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Multi-Physics Approach to the Modeling and Analysis of Molten Salt Reactors
No full textIn this book, a multi-physics approach to the modeling and analysis of nuclear reactor core behavior is presented, and applied to study the dynamics of Molten Salt Reactors (MSR). The Multi-Physics Modeling (MPM) approach is implemented in a unified simulation environment, which results able to catch the synergy between the different phenomena involved in the reactor core behavior, whose modeling would otherwise require either the adoption of existing simulation tools with drastic modifications of their structure and possible loss of significant information or the development of purpose-made numerical codes for the specific analyzed situation.
The book is organized into four chapters, which are conceived so as to be read independently of each other. Here, the rationale of the R&D activities performed at the Politecnico di Milano by the authors, and giving rise to the book contents, is briefly outlined.
In the last years, MSRs have been the subject of a growing and renewed interest from the scientific community in the framework of the Generation IV International Forum. Due to the availability of an advanced conceptual design, the Molten Salt Breeder Reactor (MSBR) developed at the Oak Ridge National Laboratory during the 1960s is usually considered in the literature as reference system for benchmark analyses and validation purposes. Coherently, the MSBR has been chosen as reference configuration for the analyses presented in the book. In such kind of Circulating Fuel Reactor (CFR), like the other MSRs, the coupling between neutronics and thermo-hydrodynamics is a key issue. This feature cannot be neglected in order to perform an adequate description of the reactor dynamic behavior, which shows peculiar aspects with respect to solid-fuelled conventional nuclear power plants. Relevant differences in terms of safety, fuel cycle and technology also distinguish the MSRs from the other nuclear reactors (Chapter 1).
The developed MPM methodology and the adopted models for neutronics and thermo-hydrodynamics required a deep investigation for what concerns the assessment and the extension of the simulation environment, represented in the specific case by the finite element COMSOL Multiphysics® software (chosen thanks to its flexible and modular numerical structure), but the same methodology can be applied to other multi-physics platforms. As far as thermo-hydrodynamics is concerned, a generalized approach was developed (Chapter 2) and exploited for the assessment of COMSOL simulations (Chapter 3), making use of a dedicated finite volume computational fluid dynamics code (FLUENT®), in order to better appreciate the differences in numerical approaches to turbulence. The generalized approach was built in order to carefully take into account the molten salt mixture specificities (i.e., a fuel that operates also as coolant), the reactor core power conditions and the heat transfer in graphite. In this context, a Nusselt number correlation was developed, which takes into account the effect of internal heat generation on fluid heat transfer characteristics. As far as neutronics is concerned, a module for the "reactor physics" was built in the COMSOL environment of simulation, and allowed to extend the potentialities of this computing platform. Numerical results were assessed by means of: (i) a code-to-code comparison with dedicated neutron transport codes, in the case of static fuel; (ii) a comparison with simplified neutron kinetics models, representative of the zero-power dynamics, in the case of circulating fuel.
After the assessment of the COMSOL capabilities to cope with the adopted models for neutronics and thermo-hydrodynamics, the MPM approach was applied to study the dynamic behavior of a single-channel representative of the average conditions of the MSBR core (Chapter 4). For this case study, the MPM approach resulted particularly suitable because of the strong non-linear coupling between the fuel motion and neutron dynamics, which requires a careful description of the time-space distribution of the physical quantities. Several different transients were analyzed, such as those driven by: reactivity variations due to control rod movements; fuel mass flow rate variations due to the change of the primary pump working conditions; presence of periodic perturbations, due to local precipitation of fissile solid compounds within the molten salt mixture. The analyses gave significant information on the MSBR dynamic behavior and highlighted the several advantages and potentialities offered by the proposed MPM approach (for instance, the "modularity", namely the possibility to include other physical phenomena and couplings). These potentialities are of more general interest in the prospect of studying the design configuration, the dynamics and the control strategy of next generation MSRs, as well as of other nuclear reactor types
Collapse of Nuclear Reactor SG Tubes Pressurized From Outside: The Influence of Imperfections
No full textSome innovative nuclear power plant proposals consider for the design tubes of considerable thickness subjected to external pressure (e.g., steam generators tubes). The collapse of thick tubes is expected to be dominated by yielding but, because of the decreasing nature of the postcollapse evolution, interaction with buckling is likely to be significant enough to demand consideration. At the present, few studies have been carried
out both experimentally and numerically, as witnessed by the really conservative attitude that codes assume for thick tubes. A numerical investigation has been performed in this context at the Politecnico di Milano, which was originally intended as a support for requesting a relaxation of American Society of Mechanical Engineers (ASME) regulations. Actually, in 2007, ASME code case N-759 was approved, permitting significant thickness saving in the tube design. Nevertheless, the numerical investigation was pursued to assess the influence of different parameters, such as eccentricity, initial stresses, and material hardening, on the collapse of tubes with diameter to thickness ratios D/t<20. Results are thought to be useful under at least two respects: first, providing some understanding on the collapse behavior in a thickness range so far unexplored; second, giving an indication on the assumptions on which computer codes ought to be based
when numerical analyses are required
Multiscale Modelling for the Fission Gas Behaviour in the TRANSURANUS Code
No full textA formulation is proposed for modelling the process of intra-granular diffusion of fission gas during irradiation of UO2 under both normal operating conditions and power transients. The concept represents a simple extension of the formulation of Speight, including an estimation of the contribution of bubble motion to fission gas diffusion. The resulting equation is formally identical to the diffusion equation adopted in most models that are based on the formulation of Speight, therefore retaining the advantages in terms of simplicity of the mathematical-numerical treatment and allowing application in integral fuel performance codes. The development of the new model proposed here relies on results obtained by means of molecular dynamics simulations as well as finite element computations. The formulation is proposed for incorporation in the TRANSURANUS fuel performance code
A Numerical Assessment of the Load Bearing Capacity of Thick Tubes Pressurized from Outside
No full textThe collapse behavior of cylindrical shells pressurized from outside is examined. Attention is focused on tubes of moderate thickness, as required by very deep water pipelines or some innovative nuclear power plant proposals. Their collapse is expected to be dominated by yielding but, because of the decreasing nature of the post-collapse evolution, interaction with instability is likely to be significant enough to demand consideration. At present, no quantitative assessment of such effect is available, because little study has been devoted to tubes in this thickness range.
