Search

Considering that system analysis codes are used for the evaluation of the performance of Passive Safety Systems (PSSs), it is important to investigate the capability of the system analysis code to reliably predict the heat transfer and natural circulation flow, which are the main phenomena governing the performance of a PSS. Since MARS-KS has been widely validated for heat transfer models, this study focuses on evaluating its capability to predict the single and two-phase pressure drops and natural circulation flow. The straight pipe simulation results indicate that the pressure drop predictions are reliable within ±5 % error margin for the single-phase flow and the errors of pressure drop up to −30 % for the two-phase flow. Through single-phase natural circulation flow analysis, it is concluded that the use of the appropriate K-factor modeling based on the flow regimes is important since the natural circulation flow rate in MARS-KS is mainly affected by the form loss factor modeling. With two-phase natural circulation flow analysis, this study emphasizes the behavior of the system could change significantly depending on the two-phase wall friction and pressure loss modeling. With the analysis results, modeling considerations for the PSS performance evaluation with the system analysis codes are proposed. more...

An autonomous system of passive removal of residual heat (PRRHS) of a reactor installation with VVER designed to ensure the safety of nuclear power plants in an accident with complete long-term blackout is considered. The system provides for the removal of heat directly from the first circuit of the reactor plant (PRRHS R). In order to increase the reliability and safety of the emergency heat sink, heat exchange equipment based on closed-type evaporation and condensation devices – two-phase thermosyphons – has been used in the system. The main feature of such heat exchangers is that their thermosiphon assemblies structurally separate the primary circuit and the auxiliary circuit of the PRRHS, which is removed outside the reactor compartment, and provide safe and efficient heat removal, reduce the risk of radioactive contamination spreading beyond safety barriers. Such autonomous passive systems will provide effective heat removal directly from the primary circuit by changing the chain of successive heat transfer sites from nuclear fuel to the final absorber and excluding from it such elements, as for example steam generators, the condition and operability of which in the emergency process of heat removal have a major impact on the safety of the reactor core. The article presents a diagram of an autonomous heat sink system; also, a description of its operation is given. The main characteristics of the course of the emergency process of removal of residual heat by the autonomous thermosiphon PRRHS R obtained by computational modeling have been considered. The advantages of an autonomous thermosiphon passive system in comparison with a passive heat removal system of a reactor installation with VVER through the second circuit are analyzed. The obtained results are proposed to solve the problems of diversification of passive safety systems of evolutionary reactor plants of nuclear power plants with VVER type reactors. more...

The safety of nuclear installations is largely determined by the tightness of fuel elements cladding. As the Fukushima nuclear accident showed, the main task in case of loss of power supply is to ensure reliable removal of residual heat release from spent fuel pool (SFP) with irradiated fuel assemblies (IFAs). The paper presents the results of calculated-experimental studies and thermal-hydraulic modeling of temperature storage modes of IFAs in SFP. Experimental studies of SFP's temperature regime and calculated evaluation of residual heat removal due to the thermal conductivity of building structures surrounding the SFP were performed. To ensure the safe operation of research reactors, it's necessary to know the IFA's residual heat power (RHP) in the reactor and SFP, which is determined depending on the operating time of fuel assemblies (FAs) and the IFAs calculated holding time. The FAs operating time depends on the reactor energy output. The IFAs calculated holding time is determined by the fuel burnup, U-235 mass in the fuel, and reactor utilization factor. The IFAs fuel burnup was calculated using the MCU-PTR program. Also presented are the RHP's calculation results using some of the empirical dependencies. The concept of a passive heat removal system (PHRS) based on thermosyphon's operating principle was proposed. more...

