The fission-fusion hybrid reactor (FFHR) constitutes a challenging playground for nuclear enginee... more The fission-fusion hybrid reactor (FFHR) constitutes a challenging playground for nuclear engineers. The CIRTEN consortium is responsible today for essentially all of the nuclear engineering research and education efforts in the Italian academia, with a spread of competences going from ITER, DEMO and, more generally, different fusion engineering applications, to fission applications like Generation IV reactors, ADS, SMR etc, including different enabling technologies and cultures, like materials and safety. In the paper a brief summary of the multi-disciplinary know-how inside CIRTEN is presented. It is argued that CIRTEN could give a non-negligible contribution to the further development of the FFHR concept, provided a suitable national and, especially, international collaboration framework could be envisaged.
This chapter summarizes some specific aspects concerning the thermal hydraulics of liquid-fueled ... more This chapter summarizes some specific aspects concerning the thermal hydraulics of liquid-fueled molten salt reactors (MSRs), including the presence of an internally heated fluid that serves both as fuel and coolant. Throughout the chapter, in order to focus the attention on some fundamentals from a physical point of view, reference is made to very simple case studies, namely, a typical graphite-moderated MSR channel and two single-phase natural circulation rectangular loops. The effects of internal heat generation on heat transfer, pressure losses, and stability of natural circulation (both in laminar and turbulent regimes) are investigated. The presented case studies also offer a useful analytical framework to assess more sophisticated numerical simulation tools, in view of their adoption for more realistic and complex 3-D analyses. Solid-fueled MSRs are not considered in this chapter as they feature different thermo-hydraulic issues, with the molten salt acting only as a coolant.
The objective of this work is to evaluate the Molten Salt Fast Reactor (MSFR) potential benefits ... more The objective of this work is to evaluate the Molten Salt Fast Reactor (MSFR) potential benefits in terms of transuranics (TRU) burning through a comparative analysis with a sodium-cooled FR. The comparison is based on TRU- and MA-burning rates, as well as on the in-core evolution of radiotoxicity and decay heat. Solubility issues limit the TRU-burning rate to 1/3 that achievable in traditional low-CR FRs (low-Conversion-Ratio Fast Reactors). The softer spectrum also determines notable radiotoxicity and decay heat of the equilibrium actinide inventory. On the other hand, the liquid fuel suggests the possibility of using a Pu-free feed composed only of Th and MA (Minor Actinides), thus maximizing the MA burning rate. This is generally not possible in traditional low-CR FRs due to safety deterioration and decay heat of reprocessed fuel. In addition, the high specific power and the lack of out-of-core cooling times foster a quick transition toward equilibrium, which improves the MSFR c...
A reference design of the fluid fuel circuit and the emergency draining system has to be defined ... more A reference design of the fluid fuel circuit and the emergency draining system has to be defined and optimized in the frame of the SAMOFAR project, together with the interaction phenomena between the reactor and the chemical plant. The present deliverable is divided in two main parts dedicated to the fuel salt circuit and the draining system. Each part comprises three sections: - a first section named ‘Feedback from EVOL and previous studies: list of identified constraints’ explains the constraints leading to some design options on the basis of past studies (typically from the EVOL project and some previous MSFR parametric studies); - a second section named ‘Proposition of initial reference design’ proposes a preliminary design, based on the constraints identified, to be used as a basis for the optimization studies foreseen in the SAMOFAR project. This design is not fixed: it will be optimized and will evolve all along the project as will this deliverable itself; - a third section named ‘Developments and studies in the frame of SAMOFAR’ in which all the partners involved in the WP1 will list the studies that are undertaken during the project. Deliverable 1.1 presents the initial reference design and operation procedures of the MSFR proposed by CNRS and the other partners of WP1 at the beginning of the project. The fuel circuit is enclosed in an internal vessel which serves as the container for the fuel salt, with 16 cooling sectors arranged circumferentially in the vessel, inserted from the top. This design results from previous physical and safety preliminary studies and aims at minimizing the fuel leakage occurrences and optimizing the use of the liquid salt both as fuel and coolant. The proposed design for the emergency draining system is based on the same idea, with a vessel containing the drained fuel salt with cooling rods to control the salt temperature on the short to long term. Such rods contain an inert salt used for its thermal inertia. The draining tank geometry has been defined in order to ensure a subcritical configuration in any situation. During the project, this description will be continuously updated by including all improvements and recommendations from the other work packages. This deliverable will thus be a living document, to follow this evolution all along the project
Volume 6B: Thermal-Hydraulics and Safety Analyses, 2018
Heat transfer phenomena involving internally heated fluid flows are of particular interest in sev... more Heat transfer phenomena involving internally heated fluid flows are of particular interest in several industrial applications, in chemical plants as in the nuclear field. This topic is relevant for the development of the Molten Salt Reactors (MSRs) since it involves the safety characteristics of the liquid molten salt fuel. In the literature, there is a lack of systematic studies on the heat transfer mechanism and correlations for flows in ducts featuring an internal heat source, apart from some analytical studies performed in Fiorina et al., “Thermal-hydraulics of internally heated molten salts and application to the MSFR”, Journal of physics, Conference series 501 (2014). In this work, the Nusselt number is computed multiplying the traditional Nu for internal flows times a corrective factor to account for the internal heat source. As a main outcome of this work, it is possible to obtain a CFD-based improved estimate of the corrective factor correlation for turbulent flow regime wi...
