), and (ii) whether a severe accident condition could be reached if the passive safety systems are damaged, namely the case of heat exchanger tube rupture. Analysis results show that the reactor coolant system remains in the hot shutdown condition without operator actions or electricity for over 72 h when the passive auxiliary feedwater systems (PAFSs) are operable without damage. However, heat exchanger tube rupture in the PAFS leads to core damage after about 18 h. Such results demonstrate that, to enhance the safety of iPOWER, maintaining the integrity of the PAFS is critical, and therefore additional protections for PAFS are necessary. To improve the reliability of iPOWER, additional battery sets are necessary for the passive safety systems using limited active components for accident mitigation under such extreme circumstances.

A comparison of fault trees and the Dynamic Flowgraph Methodology for the analysis of FPGA-based safety systems part 2: Theoretical investigations

The use of Field Programmable Gate Arrays (FPGAs) in safety-critical systems means that these systems must undergo a detailed reliability and safety analysis. Fault Tree Analysis (FTA) is a well-known method of reliability analysis, while the Dynamic Flowgraph Methodology (DFM), is a modern analysis method that includes time-dependent dynamic properties and was created to model and analyze digital control systems. This paper expands on previous work to examine the fundamental theoretical differences between common FTA methods such as: MOCUS, Binary Decision Diagrams (BDDs), and the “Method of Generalized Consensus” employed by DFM for Multiple-Valued Logic (MVL) systems. This was accomplished using a simplified feed water system. It was found that common FTA methods will not apply the necessary logical reduction operations to reduce MVL systems, resulting in many implicants being returned, and several Prime Implicants (PIs) being missed. Dynamic tests were performed showing that FTA could not explicitly include sink states and dynamic consistency rules in the model, as DFM does. Lastly, the original test system was modified and run for multiple time steps. Differences in dynamic top event probabilities, PIs, and the Fussel–Vesely importance measure are discussed, as are the potential advantages of DFM regarding FPGA-based systems.

Design and CFD performance analysis of a novel impeller for double suction centrifugal pumps

Abstract Double suction centrifugal pumps are the main devices involved in the feedwater system of nuclear power plants and they are responsible for a significant share of their energy consumption (by affecting the balance of both the gross and net electrical energy production), hence even an efficiency increase of only a few percentage points could be substantial in their economy. Herein a novel impeller designed for low-medium specific speed double suction centrifugal pumps ( n q 60 ) is proposed showing an efficiency improvement with respect to conventional designs in the order of 1–2% associated to a slip factor increase, secondary losses reduction and impeller outflow homogeneity improvement. The novel double suction impeller is characterized by a new arrangement of its flow channels, which come up alternately on the same circumferential exit even if they start from the two different sides. The flow field through the impeller is investigated via numerical simulations run by means of the open source CFD code OpenFOAM and its performance is compared against experimental results. The CFD model set-up, in terms of grid size and discretization schemes, has been previously assessed against results of consolidated CFX simulations on a conventional centrifugal pump.

Remaining useful life estimation for repairable multi-state components subjected to multiple maintenance actions

This paper discusses the methodologies for determining the reliability and remaining useful life (RUL) of repairable Multi-State Components (MSCs) subjected to different maintenance actions. By utilizing the degradation rates of the components that depend on the failure and maintenance rates, the transition intensities at the performance states and the Universal Generating Function (UGF), the availability and Mean Time To failure (MTTF) was obtained. The technique developed in this study is used to determine the expected RUL of a Feed Water System (FWS) of a power generating plant that uses three maintenance policies that include – no, minor and major maintenance actions for the components integrity management. The study also shows the influence of repeated maintenance actions on the RUL of the components and the impact on reliability at the lifecycle durations of the components and systems.

Thermal-hydraulic study of air-cooled passive decay heat removal system for APR+ under extended station blackout

The air-cooled passive decay heat removal system (APDHR) was proposed to provide the ultimate heat sink for non-LOCA accidents. The APDHR is a modified one of Passive Auxiliary Feed-water system (PAFS) installed in APR+. The PAFS has a heat exchanger in the Passive Condensate Cooling Tank (PCCT) and can remove decay heat for 8 h. After that, the heat transfer rate through the PAFS drastically decreases because the heat transfer condition changes from water to air. The APDHR with a vertical heat exchanger in PCCT will be able to remove the decay heat by air if it has sufficient natural convection in PCCT. We conducted the thermal-hydraulic simulation by the MARS code to investigate the behavior of the APR + selected as a reference plant for the simulation. The simulation contains two phases based on water depletion: the early phase and the late phase. In the early phase, the volume of water in PCCT was determined to avoid the water depletion in three days after shutdown. In the late phase, when the number of the HXs is greater than 4089 per PCCT, the MARS simulation confirmed the long-term cooling by air is possible under extended Station Blackout (SBO).

