1,721,017 research outputs found
Stability analysis of a zero-dimensional model of PWR core using non-modal stability theory
No full textIn this work, non-modal stability theory is used to study the non-linear dynamics of the Pressurised Water Reactor (PWR), using a zero-dimensional model. Usually, stability analysis for nuclear reactors is performed using modal methods. Instead, the non-modal approach used in the present work offers a different perspective. This approach considers the so-called short-term behaviour following a disturbance instead of the asymptotic (long-term) one. In the model, point-reactor kinetics with six groups delayed neutrons is used for neutronics, whereas thermal-hydraulics is treated using a two-region model (fuel and coolant). The non-orthogonality of the dynamic matrix of the obtained system shows the possibility of transient energy growth following a disturbance, and the proposed stability analysis offers proof of this. A sensitivity analysis of some parameters is then carried out to study how their variation influences the short-term response of the system. The results of this analysis allow for a ranking of the parameters, allowing the identification of the most significant ones regarding system stability
Development and implementation of a multi-physics high fidelity model of the TRIGA mark II reactor
No full textIn this paper, we develop, verify and validate a high fidelity multi-physics model of the TRIGA Mark II reactor of the University of Pavia, based on the coupling between the Serpent Monte Carlo code for neutronics and the OpenFOAM CFD toolkit for thermal-hydraulics. We get a realistic and accurate characterization of the reactor at the steady state condition for the first criticality configurations at fresh fuel. We validate the neutronics and thermal-hydraulics results with the available experimental data for criticality parameters and fuel temperature. In addition, we carry out code-to-code comparisons with the previous state-of-the-art multi-physics model and data in literature for power profiles, temperature and velocity fields. We also observe how the model has the powerful capability to characterise the effects of the control rods movements on the local fuel temperature. Future perspectives of this work include its employment to study transient analysis and complex CFD phenomena, as the natural circulation and fluid instabilities
Numerical Modeling and Simulation of Melting Phenomena for Freeze Valve Analysis in Molten Salt Reactors
Full text linkIn recent years, molten salt reactors (MSRs) have gained new momentum thanks to their potential for innovation in the nuclear industry, and several studies on their compliance with all the expected safety features are currently underway. In terms of passive safety, a strategy currently envisaged in accidental scenarios is to drain by gravity the molten salt, which acts both as fuel and coolant, in an emergency draining tank, ensuring both a subcritical geometry and proper cooling. To activate the draining system, a freeze plug, made of the same salt used in the core, is expected to open when the temperature in the core reaches high values. Up to this point, the freeze valve is still a key concept in the molten salt fast reactor (MSFR), and special attention must be paid to its analysis, given the requirement for passive safety, especially focusing on melting and solidification phenomena related to the molten salt mixture. This work aims to contribute to the macroscale modeling of melting and solidification phenomena relevant to the analysis of the freeze valve behavior. In particular, the focus is on the identification of the numerical models that can be adopted to achieve the quantitative insights needed for the design of the freeze valve. Among the ones available in the literature, the most appropriate models were selected based on a compromise between accuracy and computational efficiency. A critical look at the models allows for a synthetic and consistent formulation of the numerical models and their implementation in the open-source software OpenFOAM. The code was subsequently verified using analytical and numerical solutions already well established in the literature. A good agreement between the results produced by the developed solver and the reference solutions was obtained. In the end, the code was applied to simple case studies related to the freeze valve system, focusing on recognizing whether the developed code can model physical phenomena that can occur in a freeze valve. The results of the simulations are encouraging and show that the code can be used to model single-region melting or solidification problems. As such, this work constitutes a starting point for further development of the code, intending to achieve better quantitative predictions for the design of a freeze valve
1D modelling and preliminary analysis of the coupled DYNASTY–eDYNASTY natural circulation loop
Full text linkIn the continuous strive to improve the safety of current-generation and next-generation nuclear power plants, natural circulation can be used to design passive safety systems to remove the decay heat during the shutdown. The Molten Salt Fast Reactor (MSFR) is a peculiar type of Gen-IV nuclear facility, where the fluid fuel is homogeneously mixed with the coolant. This design leads to natural circulation in the presence of an internally distributed heat source during the shutdown. Furthermore, to shield the environment from the highly radioactive fuel, an intermediate loop between the primary and the secondary loops, able to operate in natural circulation, is required. To analyze the natural circulation with a distributed heat source and to study the natural circulation of coupled systems and the influence of the intermediate loop on the behaviour of the primary, Politecnico di Milano designed and built the DYNASTY-eDYNASTY facility. The two facilities are coupled with a double-pipe heat exchanger, which siphons heat from DYNASTY and delivers it to the eDYNASTY loop. This work focuses on modelling the coupled DYNASTY-eDYNASTY natural circulation loops using DYMOLA2023((R)), an integrated development environment based on the Modelica Object-Oriented a-causal simulation language. The 1D Modelica approach allows for building highly reusable and flexible models easing the design effort on a complex system such as the DYNASTY-eDYNASTY case without the need to rewrite the whole model from scratch. The coupled models were developed starting from the already-validated single DYNASTY model and the double-pipe heat exchanger coupling. The models were tested during the whole development process, studying the influence of the numerical integration algorithm on the simulation behaviour. A preliminary analysis of both the adiabatic and the heat loss models analyzed the effect of the secondary natural circulation loop on the behaviour of the DYNASTY loop. The simulation results showed that the eDYNASTY loop dampens the behaviour of the primary DYNASTY loop. Furthermore, a parametric analysis of the DYNASTY and the eDYNASTY coolers highlighted the influence of the cooling configuration on the facility's behaviour. Finally, the simulation results identified the most critical aspects of the models in preparation for an experimental comparison
Preliminary validation of the 1D modeling of the DYNASTY natural circulation loop against results from water experimental campaign
No full textIn the continuous improvements of the safety of nuclear power plants, the study of natural circulation is of primary importance for developing passive safety systems and Generation-IV reactors, such as the Molten Salt Fast Reactor (MSFR). In particular, removing the decay heat during the shutdown of the MSFR involves dealing with a circulating fuel driven by natural circulation; as such, the development and validation of natural circulation models are of primary importance. To validate models and provide experimental data on natural circulation with a distributed heat source, the DYNASTY (DYnamics of NAtural circulation for molten SalT internallY heated) natural circulation loop has been built on the premises of Politecnico di Milano. This paper focuses on improving the 1D modeling of natural circulation phenomena in the DYNASTY facility and validating them against the results (also reported in this work) found in the first experimental campaign, carried out with water as the working fluid. The modeling of DYNASTY is based on a 1D approach using DYMOLA®, a modeling environment based on the MODELICA simulation language. The models used are improved versions of the ones available in the literature, which better model the experimental facility as they now include the mass flow rate meter, the heat loss between the facility and the environment, and a more realistic cooler. The improved model was then used to study the outcomes of the model simulations and verify the influence of the different numerical integration algorithms on the simulation results. The first DYNASTY experimental campaign provides in-depth testing of its behavior, with different cooling fan speeds and considering all possible heating configurations. The collected results for the heating and cooling transients are the mass flow rate and four fluid temperatures measured by thermocouples placed at the beginning and the end of the four DYNASTY legs. The experimental results were compared with the model simulations, showing the good predicting capabilities of the latter one following the tuning of the heat exchange model. In particular, the comparison showed an excellent match between the model and the experimental results for the heating transient while showing improving margins for the cooling transient
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
