1,721,008 research outputs found
Multiscale thermal-hydraulic analysis of experimental facilities in support of the lead-cooled fast reactors development
Full text linkNuclear energy is a key pillar for reducing global carbon emissions and transitioning toward a sustainable energy production system. Within the framework of advanced nuclear reactors, Lead-cooled Fast Reactors (LFRs) represent one of the most promising technologies among those identified by the Generation-IV International Forum (GIF), offering significant potential due to their enhanced safety features, fuel efficiency, and waste reduction. LFRs aim to meet these objectives through to the use of Heavy Liquid Metals (HLMs) as coolants and the Mixed OXide (MOX) fuel, utilizing plutonium from reprocessed of previous generations reactors’ spent fuel. This approach reduces the proliferation risk and contributes to closing the fuel cycle, making this type of reactors even more sustainable than current by minimizing the amount of long-lived waste. To meet all the goals set by the GIF, LFRs must also be economically viable, meaning that they should be competitive with the other existing technologies. This feature is achieved mainly due to the use of HLM coolants, which simplify the design by eliminating the need to pressurize the primary system, and significantly reducing the containment pressurization in case of Loss Of Coolant Accident (LOCA) compared to Light Water Reactors (LWRs). HLM also enables operation at higher temperatures, improving the efficiency of the thermodynamic cycle and contributing to the economics of LFRs. Moreover, the good thermophysical properties of HLMs allow for extensive use of passive safety systems, that operate without external energy input.
However, LFRs present several challenges that span multiple disciplines, such as material science and coolant chemistry, particularly regarding erosion and corrosion phenomena due to prolonged exposure of structural materials to HLMs, contaminating the coolant with metal oxides. Irradiation studies are necessary to assess the resistance of structural materials to fast neutrons fluxes and their impact on material properties. Instrumentation must also be developed since it must withstand high temperatures, high values of fast spectrum neutron fluence, and a corrosive and opaque environment. The challenge object of this thesis is the HLMs Thermal-Hydraulics (TH). Phenomena like pool mixing and thermal stratification, transitions from forced to natural convection, and fuel assembly TH must be analyzed from both experimental and numerical perspectives.
Many organizations worldwide have undertaken extensive experimental activities to address these challenges. The ENEA Brasimone research center is one of the most active institutions in this area, leveraging its experimental fleet and the know-how about HLMs developed since the early 2000s. However, the analysis of the aforementioned TH phenomena must also be addressed from the numerical and simulation point of view to conduct comprehensive safety analyses for reactor licensing. The complex phenomena occurring in LFRs and their safety systems present unique challenges for simulation and modelling, since they involve both local phenomena, such as hot spots within the fuel rods, and large-scale processes such as pool thermal-hydraulics. The multiscale nature of LFR TH necessitates the development of numerical tools capable of addressing both local-level phenomena through Computational Fluid Dynamics (CFD) codes, and at system-level through System Thermal-Hydraulic (STH) codes.
Both families of codes require extensive validation since CFD codes are not currently validated for nuclear applications, while certain STH codes are validated primarily for LWRs TH analysis. This thesis addresses these challenges by developing a novel multiscale simulation tool that couples the CFD code Ansys CFX with the STH code RELAP5/Mod3.3. The coupled tool enables detailed simulation of LFR behaviour by combining system-level modelling with local, high-fidelity analysis of key components, such as the core and regions where 3D effects are dominant, as in pool configurations. After a review of existing literature, the coupling methods and the gaps in current modelling approaches, previous applications of RELAP5 and CFX in HLM environments are assessed. The coupling approaches developed in this thesis – explicit and semi-implicit time-advancing schemes, and domain decomposition and overlapping discretization approaches – are described, along with a discussion of the advantages and drawbacks of each method.
The work continues with the validation of the multiscale numerical tool against experimental data from facilities relevant to the development of LFR technologies and numerical tools. The first validation activity involves the SIRIO facility, a water-cooled loop that aims at demonstrating the feasibility of an innovative passive Decay Heat Removal system (DHR). Its working principle is based on the gradual degradation of the heat transfer capability of the Isolation Condenser (IC) thanks to non-condensable gases. This mechanism should ensure the long-term cooling (at least 24 hours) without the intervention of the operator, while preventing the early freezing of the lead. In this case, only the RELAP5 code is used for validation since the main phenomena investigated are natural circulation, pool boiling, and condensation in presence of non-condensable gases, which fall outside of the typical CFD codes application. The unique characteristics of the experiment led to the necessity of the stand-alone code development activity, since these phenomena are not typically investigated in detail with STH codes. The thesis then focuses on the application of the coupled tool to simulate the transition from forced to natural circulation in Lead-Bismuth Eutectic (LBE)-cooled facilities, such as NACIE-UP and TALL-3D. They feature similar characteristics, e.g., the coolant, the operating conditions, and the type of experiment, but differ in some relevant aspects. For instance, NACIE-UP heat source is a 19 pin wire-wrapped fuel pin bundle simulator, where fuel assembly heat transfer is a relevant aspect of the experiment, while TALL-3D features two vertical hot legs, allowing for mass flow rate inversion and oscillation from one leg to another. Study of fluid dynamics and heat transfer regimes inside a pool test section, e.g., pool mixing and thermal stratification, installed in one of the vertical legs, is the major focus of the facility.
