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[en] Highlights: •Supercritical flow instability in parallel channels is investigated. •Flow dynamics and heat transfer characteristics are analyzed. •Mass flow rate, pressure, heating power, and axial power shape have significant effects on flow instability. •Numerical results are validated with experimental results. -- Abstract: SCWR is one of the selected Gen IV reactors purposely for electricity generation in the near future. It is a promising technology with higher efficiency compared to current LWRs but without the challenges of heat transfer and its associated flow instability. Supercritical flow instability is mainly caused by sharp change in the coolant properties around the pseudo-critical point of the working fluid and research into this phenomenon is needed to address concerns of flow instability at supercritical pressures. Flow instability in parallel channels at supercritical pressures is investigated in this paper using a three dimensional (3D) numerical tool (STAR-CCM+). The dynamics characteristics such as amplitude and period of out-of-phase inlet mass flow oscillation at the heated channel inlet, and heat transfer characteristic such as maximum outlet temperature of the heated channel outlet temperature oscillation are discussed. Influences of system parameters such as axial power shape, pressure, mass flow rate, and gravity are discussed based on the obtained mass flow and temperature oscillations. The results show that the system parameters have significant effect on the amplitude of the mass flow oscillation and maximum temperature of the heated outlet temperature oscillation but have little effect on the period of the mass flow oscillation. The amplitude of mass flow oscillation and maximum temperature of the heated channel outlet temperature oscillation increase with heating power. The numerical results when compared to experiment data show that the 3D numerical tool (STAR-CCM+) could capture dynamics and heat transfer characteristics of the flow quite well and also predict flow instability in the parallel channels.
[en] Highlights: • Supercritical flow instability in parallel channels is investigated. • Flow dynamics characteristics and instability boundaries are analyzed. • Mass flow rate, pressure, and gravity have significant effects on flow instability. • Axially deceased power shape is adopted for the numerical study. • Numerical results are validated with experimental results. - Abstract: Realizing the economic viability of SCWR, a GEN IV reactor, several research activities have been carried to address challenges associated with a system operated at supercritical pressures as a result of drastic changes in fluid properties at these supercritical pressures. These challenges include enhanced heat transfer EHT, deteriorated heat transfer HTD and flow instability among many others. The research activities mostly focused on CFD and experimental studies involving single tube due to the complexity of parallel channel flow and other non-circular flow geometries. Research in parallel channels is needed to address related supercritical heat transfer challenges and to provide more realistic information to the SCWR core design. This study investigated flow instability in parallel channels with water at supercritical pressures adopting axially decreased power shape ADPS. The effects of pressure, mass flow rate, and gravity on flow instability were investigated. Sensitivity analysis of some selected turbulence models and time steps were first carried out with the aim of selecting suitable turbulence model and time step for the numerical simulations. For the system operated at system pressure of 23 MPa, inlet temperatures from 180 °C to 360 °C, and system mass flow rates of 125 kg/h and 145 kg/h, the system stability decreases with inlet temperature at the high mass flow rate with only lower threshold as instability boundary, but there is a threshold power corresponding to a particular inlet temperature below which stability decreases and above which stability increases with inlet temperature for the low mass flow rate. The system stability increases with increase of system mass flow rate at low inlet temperatures, but decreases with increase of system mass flow rate at high inlet temperatures. With the increase of system pressure at 125 kg/h to 25 MPa, there is different threshold power with particular inlet temperature below which stability decreases and above which stability increases with inlet temperature. The system operated at high pressure is more stable than that operated at low pressure. The effect of stability of a system operated with or without gravity influence is similar to that of the system operated at low pressure or at high pressure respectively. The system operated without gravity influence is more stable than that operated with gravity influence. For the system operated at system pressure of 23 MPa, inlet temperatures from 180 °C to 260 °C, and system mass flow rates of 125 kg/h and 145 kg/h, the trends of the numerical results are in agreement with the trends of the experimental results. The obtained numerical instability boundary finding that the system is more stable at larger mass flow rate is the opposite of the corresponding experimental instability boundary finding. The numerical dynamics characteristics finding that the system is more stable at low mass flow rate contradicts the corresponding experimental dynamics characteristics finding that mass flow rate has less effect on flow instability. The numerical tool predicted quite close to the experimental results at larger mass flow rate. The numerical tool adopted largely under-predicted experimental amplitude and quite well predicted experimental period of the inlet mass flow oscillations. The adopted 3D numerical tool, STAR-CCM+ code could capture dynamics characteristics of the flow quite well and also predict flow instability in the parallel channels. However, there is evidence that the presence of heating structures in the geometrical model adopted may change the predicted behavior, as shown in previous works. More relevant experiments at supercritical pressures should be carried out for validation of numerical tools adopted for similar studies.
