1,720,985 research outputs found
Improved methodology for analysis and design of Molten Salt Reactors
No full textMolten Salt Reactors is a class of advanced reactors that is typically characterised by the presence of a molten inorganic salt of a fissile material as fuel. At the moment, this class of reactors is considered one of the promising options in the long run of nuclear power.
In the context of safety assessment of the MSFR, a task on analysis of selected transients was planned by the SAMOFAR project, to which this thesis should contribute. In addition, fluids with high melting temperature bring with them challenges regarding solidification of the fluid and possible flow blockage, to which a method was implemented during this thesis to evaluate the consequences. The open core cavity with curved shape adds another issue where the core is highly turbulent and requires the use of unstructured mesh codes for routine analysis, adding uncertainty and computational burden to the analytical workflow. Using ATARI, the tool developed during this thesis, we:
1. Analyse the MSFR together with partners in support of a safety assessment task.
2. Consider the impact and how to include solidification/melting in an analytical methodology.
3. Evaluate options to manage/reduce uncertainties and the burden of routine use of high-fidelity codes.
After a careful derivation of the conservation laws and assembling the code main algorithms, ATARI undergoes verification in a limited scope using the Stefan problem, manufactured solutions and energy balances.
The nuclear cavity benchmark was performed between SAMOFAR partners showing equivalency of the codes within the scope of the benchmark. Then the participating institutions perform analysis of the MSFR for the safety assessment task, which ATARI is unable to followup with the transient analysis. The reason for failure is identified as the requirement of appropriate acceleration schemes by OpenFOAM-based multiphysics solvers, and the lack of one in ATARI.
In the lack of accurate data regarding the heat exchangers, the impact of solidification is evaluated in a few simplified cases of a pipe with square cross-section and heat exchanger. For different salt compositions and different boundary conditions, the heat exchanger is found to recover from blockage if pump maintains its momentum and some cross-flow is allowed. The recovery of cases with abrupt solidification is questionable due to the required pressure head.
The use of flow baffles is investigated as an option to structure the flow inside the reactor and reduce uncertainties originating from turbulence modelling while allowing the use of structured mesh codes for analysis. The concept is tested in a chloride based MSR, which retains its breed and burn mode while showing a quasi-1D flow appropriate for modelling with legacy codes.
We recommend that the nuclear cavity benchmark should be expanded to include a more demanding transient case, representative of MSFR transient analysis requirements. A case is made for homogenised and economical models as used in ATARI to enable the practical use of coupling methods that promote the simultaneous solution of equations. A niche for ATARI is found as a special-mission code for analysis of challenging cases with deforming structure and flow field that justifies the use of unstructured meshes and the added computational burden. Due to design uncertainties and their nature, it is recommended to consider solidification implicitly when analysing the MSFR fuel circuit.LR
Conceptual Design of a Sustainable Waste Burning Molten Salt Reactor
Get PDFFor an energy source to qualify as sustainable, it must maximize the efficiency with which it uses natural resources while minimizing the amounts of waste it produces. Currently deployed nuclear power plants however use a very limited amount of the energy contained in natural nuclear fuels such as Uranium and Thorium and produce long-lived nuclear waste that must be dealt with.
Molten Salt Reactors (MSRs) are one family of advanced reactors designs with the potential for safe and sustainable energy production due to their use of a liquid molten salt fuel. This thesis aimed at reviewing MSR designs that have been proposed in the past, investigating alternative design possibilities and proposing a concept capable of disposing of existing waste in a sustainable manner. One possibility to do so is to design a reactor to run on an isobreeding closed fuel cycle while using existing actinide waste as a start-up inventory, which is the route investigated in this thesis.
For this purpose, the EQL0D fuel cycle procedure was developed using the MATLAB environment and the Serpent 2 Monte Carlo neutron transport code to simulate the start-up, transition and equilibrium of MSRs.
The procedure was then applied to investigate the performance at equilibrium of several candidate fuel salts and moderators in an infinite lattice and in a closed Uranium-Plutonium and Thorium-Uranium fuel cycle. The results showed that only an isobreeding closed Uranium-Thorium fuel cycle can be achieved in moderated MSRs, while both isobreeding Uranium-Plutonium and Thorium-Uranium closed fuel cycles can be sustained in fast-spectrum MSRs, which additionally show a substantial reactivity margin that enable them to dispose of poorly fissile nuclides.
The transition behavior of selected fluoride- and chloride-fueled reactors was then investigated. The fluoride-fueled cores all feature a closed Thorium-Uranium fuel cycle and included two historical graphite-moderated designs (the single- and two-fluid Molten Salt Breeder Reactors) and a modern fast-spectrum design (Molten Salt Fast Reactor). The chloride-fueled cores were proposed based on the results obtained in the infinite lattice study. The results obtained indicated that the transuranic-burning capabilities of fluoride-fueled reactors are substantially limited by the low solubility of transuranic trifluorides in fluoride salt mixtures, while no such limitation was found in chloride salts. On the other hand, chloride salts necessitate substantially larger cores and inventories.