An analytical model for the study of a small Lead-cooled Fast Reactor (LFR) control-oriented dyna... more An analytical model for the study of a small Lead-cooled Fast Reactor (LFR) control-oriented dynamics has been developed aimed at providing a useful, very flexible and straightforward, though accurate, tool allowing relatively quick transient design-basis and stability analyses. A simplified lumped-parameter approach has been adopted to couple neutronics and thermalhydraulics: the point-kinetics approximation has been employed and an average-temperature heatexchange model has been implemented. The reactor transient responses following postulated accident initiators such as Unprotected Control Rod Withdrawal (UTOP), Loss of Heat Sink (ULOHS) and Loss of Flow (ULOF) have been studied for a MOX and a metal-fuelled core at the Beginning of Cycle (BoC) and End of Cycle (EoC) configurations. A benchmark analysis has been then performed by means of the SAS4A/SASSYS-1 Liquid Metal Reactor Code System, in which a core model based on three representative channels has been built with the purpo...
This report presents results of transient analyses performed for a pre-conceptual design of a Mol... more This report presents results of transient analyses performed for a pre-conceptual design of a Molten Salt Fast Reactor (MSFR) with the power of 3000MW (th). MSFR can employ a fuel containing initially U233 or/and TRUs. This reactors concept was proposed by CNRS Grenoble, France, within the EVOL project sponsored by the European Commission (EC) in the Seventh Framework Program (FP7). The safety analysis for MSFR reported in the following are related to different transient scenarios, which were simulated and analyzed by different partners within WP2 of EVOL project. The partners employed different computer codes, as well as different boundary conditions, simplifications and assumptions, for analyses of a number of transient scenarios for the pre-conceptual design of MSFR, thus providing coverage for each of chosen transients by at least two codes and gaining a deeper insight into the MSFR transient behavior
This deliverable is a description of MSFR transients that will be studied in detail in WP4 using ... more This deliverable is a description of MSFR transients that will be studied in detail in WP4 using multi-physics approaches, to investigate the safety of the MSFR under such conditions. Transients are divided into two classes: (i) transients related to the fuel circuit and (ii) transients related to the draining tank
In this work, the PSI FAST code system is extended for the modeling and simulation of Molten Salt... more In this work, the PSI FAST code system is extended for the modeling and simulation of Molten Salt Reactor (MSR) dynamics. The thermal-hydraulic code TRACE has been provided with a module for 1-D Delayed Neutron Precursor (DNP) balance and decay heat modeling in fluid fuel, and with built-in MSR materials. The reactor power is determined by means of a Point-Kinetics approach that makes use of power-weighted values of temperature and DNP distributions in the core. To validate the module, models for the comparison with experiments performed in the Molten Salt Reactor Experiment (MSRE) have been developed. The MSRE was a graphite moderated reactor built and operated in the sixties at Oak Ridge National Laboratory (ORNL). The neutronic characteristics of the reactor have been determined by means of the Monte Carlo code SERPENT. Two models of the MSRE plant with different detailing have been set up in order to determine the importance of the plant components. In particular, different descriptions of the external cooling loop have been tested and compared. Some significant transients have been considered for the module assessment, by comparison with available experimental data from ORNL, both in time and frequency domain.
