1,720,966 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
CFD - STH Code Coupling for the Thermal Hydraulic Analysis of NACIE-UP Experimental Facility
Full text linkGEN-IV Lead-cooled Fast Reactors are recognized as an economically competitive solution with intrinsic safe operation. ENEA is a member of the FALCON Consortium, which has the goal to construct the Advanced Lead-cooled Fast Reactor European Demonstrator (ALFRED) in the 30’s.
In this framework, computational tools are required to support the design and safety assessment of new facilities and reactors. Computational Fluid Dynamic (CFD) codes are able to reproduce local phenomena
(e. g., thermal stratification, fluid mixing and local distributions) by solving directly the three-dimensional Navier-Stokes equations, but at the price of high computational cost. Instead, System Thermal-Hydraulic (STH) codes solve one-dimensional equations and are more suited for system-scale analyses.
The goal of this work is to develop, validate and apply a simulation tool able to reproduce the TH behavior of Heavy Liquid Metals (HLMs) through the coupling between STH and CFD codes. The tool aims to exploit the advantages of the two families of codes and adopt a multi-scale approach for improved
simulation at component level within system analysis, with an acceptable computational time. The coupling technique is based on FORTRAN user routines implemented in Ansys CFX, i.e. the master CFD code. The STH code used in this activity is RELAP5/MOD3.3. The user routines take care of data exchange, RELAP5
execution, and error checking. The coupled simulation tool is adopted to reproduce experimental data on a forced-to-natural-circulation transition test, carried out on the NACIE-UP facility, with LBE as working fluid. Limitations of the present analysis and plans for future improvements will be discussed
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
A novel ansys CFX – RELAP5 coupling tool for the transient thermal-hydraulic analysis of liquid metal systems
No full textThermal-hydraulic analysis of three-dimensional systems and phenomena is essential for the design and safety assessment of new Generation IV reactor concepts. While System Thermal-Hydraulic (STH) codes serve as established tools for studying Light Water Reactors, their intrinsic limitations cannot allow to accurately model pool-type systems and fluids with high thermal conductivity, where three-dimensional phenomena cannot be accurately replicated. On the other hand, Computational Fluid Dynamics (CFD) codes have demonstrated significant potential in simulating components at a detailed scale and in predicting their thermal-hydraulic behaviour. To exploit the benefits of both code families, coupled tools are gaining attention as a viable solution to enhance the representation of system thermal-hydraulics, while maintaining reasonable computational times. This paper presents a novel coupling tool between Ansys CFX and RELAP5/Mod3.3 codes, developed for different numerical time advancing schemes and domain discretization approaches, and achieved through the development of in-house scripts for memory management, code synchronization, and information exchange. The tool is then applied to three different test cases with increasing modelling complexity, examining the performance and consistency of the coupling method with respect to the RELAP5/Mod3.3 standalone code. As expected, the adoption of different numerical time advancing schemes and domain discretization approaches lead to consistent results, highlighting the efficacy and limitation of each proposed solution. Furthermore, in cases where the approximations of the STH codes are valid (test cases 1 and 2), the coupled tool and the RELAP5/Mod3.3 standalone simulations give similar results. Instead, an application of the tool where RELAP5 is expected to be not reliable is proposed in test case 3, i.e., 3D flow mixing in a large volume, highlighting the potentiality of the tool and the conditions where a TH analysis can benefit from a STH/CFD coupled calculation
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
- …