Finally, the possibility of operating MSRs on two types of open cycles without fuel processing, breed-and-burn and once-through, was investigated to alleviate concerns related to the uncertainties related to the technical feasibility of molten salt fuel processing in closed fuel cycles. The results obtained show that it is possible to operate chloride-fueled MSRs in a sustainable breed-and-burn fuel cycle and also on an open cycle with appreciable burn-ups achieved.LR
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
* Thorium Energy Conference 2013 (ThEC13) * CERN Globe of Science and Innovation, Geneva, Switzerland
No full textVariations on the Author
Get PDF“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
Get PDFWe 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
Kaksoisrakenteinen sulasuolareaktori: Suunnittelu ja sovellukset kloridisuolilla
No full textThe purpose of this thesis was to study neutronics of a two-fluid molten salt reactor. In particular, far more efficient resource utilization and achievable burnups in modified open fuel cycles were examined. Fuel in liquid form and possibility to operate with natural uranium feed would simplify the nuclear fuel cycle. A simple two-fluid reactor model was generated and studied with two different chloride salt (NaCl-UCl3 and NaCl-ThCl4-UCl3). Capability of a reactor to maintain criticality and convert its new fissile fuel were under the main interest.
Applied chloride salts moderate neutrons less than fluorides and can accommodate more actinides in a solution. Material problems, such as high corrosion, require meticulous research and refractory metal alloys or composites as reactor materials are necessary.
Fuel burnup constitutes a non-linear problem and solutions must be found by iteration between the solvers. In this study, Monte Carlo based Serpent -code was combined with EQL0D procedure. Serpent was used for the neutron flux calculations at discrete timesteps and to determine the microscopic cross sections. Burnup equations and processing of the fuel were modelled with EQL0D. In contrary to solid fuels, liquid fuel is in continuous flow and fuel burnup is rather constant within the core. This homogeneity reduces the computational time as burnup equations can be solved only once for each core region. Thermohydraulic model was not implemented, but could be considered in further studies in order to model the flux more accurately.
Disadvantages of a two-fluid reactor are its more complex structure and possibility to extract weapons-grade material from its lower burnup blanket region. Slightly higher achievable burnups and improved resource utilization may not reason a two-fluid reactor type enough. In addition, moderation of the blanket improved the performance only marginally. However, optimization of the moderator geometry and its use in smaller cores may give some gains. Unfortunately, the most widely used moderator material graphite has durability issues.
A major challenge is to develop reliable fuel processing systems. For example, noble gases release the fuel salt during operation, having a significant radioactive inventory. Nevertheless, molten salt reactor type is unique with its liquid fuel design. Experimental research projects would be important to launch after decades of inactivity. Many important phenomena can be already simulated, but operational experience is highly needed to validate the models in practice.Työn tavoitteena oli tutkia sulasuolareaktorin neutroniikkaa ja saavutettavia polttoaineen palamia avoimessa polttoainekierrossa. Luonnonvaroja aiempaa merkittävästi tehokkaammin hyödyntävä reaktorikonsepti oli tarkastelun kohteena. Uraanin rikastamisen sekä kiinteiden polttoaine-elementtien puuttuminen voisivat suoraviivaistaa nykyistä ydinpolttoainekiertoa. Kaksoisrakenteisesta reaktorista tehtiin yksinkertaistettu malli ja kahdella eri kloorisuolalla (NaCl-UCl3 ja NaCl-ThCl4-UCl3) tutkittiin reaktorin kykyä säilyttää kriittisyys ja konvertoida uutta polttoainetta.
Valitut kloorisuolat hidastavat neutroneita fluoridisuolia vähemmän ja aktinidien liukoisuudet ovat suuremmat. Materiaaliongelmat, kuten nopea korroosio, vaativat huolellista tutkimusta ja reaktoriastian materiaaleina erikoisseokset tai komposiitit ovat tarpeen.
Polttoaineen palama muodostaa epälineaarisen ongelman, jolloin ratkaisut on löydettävä iteroimalla. Työssä yhdistettiin Monte Carlo pohjainen Serpent -koodi ja nestemäisen polttoaineen palamalaskuja varten kehitetty EQL0D proseduuri. Serpentillä laskettiin neutronivuo eri ajanhetkillä ja määritettiin mikroskooppiset vaikutusalat eri isotoopeille. Itse palamayhtälöt ja polttoaineen prosessointi mallinnettiin EQL0D proseduurilla. Toisin kuin kiinteä polttoaine, nestemäinen polttoaine virtaa vapaasti ja palamaa voidaan pitää lähes vakiona reaktorin eri osissa. Tämä homogeenisyys nopeuttaa laskentaa, koska palamayhtälöä ei tarvitse ratkaista usealle eri reaktorin alueelle. Termohydrauliikan lisäys malliin voisi tarkentaa vuon laskentaa, mutta sitä ei työhön sisällytetty.
Kaksoisrakenteisen reaktorin ongelmina voidaan nähdä monimutkaisempi rakenne ja mahdollisuus erottaa asekelpoista materiaalia matalamman palaman ulkokerroksesta. Hieman suurempi palama ja luonnonvarojen tehokkaampi käyttö eivät riittävästi puolla kaksoisrakennetta. Neutronihidastimen käyttö kaksoisrakenteisen reaktorin ulommaisessa kerroksessa ei myöskään juuri nostanut palamaa. Toisaalta hidastinaineen käyttö pienemmissä reaktorisydämissä tai reaktorigeometrian optimointi voisivat tuottaa pieniä hyötyjä. Silloinkin laajimmin tutkitun hidastinaineen, grafiitin, kestävyysongelmat toisivat rajoitteita.
Merkittävä haaste on kehittää luotettava polttoaineen prosessointijärjestelmä mm. käytön aikana vapautuville korkeasti radioaktiivisille jalokaasuille. Reaktorityyppi on haasteistaan huolimatta monella tapaa erityinen ja kehitysprojekteja olisi tärkeää saada aloitettua vuosikymmenien hiljaiselon jälkeen. Tietokonemallinnuksilla saadaan arvokasta tietoa ja monia ilmiöitä pystytään jo simuloimaan. Käytännön kokemuksia tarvitaan kuitenkin lisää mallien validointiin
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
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