Plasticity–instability interaction is activated by imperfections and to assess their influence on a systematic numerical study is undertaken. Computations produce a meaningful measure of the collapse pressure and it is proposed that the allowable pressure be determined on its basis, by introducing a suitable safety factor. This is chosen so that results reproduce those provided by presently accepted procedures in the well explored and reliable range of medium-thin tubes. When the same factor is applied
to thicker tubes, the resulting allowable pressure is significantly higher than the values suggested by codes, which apparently react to the present lack of knowledge by assuming an extremely conservative attitude
The Multi-Physics Modelling Approach Oriented to Safety Analysis of Innovative Nuclear Reactors
No full textIn this chapter, the Multi-Physics Modelling (MPM) approach to the study of nuclear reactor dynamics is discussed. Based on the several activities (of development and validation) carried out at the Politecnico di Milano, MPM reveals a useful tool when dealing with reactor safety and control, and can be considered as an integrative analysis implement in the design process of reactor layout. The MPM fundamental feature is represented by the capability to simultaneously solve the coupled equations, which govern the different physical phenomena occurring in a nuclear reactor in the same simulation environment. This prevents the safety analyst from implementing additional code pieces for coupling neutron kinetics and thermal-hydraulics codes. Moreover, the Coupled Code Techniques (CCT) can require significant modifications of the considered codes. On the contrary, the adoption of a MPM tool allows the analyst to minimize the programming burden and to focus on the physical aspects of the problem. An important advantage given by the employment of such MPM approach is the possibility to perform more accurate thermal-hydraulic simulation by using computational fluid dynamics schemes comprising the description of turbulent flows, which occur in the majority of nuclear reactor cores.
The approach discussed in this chapter is applied to different case studies in order to demonstrate the MPM adequacy to perform preliminary safety analysis. In particular, two innovative Generation IV systems are taken into account, namely the Molten Salt Reactor (MSR) and the Lead-cooled Fast Reactor (LFR). The MPM approach is particularly well suited to the MSR, which features a liquid fuel that also serves as coolant. For this system, two of the main accident transients to be considered in the safety analysis are the unprotected loss of flow (ULOF) and the unprotected transient overpower (UTOP). Both transients are simulated showing that the multi-physics modelling gives additional information when compared to simplified thermal-hydraulic description. As for the LFR, the ULOF transient represents a minor accident because of the weaker coupling between thermal-hydraulic conditions and neutronics, whereas the insertion of prompt reactivity must be considered as a main accident scenario in the safety analysis.
In this chapter, the MPM theoretical basis is first summarized, and compared with the classical CCT approach. Then, a brief system description along with general remarks about the safety features is separately given for each studied reactor, showing the adopted multi-physics models and presenting the performed transient analyses
Transfer Function Modeling of Zero-Power Dynamics of Circulating Fuel Reactors
No full textIn this paper, the zero-power behavior of circulating fuel reactors (CFRs) has been investigated by means of a zero-dimensional neutron kinetics model that provides a simplified but useful approach to the simulation of the dynamics of this class of nuclear
reactors. Among CFRs, the most promising is the molten salt reactor (MSR), which is one of the six innovative concepts of reactor proposed by the “Generation IV International Forum” for future nuclear energy supply. One of the key features of CFRs is represented by the fission material, which is dissolved in a liquid mixture that serves both as fuel and coolant. This causes a relevant coupling between neutronics and thermo-hydrodynamics,
so that fuel velocity plays a relevant role in determining the dynamic performance of such systems. In the present study, a preliminary model has been developed that is based on
the zero-power kinetics equations (i.e., reactivity feedbacks due to temperature change are neglected), modified in order to take into account the effects of the molten salt circulation on the drift of delayed neutron precursors. The system dynamic behavior has
been analyzed using the theory of linear systems, and the transfer functions of the neutron density with respect to both reactivity and fuel velocity have been calculated. The developed
model has been assessed on the basis of the available experimental data from the molten salt reactor experiment (MSRE) provided by the Oak Ridge National Laboratory.
The results of the present work show that the developed simplified theoretical model is well descriptive of the MSRE zero-power dynamics, allowing a preliminary evaluation of the effects due to the circulation of the fuel salt on the neutronics of the system. Moreover, the model is of general validity for any kind of CFRs, and hence is applicable to study other MSR concepts in order to have some indications on the control strategy to be
adopted in the MSR development envisaged by Generation IV
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
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
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