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1	Investigation of coarse-grid CFD approach for nuclear engineering application / Undersökning av CFD-metod med grovt nät för kärnteknisk tillämpning Casarella, Michela January 2023 (has links) In this thesis, an innovative coarse grid CFD approach is developed that aims toexploit the capabilities of sub-channel codes and CFD methods while overcoming theirlimitations. In the approach, a very coarse mesh is implemented in the CFD softwareOpenFOAM and a new wall treatment, based on the traditional concept of the wallfunction, is applied to the wall boundary conditions of the domain to take into accountthe low resolution of the grid which does not allow to effectively capture the effect of thesolid walls on the thermo-hydraulics of the flow. To investigate the performance of thenew approach, the method is implemented first in three simple test cases for whichthe sub-channel codes are the state-of-the-art thermo-hydraulic analysis since theyare single-phase flow problems in which there are no prevailing 3D flow conditions.An additional test case representing a 2x2 fuel bundle with three full-length rods andone half-length rod is investigated to verify the behavior of the new approach in caseswhere secondary flows are present. The results for the pressure fields are comparedwith the analytical pressure profiles for the four test cases that well represent the onesthat would be obtained with sub-channel code analysis, while the results for the wallshear stresses obtained in the four test cases are compared with the ones obtained witha more refined mesh in which the traditional wall function approach is implementedsince they should be the best estimation of the actual wall shear stresses at the walldomain. For the first two cases, the developed approach produces reasonable resultswith a good agreement to the analytical pressure profiles while the other two testcases show that the methodology has a limited applicability and, before proceedingwith the extension of the new approach to single-phase problems with 3D prevailingphenomena and two-phase problems, it is necessary to solve the issues that emerge forsome types of cases. / I denna avhandling utvecklas en innovativ CFD-metod med grovt rutnät som syftar till att utnyttja kapaciteten hos underkanalskoder och CFD-metoder och samtidigt övervinna deras begränsningar. I metoden implementeras ett mycket grovt nät i CFD-programvaran OpenFOAM och en ny väggbehandling, baserad på det traditionella konceptet med vägg väggfunktion, tillämpas på domänens vägggränsvillkor för att ta hänsyn till den låga upplösningen av nätet som inte tillåter att effektivt fånga effekten av de solida väggar på flödets termo-hydraulik. För att undersöka prestandan hos den nya tillvägagångssättet implementeras metoden först i tre enkla testfall för vilka subkanalskoderna är den senaste termo-hydrauliska analysen eftersom de är enfasflödesproblem där det inte finns några rådande 3D-flödesförhållanden.Ett ytterligare testfall som representerar ett 2x2 bränsleknippe med tre fullängdsstavar och en halvlång stav undersöks för att verifiera beteendet hos den nya metoden i fall där sekundära flöden förekommer. Resultaten för tryckfälten jämförs med de analytiska tryckprofilerna för de fyra testfall som väl representerar de som som skulle erhållas med kodanalys av underkanalen, medan resultaten för väggskjuvspänningarna skjuvspänningar som erhållits i de fyra testfallen jämförs med de som erhållits med ett mer förfinat nät i vilket den traditionella väggfunktionsmetoden är implementerad eftersom de bör vara den bästa uppskattningen av de faktiska väggskjuvspänningarna vid väggens domän. För de två första fallen ger den utvecklade metoden rimliga resultat med en god överensstämmelse med de analytiska tryckprofilerna medan de andra två visar att metoden har en begränsad tillämplighet och, innan man går vidare med utvidgningen av den nya metoden till enfasproblem med 3D-fenomen och två fenomen och tvåfasproblem, är det nödvändigt att lösa de problem som uppstår för vissa vissa typer av fall. Thermal-hydraulics Nuclear reactor CFD Sub-channel Coarse-grid Turbulence Pressure drop Termisk-hydraulik kärnreaktor CFD underkanal grovkornigt nät turbulens tryckfall Physical Sciences Fysik
2	Modellering, simulering och analys av kärnreaktorn BWRX-300 Backlund, Erik January 2023 (has links) The demand for fossil-free energy production is rising due to electrification and increased consumption in the energy system. There are also multiple climate goals to reach, to preserve the possibilities of a sustainable future. A response to this is the BWRX-300, a natural circulation boiling water small modular reactor (SMR) concept developed by GE Hitachi Nuclear Energy. It is currently at the forefront of study for many power utility companies around the world. For decision making it is of interest to investigate the capabilities of new facilities. This Master Thesis work's aim is to study the BWRX-300 reactor's feasibility together with evaluating and optimizing its performance using the core simulation softwares Casmo5 and Simulate5. This is carried out by first verifying Simulate5's