For the assessment of the safety of the MSFR during various transient scenarios, multi-physics to... more For the assessment of the safety of the MSFR during various transient scenarios, multi-physics tools are necessary to capture the appropriate physics of the reactor system. Classical codes used in reactor physics cannot be used as they do not allow for the key features of the MSFR. The special features of the MSFR include the movement of precursors with the moving fuel, the strong coupling between the neutronics and the thermal-hydraulics due to the use of liquid fuel, the internal heat generation and the shape of the core having no fuel pins as a repeated structure. These features cause a variety of phenomena occurring during transients that are particular to the MSFR. Dedicated tools have been developed before and during this project specifically for this purpose. The present document represents a basic description of the tools used in the SAMOFAR project. The capabilities of these codes include detailed computational fluid dynamics analysis, (2D, 3D, geometrically flexible) detailed neutronics (diffusion, transport), and incorporation of complex physics such as gas bubbling, melting and solidification. Specifically the code systems of CNRS, KIT/EdF, TUD, PoliMi and PSI are described
In this work, an analytical study of thermal-hydraulic instabilities in a supercritical channel i... more In this work, an analytical study of thermal-hydraulic instabilities in a supercritical channel is presented. A particular focus was devoted to density wave oscillations (DWOs), which are the most common instability phenomenon in supercritical flows. The physics of DWOs are studied from the perspective of linear dynamics, to point out the feedback mechanisms that lead to the onset of the oscillations. Starting from this analysis, a linear simulation tool is developed to assess the effect of the operational parameters of a supercritical channel on the system stability. The results are compared with previous stability analysis to verify the accuracy of the present methodology, obtaining a good agreement with just modest computational requirements
Investigation of fuel compressibility effects on the Molten Salt Fast Reactor (MSFR) dynamics. Mo... more Investigation of fuel compressibility effects on the Molten Salt Fast Reactor (MSFR) dynamics. Modelling of the MSFR helium bubbling system. Development of a coupled neutronics and fluid dynamics model for the MSFR. Modelling of both liquid fuel and helium bubbles as compressible fluids. Effects on compressibility due to presence and distribution of helium bubbles are investigated.
This paper presents a CFD-based simulation tool aimed at analysing the dynamics of natural circul... more This paper presents a CFD-based simulation tool aimed at analysing the dynamics of natural circulation by means of the techniques provided by the linear stability theory. The tool can be adopted for a comprehensive stability analysis of free convection systems, giving information about the asymptotic behaviour, the spatial locations where perturbations have the largest impact on stability (i.e., a sensitivity analysis), and the so-called "transient energy growth", as well. The CFD-based approach overcomes the simplified hypotheses and the constitutive relations usually adopted to study the dynamic behaviour of natural circulation systems (i.e., one-dimensional approximation, correlations for pressure drops and heat transfer, …), and allows extending the analysis to complex geometries. The developed tool, that is based on a MATLAB ® -OpenFOAM ® coupling, has been assessed in a classic buoyancy-driven benchmark (i.e., the "differentially heated 2D cavity" test case), comparing the results with a classic Arnoldiiteration approach and literature results. Subsequently, the study of the linear stability of a natural circulation loop facility has been performed, considering for the first time also the 3D representation of the system. The results have turned out to be satisfactory, being able to give proper information about the stability of natural circulation systems.
In this paper, semi-analytical and numerical models developed in our previous works to study the ... more In this paper, semi-analytical and numerical models developed in our previous works to study the dynamic behaviour of natural convection are assessed against the experimental data obtained by means of the L2 Natural Circulation Loop (NCL) of DIME-Tec Labs (University of Genoa). As for the experimental campaign, reference is made to a set of nine experiments performed using water as working fluid and providing a thermal power of 2 kW. This set of data is firstly adopted for the validation of a semianalytical linear analysis tool aimed at studying the asymptotic behaviour of NCLs through the definition of dimensionless stability maps. Then, two different numerical models (adopted in our previous work to confirm the linear analysis) are assessed, namely an Object-Oriented (O-O) one-dimensional model and a three-dimensional Computational Fluid Dynamics (CFD) model. In this regard, the O-O model represents a fast tool for the evaluation of the most important quantities, such as the velocity and the temperature fields in the loop along the axial coordinate. On the other hand, the CFD tool, which is intended as a support to the 1D analysis, is characterised by a high computational burden, but allows highlighting interesting 3D spatial effects. The validation of these tools is not secondary with respect to that of the stability maps. Actually, the numerical approach is fundamental to study the time-dependent behaviour of both stable and unstable natural circulation regimes, for which the stability maps do not provide information. As for the achieved results, the developed models are able to catch the behaviour of the experimental data. In particular, this outcome is possible if an accurate modelling of both the heatexchanger section and the piping thermal inertia is considered.
