Theses

Doctoral Thesis 2024

Diffusion of volatile fission products in very heavy reactor fuel matrices.pdf (pdf 5.1 MB)

Abstract

The nuclear reactors of the future require materials that are exceptionally resistant to irradiation-induced degradation. This study presents a theoretical framework, combining density functional theory and interatomic potential methods, to predict microstructural evolution in Fe-based alloys and oxides (Al2O3) subjected to damaging irradiation. Our research employs a powerful creation-relaxation algorithm to simulate defect formation and microstructure development under intense irradiation.We present the pioneering implementation of a first-principles quantum mechanical approach for directly modelling the microstructural evolution ofmagnetic materials and ceramics under irradiation. A crucial aspect of studies involves investigating the influence of the spatial distribution of Frenkel-pairs (FPs) on the microstructural evolution in Fe. Our findings reveal that spatially localised FP distributions, replicating low-energy transfer irradiation conditions, predict a significantly more moderate microstructure development compared to uniform distributions. This highlights the importance of considering the FP distribution for an accurate prediction of the formation and growth of the dislocation segments under low-energy irradiation conditions. Furthermore, first principles calculations suggest that irradiation induced excess energy can trigger polymorphism in bcc Fe, leading to magnetic instabilities, localised structural constriction, and ultimately local phase transformations. Consequently, under extreme conditions, α-Fe undergoes local transformations into three-dimensional, non-parallel C15 Laves phase structures with highly close-packed stacking and internal short range ferromagnetism. Notably, the inclusion of antiferromagnetic chromium in bcc Fe significantly enhances the stability of C15 interstitial clusters in concentrated FeCr alloys. Beyond these structural insights, the investigation delves into the intricate interplay between atomic constituents and their profound impact on the non-linear magnetic properties of FeCr systems under irradiation. A striking correlation emerges, revealing that the chromium content directly influences the appearance of swelling, a key phenomenon following irradiation-induced damage. Increasing the chromium content mitigates irradiation induced swellingby approximately 40%, compared to pure Fe, highlighting the profound effectof alloying in Fe-based alloys. In addition, our first principles simulations of irradiation induced damagein bcc FeCrAl and hcp Al2O3 predict that while there are relatively small differences in total defect number densities among bcc Fe and its alloys, there aresignificant discrepancies in defect concentrations between these bcc structuresand hexagonal Al2O3. Notably, the surviving FP content in alumina is seven times higher than that recorded for FeCrAl alloys. Consequently, the different build-up of surviving damage in Fe alloys and alumina leads to diverse levels of swelling in the irradiated materials, with a remarkable three times higher swelling observed in alumina upon reaching a saturation state after an irradiation dose of approximately 1 displacement per atom (dpa). Furthermore, our observations of amorphous phase formation in damaged corundum alumina, as predicted in this study, corroborate that there are significant irradiation induced effects in alumina. These findings not only deepen our fundamental understanding of the responses of structural materials to irradiation, but also pave the way for advanced materials engineering with potential applications in near-future nuclear reactor components.

Diffusion of volatile fission products in very heavy reactor fuel matrices

Nils Wikström

Master Thesis 2024 (Uppsala University)

Abstract

The interplay of nuclear fuel with fission products is key for safe and efficient nuclear power operation. The diffusion of volatile fission products in very heavy reactor fuel matrices was investigated by analysing Zirconium Dioxide and Uranium Nitride, implanted with different ions. The samples were implanted using the 350kV Ion Implanter available at Uppsala University. Zirconium Dioxide was implanted with Xenon, Krypton and Iron, and Uranium Nitride was implanted with Krypton and Zirconium. The samples were then analysed using Time of Flight Elastic Recoil Detection Analysis (ToF-ERDA), Rutherford Backscattering Spectrometry (RBS), Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD). After the implantation and analysis, the samples were annealed at different times and temperatures. The annealing times were predicted by solving Fick’s second law with numerical methods and using Stopping and Range of Ions in Matter (SRIM) as an initial guess. The results show that annealing times can be predicted by solving Fick’s second law, to first order, and that ion implantation effects the stoichiometry of the samples. Future improvements could include improvement of underlying physics in the annealing predictions, and more extensive measurements performed on a wider range of samples.