Fukushima Daiichi accidents led to the worldwide update of the NPP safety requirements. Particular attention was paid to severe accidents both during the design of new NPP units and during the safety reassessment of existing ones. Performed safety reassessment of operating units was focused on development and implementation of technical means and strategies directed to prevention and/or mitigation of severe accidents. Technical solutions that are used in modern designs of reactor facilities involve several options for providing of heat removal from the core and containment. Such options include heat removal systems from the reactor core and from the containment of reactor facility. Most of such systems are based on passive principals of operation and do not require or require a minimum action of operating personnel. At the same time, reactor facilities that are operated in Ukraine are based on old VVER designs, which do not include passive safety systems for long-term heat removal (especially for the case of total station blackout, which was in the focus during safety reassessments, so-called “stress tests”). Thus, the necessity of such systems at Ukrainian NPPs with VVER-1000 and VVER-440 was discussed together with analysis of existing types of passive systems and their prevalence. Based on the results of performed analysis the feasibility of implementation at Ukrainian NPPs of a passive heat removal system from the core for VVER-1000 and from the confinement for the VVER-440 was identified. The possibility of implementation of such systems at Ukrainian NPPs requires a separate analysis. In addition, a general information on current state of passive systems reliability assessment was described. It should be noted that despite the reliability of passive safety systems is one of the important issues it is still poorly studied and require additional analyses. [ABSTRACT FROM AUTHOR] more...

The paper considers structural features of components in a passive heat removal system (PHRS) used for the first time in Russia in the VVER-1200 reactor. It describes the design deficiencies of the system's air gate valves and regulators detected in the process of pre-commissioning activities and making it impossible for the system to perform its design functions. The deficiencies are explained by unpredictable effects of the working fluid's thermodynamic processes on the movable parts of the equipment. The equipment deficiencies were eliminated through the installation of additional equipment retainers which respond to the system actuation and by additional debugging of components. The retainer allows keeping the air gate valves fully opened and prevents their uncontrolled closure under the action of a rapidly ascending hot air flow inside the PHRS shell. During the commissioning stage of Novovoronezh NPP II's unit 1, following an update of the equipment design, the PHRS proved to be indispensible as the means to remove the reactor core heat in conditions of an unexpected scram and has confirmed its efficiency as part of an integrated test at 75% of the reactor power. The PHRS heat exchanger total heat removing capacity was over 100 MW at the ambient air temperature of –13 °C, and the time for the PHRS to reach full capacity did not exceed 90s, which corresponds to the system's design performance. more...

GE2M-PG test facility was constructed at the JSC “SSC RF-IPPE” for assessment of efficiency of VVER steam generator and investigation of effects of initial conditions of the accident on the processes of heat exchange in the steam generator (SG) tube bundle assembly. The facility was used in two series of experiments with and without removal of the steam–gas mixture (SGM) from steam generator cold collector. Functionality of the steam generator model in condensation mode with different concentrations of non-condensable gases in steam at the SG model inlet and different flow rates of steam–gas mixture removed from cold collector was investigated during the first phase of experiment. Effects of main factors of operational modes on the efficiency of heat exchange processes in the steam generator tube bundle assembly was investigated during the second phase of experiments conducted without removal of steam–gas mixture. The obtained results can be applied for verification of computer codes used in calculation simulation of emergency processes in VVER reactor facility. more...

To get an insight into the operating characteristics of the passive residual heat removal system of molten salt reactors, a two-phase natural circulation test facility was constructed. The system consists of a boiling loop absorbing the heat from the drain tank, a condensing loop consuming the heat, and a steam drum. A steady-state experiment was carried out, in which the thimble temperature ranged from 450°C to 700°C and the system pressure was controlled at levels below 150 kPa. When reaching a steady state, the system was operated under saturated conditions. Some important parameters, including heat power, system resistance, and water level in the steam drum and water tank were investigated. The experimental results showed that the natural circulation system is feasible in removing the decay heat, even though some fluctuations may occur in the operation. The uneven temperature distribution in the water tank may be inevitable because convection occurs on the outside of the condensing tube besides boiling with decreasing the decay power. The instabilities in the natural circulation loop are sensitive to heat flux and system resistance rather than the water level in the steam drum and water tank. RELAP5 code shows reasonable results compared with experimental data. more...