ABSTRACT In this work, the development of a control-oriented dynamics simulator for a Generation ... more ABSTRACT In this work, the development of a control-oriented dynamics simulator for a Generation IV Lead-cooled Fast Reactor (LFR) demonstrator has been undertaken aimed at providing a very flexible and straightforward – though accurate – fast-running tool allowing to perform transient design-basis and stability analyses, and laying the foundations for the study of the system control strategy. The simulator, realized in the MATLAB/SIMULINK® environment, is composed of five essential parts: core, steam generator, primary pump, collectors, and coolant cold pool. An analytical lumped-parameter core model has been developed to treat the coupling between neutronics and thermal-hydraulics. For the steam generator a moving boundary approach has been adopted, allowing to get the physical behavior while satisfying the controller specifications, besides assuring coherence with the zero-dimensional core modeling. The complete primary loop model has been assembled by connecting the above-mentioned main subsystems through the hot and cold collectors, and by adding the remaining components blocks. Five different transients have been then simulated to analyze the whole system dynamic behavior in a control-oriented perspective: three scenarios have been initiated by acting on the secondary water side (i.e., enhancement of feedwater mass flow rate and temperature, and turbine admission valve coefficient variation), and two by perturbing the primary side nominal state (i.e., simulations of Unprotected Loss of Flow, ULOF, and of Unprotected Transient of OverPower, UTOP). As a major outcome, it can be stated that the free dynamics simulations results are very satisfactory, and they may constitute the basis and provide the means for conceiving suitable control strategies for the innovative small-size LFR systems currently under development.
The fission-fusion hybrid reactor (FFHR) constitutes a challenging playground for nuclear enginee... more The fission-fusion hybrid reactor (FFHR) constitutes a challenging playground for nuclear engineers. The CIRTEN consortium is responsible today for essentially all of the nuclear engineering research and education efforts in the Italian academia, with a spread of competences going from ITER, DEMO and, more generally, different fusion engineering applications, to fission applications like Generation IV reactors, ADS, SMR etc, including different enabling technologies and cultures, like materials and safety. In the paper a brief summary of the multi-disciplinary know-how inside CIRTEN is presented. It is argued that CIRTEN could give a non-negligible contribution to the further development of the FFHR concept, provided a suitable national and, especially, international collaboration framework could be envisaged.
This chapter summarizes some specific aspects concerning the thermal hydraulics of liquid-fueled ... more This chapter summarizes some specific aspects concerning the thermal hydraulics of liquid-fueled molten salt reactors (MSRs), including the presence of an internally heated fluid that serves both as fuel and coolant. Throughout the chapter, in order to focus the attention on some fundamentals from a physical point of view, reference is made to very simple case studies, namely, a typical graphite-moderated MSR channel and two single-phase natural circulation rectangular loops. The effects of internal heat generation on heat transfer, pressure losses, and stability of natural circulation (both in laminar and turbulent regimes) are investigated. The presented case studies also offer a useful analytical framework to assess more sophisticated numerical simulation tools, in view of their adoption for more realistic and complex 3-D analyses. Solid-fueled MSRs are not considered in this chapter as they feature different thermo-hydraulic issues, with the molten salt acting only as a coolant.
The objective of this work is to evaluate the Molten Salt Fast Reactor (MSFR) potential benefits ... more The objective of this work is to evaluate the Molten Salt Fast Reactor (MSFR) potential benefits in terms of transuranics (TRU) burning through a comparative analysis with a sodium-cooled FR. The comparison is based on TRU- and MA-burning rates, as well as on the in-core evolution of radiotoxicity and decay heat. Solubility issues limit the TRU-burning rate to 1/3 that achievable in traditional low-CR FRs (low-Conversion-Ratio Fast Reactors). The softer spectrum also determines notable radiotoxicity and decay heat of the equilibrium actinide inventory. On the other hand, the liquid fuel suggests the possibility of using a Pu-free feed composed only of Th and MA (Minor Actinides), thus maximizing the MA burning rate. This is generally not possible in traditional low-CR FRs due to safety deterioration and decay heat of reprocessed fuel. In addition, the high specific power and the lack of out-of-core cooling times foster a quick transition toward equilibrium, which improves the MSFR c...