The validated coupled tool is then applied to the TH analysis of the ATHENA experimental facility. ATHENA, currently under construction, is representative of a typical LFR pool-type configuration, and it significantly differs from the previously described facilities because of its size and the HLM inventory. While NACIE-UP and TALL-3D contain less than 2 tons of LBE, ATHENA will host approximately 800 tons of lead. ATHENA enables studies related to coolant chemistry and oxygen control in a large pool environment, as well as the TH of a multi-assembly core simulator, mechanical pump and steam generator performances. Following the presentation of the numerical models and the design of the ATHENA Main Heat eXchanger (MHX), the steady state condition and two reference transients, i.e., the Loss Of Heat Sink (LOHS) and Loss Of Flow Accident (LOFA), are analyzed through the RELAP5 code. The LOFA transient is simulated also using the coupled tool because it is expected to benefit most from the contribution of the CFD code. In particular, low velocities inside the core make phenomena such as flow distribution among assemblies and heat conduction within the fluid – neglected by RELAP5 – more relevant.
Future activities for the development of this coupling tool will include testing and comparing its results with experimental data from ATHENA and CIRCE, which is an LBE pool-type facility belonging to the ENEA experimental fleet. Being two HLM-cooled Integral Effect Test (IET) facilities, they will allow further refinement and validation of the tool, enhancing its accuracy and capabilities to provide support for reactor-scale analyses
Thermal hydraulic design of DEMO Water Cooled Lithium Lead Breeding Blanket and integration with primary system and balance of plant
Full text linkThe Ph.D. work, conducted at ENEA C.R. Brasimone, is carried out in the framework of the European Power Plant and Physics Technological Programme, under the coordination of the EUROfusion Consortium, and it was co-funded by the EUROfusion Engineering Grant.
The aim of this Ph.D. thesis work is the development of the conceptual design of the Water Cooled Lithium Lead breeding blanket and its Primary Heat Transfer System, as well as their integration, demonstrating the compatibility with the DEMO requirements.
The activity is focused on the thermal-hydraulic design, the sizing and analyses of the Breeding Blanket system and of the main components of the Primary Heat Transfer System, Energy Storage System and Power Conversion System. This has been pursued through engineering approaches and the application of numerical tools, such as thermal-hydraulic system codes and CFD codes.
The conceptual design of WCLL blanket was developed starting from a review of previous designs. A preliminary layout of the coolant systems (first wall and breeding zone) and the main parameters were defined, through engineering tools, to provide input data for the development of the CAD model.
In order to verify the thermal-hydraulic performances of the WCLL blanket system, a complete three-dimensional finite volume model of the breeding blanket was set-up, using ANSYS CFXv15.0 code. The model includes solid and fluid domains, and represents, in detail, an elementary cell of the blanket (i.e. breeding unit). CFD analyses have been carried out to investigate thermal and fluid-dynamic behavior of the breeding blanket, evaluating the efficiency of the first wall and breeding zone water coolant systems, and identifying key issues and areas of improvements. Several configurations were analyzed, and it was identified a promising coolant system layout which ensures a symmetric thermal field of the breeding blanket with maximum temperature of solid structures of 430 °C.
One of the main functions of the breeding blanket is the conversion of the energy coming from the plasma in thermal energy suitable for power generation systems, ensuring an efficient power conversion. It requires large external auxiliary systems to perform its function, namely the Primary Heat Transfer System and the Power Conversion System, and it has to be integrated with them in a complete Balance of Plant, which satisfies DEMO tokomak constraints and requirements. This objective is pursued, for the WCLL breeding blanket, in the second part of the research activity, investigating the postulated operation of DEMO power plant through thermal hydraulic analyses of the Primary Heat Transfer System and Power Conversion System design solutions and components. Moreover, considering the pulsed nature of DEMO reactor, with energy generated for 120 min (burn time) followed by the reactor dwell time (estimated to last 10-30 min), an Intermediate Heat Transfer System equipped with an Energy Storage System, is being investigated to mitigate the impact of plasma pulsing on PCS equipment (e.g. in the steam turbine) and in electrical grid.
The research activity led to the definition of the Primary Heat Transfer System and of the DEMO WCLL BB. The selected configuration relies on two separate systems connected with the breeding zone and the first wall, respectively. The Energy Storage System is foreseen, to accumulate energy during pulse time, using HITEC molten salt as fluid, and to deliver power to the Power Conversion System during dwell time. The main components (e.g. steam generators, circulators/ pumps, pipes, collectors) were sized, and the data were used to develop and integrate the CAD model into the DEMO baseline. A preliminary Gate-cycleTM analyses were carried out, presenting an average gross electrical efficiency of about 37.1%, considering both pulse and dwell phases.
In order to develop a dynamic model of the systems, an extended version of RELAP5/Mod3.3 code was set-up with the implementation of the PbLi and HITEC fluid properties. This had allowed to develop a thermal-hydraulic system model of the first wall and breeding zone primary systems. The model includes the in-vessel and ex vessel components of the primary side and the secondary side of the FW PHTS and BZ PHTS, and it will be used to perform thermal-hydraulic system analyses
Natural and Gas Injection Enhanced Circulation in a Loop with Variable Friction, GENES4/ANP Int. Conf., Kyoto (Japan) Sept. 15-19, 2003
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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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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