Markov/CCMT Dynamic Reliability Analysis of the Main and Startup Feedwater Control System in Nuclear Power Plant

The dynamic reliability of the main and startup feedwater control system in nuclear power plant is evaluated by conducting the Markov cell-to-cell mapping technology (Markov/CCMT) methodology. All the equipment failure modes and potential failure states within the system are analyzed. This process illustrates the uncertainty in the physical process of the system. Furthermore, the failure probability and cut-set of the system can be computed to provide a more comprehensive and accurate response to the system characteristics and reflect the two types of interaction within the system. In contrast to the traditional static probability safety assessment, the Markov/CCMT methodology remedies the defect in terms of event sequence setting, control loop, multiple top event competition, uncertainty of the analysis result, as well as the insufficient analysis of human-caused failure. The reliability analysis of the main and startup feedwater control system (FWCS) based on the self-developed Markov/CCMT reliability analysis software verifies the feasibility and engineering application value of the methodology and software.

소형화된 피동응축용 냉각탱크 내 온도분포와 비등현상에 관한 실험적 연구

An experimental study was conducted to investigate the water temperature distribution and saturated boiling phenomenon in a small-scale passive condensate cooling tank that is part of an emergency passive auxiliary feedwater system used in nuclear power plants. During the heating process using an electric heater inserted in the tank, the temperature of water was measured at 36 locations. Isothermal line contours were generated based on the experimental data. The contours indicated that the water above the heater location had a nearly uniform temperature due to free convection flow. Conversely, a strong temperature gradient was observed in the water below the heater location. The bubble departure diameters at the cylindrical heater surface were measured using images captured by a high-speed camera. The diameters were observed to be within the range of values predicted using existing correlations.

Mitigating the aftermath of Malaysia's worst flood in 50 years: emergency drinking water supply with ultrafiltration

Abstract In December 2014, a massive flood hit northern states of Peninsular Malaysia causing severe damage to thousands of houses and leaving the area in a disastrous condition. Electricity and public piped water supplies were cut-off, as sub-stations and municipal water treatment plants were submerged in flood water. As part of Techkem Water's corporate social responsibility initiatives, mobile ultrafiltration membrane water treatment systems were deployed to provide emergency drinking water supplies to flood victims. Four types of mobile system were evaluated in this case study. The operational parameters of the systems such as feed water turbidity, system capacity, filtrate quality, filtration flux, footprint required and specific energy requirement details are presented. Each system has distinctive features that help to accommodate different site conditions.

SGTR analysis on CPR1000 with a passive safety system

As the development of the nuclear industry, passive technology turns out to be a remarkable characteristic in the design of advanced nuclear power plants. Since the 20th century, much effort has been given to the passive technology, and a number of evolutionary passive systems have been developed. The CPR1000 plant, which is one kind of mature pressurized water plants in China, is proposed to be improved with some passive systems to enhance safety. The proposed passive systems include: (1) the RMT (reactor makeup tank); (2) the ACC (accumulator); (3) the IRWST (in-containment refueling water storage tank); (4) the PEFS (passive emergency feed water system), which is installed on the secondary side of SGs; (5) the PDS (passive depressurization system). In modifying the passive safety system, it is found that the actuation conditions of the passive safety system and the accident assumptions bring great effects on the accidental mitigation. It is necessary to study the influence induced by them. In this paper, the transient phenomenon of SGTR is analyzed to show the process in the modification of the actuation conditions and the assumptions. Finally, the most proper assumptions and actuation conditions are determined. Through the sensitivity analysis, it is found that the RMTs’ and PEFSs’ behaviors bring a great impact on the accident transient. The strong mutual effect between each passive safety component in the SGTR is the reason of choosing this accident to analyze in this paper.

Regular Environmental Investigation for Legionella and Countermeasures to Prevent Contamination by Legionella pneumophila Serogroup 5 in the Feed-water System of Our Hospital

Regular Environmental Investigation for Legionella and Countermeasures to Prevent Contamination by <i>Legionella pneumophila</i> Serogroup 5 in the Feed-water System of Our Hospital