[en] Ghana is preparing to add nuclear power to its electricity generation mix in order to tackle its perennial electricity problems and stimulate rapid industrialization and growth of national economy. In addressing the workforce management issues, promotion of a strong culture for safety is one of the requirements. Effective management system must be used to foster strong leadership and culture for safety. Therefore, this presentation will discuss the Ghana Nuclear Power Programme Organisation’s (GNPPO) effort towards strengthening culture for safety. This paper will discuss the GNPPO concept of fostering a strong safety culture based on the following: 1) evaluation of the status of the safety culture in various Ghanaian organizations, 2) identification of good practices in safety culture 3) seeking expert’s advice in nuclear safety culture from IAEA experts and 4) building a strong safety culture for Ghana nuclear power programme that incorporates both local and international best practices. To achieve the above, a questionnaire is currently being piloted in association with the Ghana Institute of Safety and Environment (GhISE). The results and trends observed from the survey would be presented in the paper. Another activity being planned is the hosting of a national conference on safety culture to be held in Accra, Ghana in October 2018. The conference is being organized in collaboration with the Ghana Institution of Engineering (GhIE) and the GhISE. This conference will bring together various international (including IAEA experts) and national experts on occupational safety and safety culture to discuss the status of the safety in critical organizations in the country. (author)
[en] Highlights: • Supercritical flow instability in parallel channels is investigated. • Analysis of flow dynamics characteristics and instability boundaries. • Mass flow rate, pressure, and gravity significance on flow instability. • Type of axial power shape adopted significance on stability of the system. • Validated Numerical results with experimental results. - Abstract: Stability of the operation of SCWR at the supercritical conditions has become a major concern to the nuclear engineers worldwide, especially around the pseudo-critical point where dramatic changes of the fluid properties are experienced. This concern of addressing supercritical flow instability is due to various studies showing that the efficiency of light water reactors can be improved considerably from 33% at subcritical pressures to 45% at supercritical pressures. This work investigates effect of axial power distribution on flow instability in parallel channels with water at supercritical pressures. The axial power distributions considered include axially decreased and homogeneous axial power shapes. The study also seeks to examine the performance of the 3D numerical tool STAR-CCM+ in predicting flow instability at supercritical pressures. The effects of parameters such as mass flow rate, pressure and gravity on flow instability were also discussed. Sensitivity analysis of some selected turbulence models and time steps were initially carried out with the aim of selecting suitable turbulence model and time step for the numerical simulations. For a system operated exclusively at two different pressures 23 MPa and 25 MPa with 125 kg/h mass flow rate, inlet temperatures from 180 °C to 360 °C and with gravity influence, stability of the system with axially decreased or homogeneous power shape decreases and increases respectively below and above a particular threshold power with inlet temperature. The system was found more stable with homogeneous Axial Power Shape (HAPS) than that of axially decreased power shape (ADPS). Similar observation was made for a system operated at mass flow rate of 125 kg/h, system pressure of 23 MPa, and with or without gravity influence. When the system is operated at 23 MPa pressure, 145 kg/h mass flow rate and with gravity influence, there is a threshold power below which stability decreases and above which stability increases with inlet temperature for HAPS. In this case, there is no inflection point for ADPS and the stability deceases with inlet temperature. At low inlet temperatures, the system is more stable with ADPS but at high inlet temperatures the system is more stable with HAPS for the system operated at 23 MPa pressure and 145 kg/h mass flow rate. The system with ADPS or HAPS becomes more stable with the change of the system operated with gravity influence to the system operated without gravity influence, and also with the change of system pressure from 23 MPa to 25 MPa. With the change of the system mass flow rate from 125 kg/h to 145vkg/h, the system with ADPS becomes more stable at low inlet temperatures and less stable at high inlet temperatures, whereas the system with