A reference design of the fluid fuel circuit and the emergency draining system has to be defined ... more A reference design of the fluid fuel circuit and the emergency draining system has to be defined and optimized in the frame of the SAMOFAR project, together with the interaction phenomena between the reactor and the chemical plant. The present deliverable is divided in two main parts dedicated to the fuel salt circuit and the draining system. Each part comprises three sections: - a first section named ‘Feedback from EVOL and previous studies: list of identified constraints’ explains the constraints leading to some design options on the basis of past studies (typically from the EVOL project and some previous MSFR parametric studies); - a second section named ‘Proposition of initial reference design’ proposes a preliminary design, based on the constraints identified, to be used as a basis for the optimization studies foreseen in the SAMOFAR project. This design is not fixed: it will be optimized and will evolve all along the project as will this deliverable itself; - a third section named ‘Developments and studies in the frame of SAMOFAR’ in which all the partners involved in the WP1 will list the studies that are undertaken during the project. Deliverable 1.1 presents the initial reference design and operation procedures of the MSFR proposed by CNRS and the other partners of WP1 at the beginning of the project. The fuel circuit is enclosed in an internal vessel which serves as the container for the fuel salt, with 16 cooling sectors arranged circumferentially in the vessel, inserted from the top. This design results from previous physical and safety preliminary studies and aims at minimizing the fuel leakage occurrences and optimizing the use of the liquid salt both as fuel and coolant. The proposed design for the emergency draining system is based on the same idea, with a vessel containing the drained fuel salt with cooling rods to control the salt temperature on the short to long term. Such rods contain an inert salt used for its thermal inertia. The draining tank geometry has been defined in order to ensure a subcritical configuration in any situation. During the project, this description will be continuously updated by including all improvements and recommendations from the other work packages. This deliverable will thus be a living document, to follow this evolution all along the project
Volume 6B: Thermal-Hydraulics and Safety Analyses, 2018
Heat transfer phenomena involving internally heated fluid flows are of particular interest in sev... more Heat transfer phenomena involving internally heated fluid flows are of particular interest in several industrial applications, in chemical plants as in the nuclear field. This topic is relevant for the development of the Molten Salt Reactors (MSRs) since it involves the safety characteristics of the liquid molten salt fuel. In the literature, there is a lack of systematic studies on the heat transfer mechanism and correlations for flows in ducts featuring an internal heat source, apart from some analytical studies performed in Fiorina et al., “Thermal-hydraulics of internally heated molten salts and application to the MSFR”, Journal of physics, Conference series 501 (2014). In this work, the Nusselt number is computed multiplying the traditional Nu for internal flows times a corrective factor to account for the internal heat source. As a main outcome of this work, it is possible to obtain a CFD-based improved estimate of the corrective factor correlation for turbulent flow regime wi...
An analytical model for the study of a small Lead-cooled Fast Reactor (LFR) control-oriented dyna... more An analytical model for the study of a small Lead-cooled Fast Reactor (LFR) control-oriented dynamics has been developed aimed at providing a useful, very flexible and straightforward, though accurate, tool allowing relatively quick transient design-basis and stability analyses. A simplified lumped-parameter approach has been adopted to couple neutronics and thermalhydraulics: the point-kinetics approximation has been employed and an average-temperature heatexchange model has been implemented. The reactor transient responses following postulated accident initiators such as Unprotected Control Rod Withdrawal (UTOP), Loss of Heat Sink (ULOHS) and Loss of Flow (ULOF) have been studied for a MOX and a metal-fuelled core at the Beginning of Cycle (BoC) and End of Cycle (EoC) configurations. A benchmark analysis has been then performed by means of the SAS4A/SASSYS-1 Liquid Metal Reactor Code System, in which a core model based on three representative channels has been built with the purpo...