The equal-height-difference passive heat removal system (EHDPHRS) utilizes the ocean environment as an infinite heat sink that can better suit ocean nuclear power plants (ONPPs). However, a structure with an equal-height-difference can easily cause reverse flow of subcooled water, leading to serious condensation-induced water hammer (CIWH). In this paper, a fully visual experimental facility with same geometric characteristics as the EHDPHRS was established with the aim of studying the CIWH phenomenon. First, two kinds of CIWH modes were observed by the results of the visual experiment, namely the intermittent CIWH and the continuous CIWH. The typical feature of the intermittent CIWH is that the single-phase natural circulation occurs intermittently. Then, the converging-diverging structure (CDS) was designed to avoid the CIWH events in the EHDPHRS. The effectiveness of CIWH prevention was evaluated employing the velocity field, which was obtained by the PIV experimental system. It was found that the CDS can effectively inhibit the further reverse flow of subcooled water and thus eliminate the triggering mechanism of CIWH events. Finally, a dimensionless number was used to balance CIWH prevention and natural circulation capacity. It was shown that when the dimensionless number is in the range of 0.67–0.80, the CDS can eliminate the CIWH events while ensuring the natural circulation capacity. • A fully visualized experimental device is built to study the CIWH phenomenon in the EHDPHRS. • Two types of CIWH modes are determined by combining visual image with thermal–hydraulic parameters. • Typical characteristics of intermittent CIWH is the intermittent occurrence of the single-phase natural circulation. • The CDS is designed to avoid the CIWH events and experimental verifications are carried out. • CDS effectively inhibits the further reverse flow of subcooled water and eliminate the CIWH formation. [ABSTRACT FROM AUTHOR] more...

The article presents the results of the experimental study of heat and mass transfer processes in an NPP steam generator during the operation of passive safety systems of new-generation VVER reactor installations. At the GE2M-PG test rig in the Leypunsky Institute for Physics and Power Engineering, two series of experiments corresponding to different stages of the accident were completed. In these experiments, the performance of VVER steam generator in the condensing mode with and without the removal of gas-vapor mixture from the 'cold' header has been studied. As a result of the first series of experiments, it was found that, for any of the parameters of the emergency process, the steam generator's power does not drop below 80% of the original value. Furthermore, we revealed that the composition and physical properties of gases in the investigated concentration range did not notably affect the processes in the steam generator. In the second series of experiments without removal of noncondensable gases, the influence of parameters of the emergency process on the efficiency of heat transfer in the steam generator operating in the condensing mode was investigated. In order to study the heat transfer processes, we studied the change of the temperature difference between the media of the first and second circuits in our experiments. We found that the value of the temperature difference depends on both the mass of noncondensable gases accumulated in the tube bundle and their accumulation rate. The accumulation rate is determined by the power of the steam generator and the concentration of gases entering the steam generator. As a result of the analysis of experimental data, we obtained the analytical dependence reflecting change in the power of the steam generator operating in the emergency condensing mode. [ABSTRACT FROM AUTHOR] more...

• The change of the internal/external environment of the system can have an effect on the performance of passive safety system. • The performance issues that can affect the system operation are investigated and selected through the research related to safety regulation of passive safety systems. • The effect of each performance issue on the system has analyzed by using conceptual problem for passive safety system. • In this paper, it is qualitatively confirmed how the performance issues of the passive system actually affect the system through the performance evaluation. The passive safety system has advantages over active safety system since it has low dependence on the operator and external power supply. However, because the passive safety system operates with natural phenomena such as gravity, density difference, pressure difference, etc., the driving force is very low compared with active safety system. Therefore, the change of the internal/external environment of the system can have an effect on the performance of system. For this reason, it is necessary to evaluate whether the system can sufficiently perform the safety function in various accident situations. Since the existing safety standards for safety system have been developed for active safety systems, it is required to develop standards for performance evaluation of passive safety systems. For this reason, in this study, performance evaluation method for passive safety system is being developed. Since the identifying major performance issues is important for developing methodology, performance issues that can affect the natural circulation flow of the system, such as friction and heat loss of pipe within the system, are investigated and selected through the research related to safety regulation of passive safety systems. The effect of each performance issue on the system has analyzed by using conceptual problem for PHRS type passive safety system. For the effect analysis, passive safety systems using single/two phase natural circulation flow are conceptually designed by referring to the concept of the existing systems. For the effect evaluation, MARS-KS, which is system analysis code developed by KAERI, is used in this study. The main conclusions on the effect of each performance issues are drawn through the analysis. In addition, after the individual effect analysis of performance issues on passive safety system, a comprehensive assessment has performed to analyze the performance of system when several issues acted simultaneously. [ABSTRACT FROM AUTHOR] more...