HAPS becomes less stable for the various inlet temperatures. For a system operated under experimental conditions of 125 kg/h mass flow rate, 23 MPa pressure, inlet temperatures from 180 °C to 260 °C and with gravity influence, the trends of the numerical instability boundary results agree quite well with the trends of the experimental instability boundary results for most of the inlet temperatures, but there is no inflection point for experimental HAPS as it is the case for numerical HAPS. The system with HAPS is more stable than that with ADPS, a finding also obtained for the experimental results. The dynamics characteristics such as amplitude and period were also compared with experimental results for ADPS at mass flow rate of 125 kg/h, 23 MPa, and with gravity influence. The experimental amplitude was largely under-predicted and the experimental period was quite well predicted by the numerical tool adopted. The results of this study show that the type of axial power shape adopted in supplying heat to the fluid flowing through heat transfer system has significant effect on the stability of the system. From the comparison of the numerical and experimental results, the 3D numerical tool, STAR-CCM+ code could predict flow instability in the parallel channels irrespective of the type of axial power shape adopted. However, there is an evidence that the presence of heating structures in the geometrical model adopted may change the predicted behavior, as shown in previous works. More relevant experiments at supercritical pressures should be carried out for validation of numerical tools adopted for similar studies.
[en] Highlights: ► Analysis by a system code of natural circulation stability in an experimental loop containing water at supercritical pressure. ► Investigation of the basic features of the phenomenon. ► Dimensionless parameters to be used in an idealized case are assessed by the results of a system code are proposed. ► The analyses of the idealized loop and of the real one provide information on different important effects related to stability. - Abstract: The paper presents a methodology for the analysis of the flow stability in natural circulation loops containing fluids at supercritical pressure. The work was made possible by the availability of experimental data collected on an experimental apparatus at the China Institute for Atomic Energy, showing the onset of unstable behaviour. The RELAP5 code is firstly used for predicting the observed phenomena, obtaining a qualitative prediction of instabilities with quantitative discrepancies in relation to the conditions observed for the onset of unstable behaviour. In the aim to allow for a systematic discussion of stability phenomena, dimensionless parameters previously proposed to analyse the stability of single heated channels with fluids at supercritical pressures are now adapted and applied to natural circulation loops. The proposed dimensionless numbers address purely thermal–hydraulic phenomena, disregarding the presence of heating structures. The combined analyses with the system code and an in-house code written in dimensionless form and based on the mentioned parameters allowed highlighting the importance of heat transfer to heating structures as a phenomenon contributing to stabilise natural circulation loops
[en] Highlights: • A new model for transient and linear stability analysis of natural circulation loops with supercritical fluids is presented. • A code-to-code assessment addressing the effect of the presence of heating structures is performed. • The new model is also applied in the analysis of an experimental loop. • Conclusions are drawn on different parametric effects. - Abstract: The paper describes the characteristics and the first applications of models recently developed to analyse both transient and linear stability of natural circulation loops, also accounting for the presence of heat structures. The models were conceived to be applied to loops containing fluids at supercritical pressure, though in principle they can be applied to a variety of fluids, even in usual single-phase and two-phase conditions. With respect to available system codes and already published models, the advantage of the twin programs set up in this work is to make use exactly of the same numerically discretised equations for transient and linear stability analyses, giving a clearer perspective of the obtained results, also including the effect of heat structures. For purpose of assessing the new models, a systematic comparison with the results obtained by the RELAP5 code results is performed, showing a very high level of coherence in very different conditions, including or not heat structures. The models are then applied to an experiment already addressed in previous work, better justifying the obtained conclusions