This report presents results of transient analyses performed for a pre-conceptual design of a Mol... more This report presents results of transient analyses performed for a pre-conceptual design of a Molten Salt Fast Reactor (MSFR) with the power of 3000MW (th). MSFR can employ a fuel containing initially U233 or/and TRUs. This reactors concept was proposed by CNRS Grenoble, France, within the EVOL project sponsored by the European Commission (EC) in the Seventh Framework Program (FP7). The safety analysis for MSFR reported in the following are related to different transient scenarios, which were simulated and analyzed by different partners within WP2 of EVOL project. The partners employed different computer codes, as well as different boundary conditions, simplifications and assumptions, for analyses of a number of transient scenarios for the pre-conceptual design of MSFR, thus providing coverage for each of chosen transients by at least two codes and gaining a deeper insight into the MSFR transient behavior
This deliverable is a description of MSFR transients that will be studied in detail in WP4 using ... more This deliverable is a description of MSFR transients that will be studied in detail in WP4 using multi-physics approaches, to investigate the safety of the MSFR under such conditions. Transients are divided into two classes: (i) transients related to the fuel circuit and (ii) transients related to the draining tank
In this work, the PSI FAST code system is extended for the modeling and simulation of Molten Salt... more In this work, the PSI FAST code system is extended for the modeling and simulation of Molten Salt Reactor (MSR) dynamics. The thermal-hydraulic code TRACE has been provided with a module for 1-D Delayed Neutron Precursor (DNP) balance and decay heat modeling in fluid fuel, and with built-in MSR materials. The reactor power is determined by means of a Point-Kinetics approach that makes use of power-weighted values of temperature and DNP distributions in the core. To validate the module, models for the comparison with experiments performed in the Molten Salt Reactor Experiment (MSRE) have been developed. The MSRE was a graphite moderated reactor built and operated in the sixties at Oak Ridge National Laboratory (ORNL). The neutronic characteristics of the reactor have been determined by means of the Monte Carlo code SERPENT. Two models of the MSRE plant with different detailing have been set up in order to determine the importance of the plant components. In particular, different descriptions of the external cooling loop have been tested and compared. Some significant transients have been considered for the module assessment, by comparison with available experimental data from ORNL, both in time and frequency domain.
For the assessment of the safety of the MSFR during various transient scenarios, multi-physics to... more For the assessment of the safety of the MSFR during various transient scenarios, multi-physics tools are necessary to capture the appropriate physics of the reactor system. Classical codes used in reactor physics cannot be used as they do not allow for the key features of the MSFR. The special features of the MSFR include the movement of precursors with the moving fuel, the strong coupling between the neutronics and the thermal-hydraulics due to the use of liquid fuel, the internal heat generation and the shape of the core having no fuel pins as a repeated structure. These features cause a variety of phenomena occurring during transients that are particular to the MSFR. Dedicated tools have been developed before and during this project specifically for this purpose. The present document represents a basic description of the tools used in the SAMOFAR project. The capabilities of these codes include detailed computational fluid dynamics analysis, (2D, 3D, geometrically flexible) detailed neutronics (diffusion, transport), and incorporation of complex physics such as gas bubbling, melting and solidification. Specifically the code systems of CNRS, KIT/EdF, TUD, PoliMi and PSI are described
In this work, an analytical study of thermal-hydraulic instabilities in a supercritical channel i... more In this work, an analytical study of thermal-hydraulic instabilities in a supercritical channel is presented. A particular focus was devoted to density wave oscillations (DWOs), which are the most common instability phenomenon in supercritical flows. The physics of DWOs are studied from the perspective of linear dynamics, to point out the feedback mechanisms that lead to the onset of the oscillations. Starting from this analysis, a linear simulation tool is developed to assess the effect of the operational parameters of a supercritical channel on the system stability. The results are compared with previous stability analysis to verify the accuracy of the present methodology, obtaining a good agreement with just modest computational requirements
Investigation of fuel compressibility effects on the Molten Salt Fast Reactor (MSFR) dynamics. Mo... more Investigation of fuel compressibility effects on the Molten Salt Fast Reactor (MSFR) dynamics. Modelling of the MSFR helium bubbling system. Development of a coupled neutronics and fluid dynamics model for the MSFR. Modelling of both liquid fuel and helium bubbles as compressible fluids. Effects on compressibility due to presence and distribution of helium bubbles are investigated.