natural circulation capabilities by modifying an existing forced-circulation reactor to natural circulation, then comparing simulation results to real world data. Next a comprehensive model of the BWRX-300 reactor pressure vessel is modelled and validated. Equilibrium cores for 12- and 24-month cycle lengths are then simulated where key reactor performance metrics such as fuel economy, safety margins, axial profiles (of voids and pressure drop) and reactor characteristics are extracted. The effect of different fuel assembly designs in the BWRX-300 reactor core is investigated to find first core design optimums. Furthermore the decay heat removal system in the BWRX-300 is investigated. Lastly the results are used to evaluate the optimal operating mode given the current and future more dynamic projected state of the energy system. The results show that there are no real technical difficulties while operating the BWRX-300 reactor for 12 or 24 months. The decay heat removal system and core flow characteristics provide abundant coolant flow to maintain long term fuel integrity during both normal and abnormal operation modes. More or less routine core design optimization work is required to obtain sufficient safety margins and improve fuel economy. It is observed that the smaller reactor core requires an increase in average fuel enrichment to maintain criticality throughout the cycle, potentially creating an incentive to raise the current licensing limit. However it is deemed possible to avoid this by conducting further fuel design optimization work. Kärnkraft SMR små modulära reaktorer liten modulär reaktor kärnreaktor BWR BWRX-300 självcirkulation passiv säkerhet GE Hitachi cykellängd härddesign nukleär design Simulate5 GNF2 kärnbränsle reaktormodell Energy Engineering Energiteknik Energy Systems Energisystem
3	Validating results from the Molten Salt Reactor Experiment by use of turbulent CFD simulations : A study of a modified U-tube shell-and-tube primary heat exchanger and radiator with molten salts Akner, Malcolm January 2021 (has links) Background Nuclear reactors utilizing molten fuels rather than solid fuels show a massive advantage in energy yield, waste handling and safety features. The only successful reactor utilizing a molten fuel was called the ‘Molten Salt Reactor Experiment’ (MSRE), built and operated in the Oak Ridge national laboratory (ORNL) in Tennessee, U.S.A. during the 1960s. The molten salts in question are fluoride compounds under the name of “FLiBe”. In this thesis, the heat exchangers of the MSRE are modelled and simulated, with the aim to test whether current computational fluid dynamics (CFD) software and mathematical models can accurately predict molten salt heat transfer behaviour. Methods All programs used are open-source and/or free-access to facilitate open collaboration between researchers in this growing field. All models and findings produced in this thesis are free to use for future research. The program Onshape was used to draw CAD-models based on hand-drawn technical documents released by ORNL. Several programs, e.g., Simscale and Salome, were used to create high detailed meshes of the heat exchangers. The CFD software Simscale and OpenFOAM have been used to simulate the heat exchangers, using the 𝑘 − 𝜔 𝑆𝑆𝑇 Reynolds averaged Navier-Stokes (RANS) turbulence model to perform a multiregion conjugate heat transfer (CHT) analysis. The program Paraview has been used for all post-processing on the large datasets. Results A working toolchain with open-source programs for CFD has been identified. Highly detailed, full-scale and accurate CAD-drawings of the two heat exchangers have been produced. Models have been finely meshed, containing tens of millions of cells, with good quality measures. The simulations produced physically sound and valuable data: Great heat transfer predictive capability with high accuracy to the data presented by ORNL. Pressure data showed a consistent over-prediction with a factor of ~2. Possibility of error within the MSRE measurement. Conclusions CHT using modern turbulence methods work well for the intended purpose and can be used by industry to simulate molten salt heat transfer. Open-source programs perform well and can be used by researchers to share ideas and progress. Doubts around certain measurements from the MSRE, showing large uncertainties. Future projects have been outlined to continue the work performed in this thesis. Molten salt reactors show fantastic promise as an energy generation method and should be seriously considered for the future of clean, reliable energy. CFD MSRE CAD CHT experiment molten liquid salt fluid mechanics mechanics fluid LTU nuclear reactor nuclear reactor generation four generation IV copenhagen atomics copenhagen atomics space engineering space engineering aerospace thesis master's thesis master's Malcolm Akner Erik CFD MSRE CAD CHT flytande salt strömningslära strömningsmekanik fluid fluidmekanik LTU kärnreaktor generation fyra generation IV copenhagen atomics rymdteknik flygteknik ingenjör civilingenjör examensarbete Malcolm Akner Erik Aerospace Engineering Rymd- och flygteknik

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