This paper presents a CFD-based simulation tool aimed at analysing the dynamics of natural circul... more This paper presents a CFD-based simulation tool aimed at analysing the dynamics of natural circulation by means of the techniques provided by the linear stability theory. The tool can be adopted for a comprehensive stability analysis of free convection systems, giving information about the asymptotic behaviour, the spatial locations where perturbations have the largest impact on stability (i.e., a sensitivity analysis), and the so-called "transient energy growth", as well. The CFD-based approach overcomes the simplified hypotheses and the constitutive relations usually adopted to study the dynamic behaviour of natural circulation systems (i.e., one-dimensional approximation, correlations for pressure drops and heat transfer, …), and allows extending the analysis to complex geometries. The developed tool, that is based on a MATLAB ® -OpenFOAM ® coupling, has been assessed in a classic buoyancy-driven benchmark (i.e., the "differentially heated 2D cavity" test case), comparing the results with a classic Arnoldiiteration approach and literature results. Subsequently, the study of the linear stability of a natural circulation loop facility has been performed, considering for the first time also the 3D representation of the system. The results have turned out to be satisfactory, being able to give proper information about the stability of natural circulation systems.
In this paper, semi-analytical and numerical models developed in our previous works to study the ... more In this paper, semi-analytical and numerical models developed in our previous works to study the dynamic behaviour of natural convection are assessed against the experimental data obtained by means of the L2 Natural Circulation Loop (NCL) of DIME-Tec Labs (University of Genoa). As for the experimental campaign, reference is made to a set of nine experiments performed using water as working fluid and providing a thermal power of 2 kW. This set of data is firstly adopted for the validation of a semianalytical linear analysis tool aimed at studying the asymptotic behaviour of NCLs through the definition of dimensionless stability maps. Then, two different numerical models (adopted in our previous work to confirm the linear analysis) are assessed, namely an Object-Oriented (O-O) one-dimensional model and a three-dimensional Computational Fluid Dynamics (CFD) model. In this regard, the O-O model represents a fast tool for the evaluation of the most important quantities, such as the velocity and the temperature fields in the loop along the axial coordinate. On the other hand, the CFD tool, which is intended as a support to the 1D analysis, is characterised by a high computational burden, but allows highlighting interesting 3D spatial effects. The validation of these tools is not secondary with respect to that of the stability maps. Actually, the numerical approach is fundamental to study the time-dependent behaviour of both stable and unstable natural circulation regimes, for which the stability maps do not provide information. As for the achieved results, the developed models are able to catch the behaviour of the experimental data. In particular, this outcome is possible if an accurate modelling of both the heatexchanger section and the piping thermal inertia is considered.
ABSTRACT In this work, the development of a control-oriented dynamics simulator for a Generation ... more ABSTRACT In this work, the development of a control-oriented dynamics simulator for a Generation IV Lead-cooled Fast Reactor (LFR) demonstrator has been undertaken aimed at providing a very flexible and straightforward – though accurate – fast-running tool allowing to perform transient design-basis and stability analyses, and laying the foundations for the study of the system control strategy. The simulator, realized in the MATLAB/SIMULINK® environment, is composed of five essential parts: core, steam generator, primary pump, collectors, and coolant cold pool. An analytical lumped-parameter core model has been developed to treat the coupling between neutronics and thermal-hydraulics. For the steam generator a moving boundary approach has been adopted, allowing to get the physical behavior while satisfying the controller specifications, besides assuring coherence with the zero-dimensional core modeling. The complete primary loop model has been assembled by connecting the above-mentioned main subsystems through the hot and cold collectors, and by adding the remaining components blocks. Five different transients have been then simulated to analyze the whole system dynamic behavior in a control-oriented perspective: three scenarios have been initiated by acting on the secondary water side (i.e., enhancement of feedwater mass flow rate and temperature, and turbine admission valve coefficient variation), and two by perturbing the primary side nominal state (i.e., simulations of Unprotected Loss of Flow, ULOF, and of Unprotected Transient of OverPower, UTOP). As a major outcome, it can be stated that the free dynamics simulations results are very satisfactory, and they may constitute the basis and provide the means for conceiving suitable control strategies for the innovative small-size LFR systems currently under development.
Uploads
Papers by Antonio Cammi