Assessment of Ba-N and Sr-N Systems in nitride fuel fission product inventory

Pierluigi Faustini

Master Thesis 2024

Abstract

The primary aim of this thesis is to assess the interaction of barium and strontium with nitrogenas part of a comprehensive study on the behavior of fission products in nitride nuclear fuels.The proposed presence of the Sr3N2 and Ba3N2 nitrides in irradiated fuel is investigated throughsynthesis experiments conducted in a nitrogen environment under temperature conditions thatare representative of the end-of-life (EoL) scenarios.Despite extensive efforts, no conclusive identification of any Me3N2 phase ( Me= Ba, Sr) isachieved. However, the results obtained suggest a significantly higher likelihood of the presenceof the corresponding subnitrides Sr2N and Ba2N.Further efforts are directed toward producing Sr2N and Ba2N compacts to acquire data on theirpotential sintering processes, allowing for future characterization. The melting point for Sr2Nis determined, along with thermal diffusivity measurement in the range 100-600°C.These results indicate the need to re-assess the Ba-N and Sr-N systems in the fission productsinventory. A definitive and contemporary demonstration of the synthesis of these materialsis crucial before further modeling of nitride fuel fission products can continue to assume theMe3N2 scenario.

Experimental Investigation of Physical and Mechanical Properties of (U,Zr), (U,Th), and (U,Th,Zr) Metallic and Nitride Fuels

Kaitlyn Bullock

Master Thesis 2024

Abstract

Metallic fuels were produced through arc-melting. As-cast phases, microstructures and selected mechanical properties were investigated for UZr, U-Th, and U-Th-Zr systems. For each system, two compositions were investigated, with approximately 5 at. % and 20 at. % solute material, for a total of six alloys. As-cast alloy microstructures were assessed in the contextof their equilibrium systems and compared to relevant published works where applicable. Mechanical testing revealed increased hardness with increasingsolute concentration, compared to the reference materials. The results support the conclusion that solid solution strengthening is the primary mechanism enabling this change in each binary system. Additionally, (U,Zr)N fuel was synthesized. This work exemplified aprocess to produce fuel with a homogeneous distribution of zirconium in the fuel matrix, thus representing a simulated burn-up distribution of zirconium. Refinements can be made to further improve this process in future work. These findings will support a broader separate effects testing campaign underway by the SUNRISE centre.

Advanced PINN Integration with Multiple PINN Methods

Hanseul Kang

Master Thesis 2024

Abstract

This thesis evaluates the efficacy of Physics-Informed Neural Networks (PINNs) in simulating fluid dynamics challenges, focusing on the Burgers' equation and the lid-driven cavity problem, to develop a robust PINN framework for nuclear engineering applications such as the Sustainable Nuclear Energy Research In Sweden (SUNRISE) project. The research compares various PINN models to traditional Computational Fluid Dynamics (CFD) simulations to enhance predictive accuracy and computational efficiency for reactor design.

The study analyses and optimises diverse PINN configurations, employing automatic and numerical differentiation techniques and their integrative approaches, while investigating the incorporation of advanced artificial viscosity methods to augment model robustness and address limitations of standalone PINN methods.

Results show that enhanced PINN strategies achieve superior accuracy in solving the Burgers' equation and the lid-driven cavity problem at increased Reynolds numbers. For the Burgers' equation, one method with artificial viscosity achieved a Mean Squared Error (MSE) of 1.19⨉10⁻³. For the lid-driven cavity problem at Re 1000, another method without artificial viscosity yielded MSEs of 2.27⨉10⁻⁴, 9.54⨉10⁻⁵, and 1.81⨉10⁻⁵ for u, v, and p, respectively. These advancements highlight the potential of PINNs in nuclear engineering applications, particularly in tackling flow-accelerated corrosion and erosion in lead-cooled fast reactors within the SUNRISE project.

Uranium nitride synthesis by gas/gas reaction of UF6 and NH3

Serena Ambrosino

Master Thesis 2024

Abstract

This thesis project aims to develop an innovative technique for the production of high-purity uranium nitride (UN) through the ammonolysis of fluorides. The desired objective is to perform a controlled gas/gas reaction between uranium hexafluoride (UF6) and ammonia (NH3) at 800°C. The intermediate product thereby obtained (uranium dinitride, UN2) is subjected to further heating up to 1100°C under argon atmosphere, to ultimately produce UN. An inherent challenge faced in previous experiments was related to the dissociation of ammonia, which is alimiting factor for upscaling. Therefore, in this project a new setup is invented to address this challenge and it is proved experimentally: the idea is to achieve a coaxial laminar flow of UF6 and a carrier gas, where a central stream of the former is shielded by the latter so that the tworeacting gasses mix only in the hot point of the furnace, where the desired reaction can happen. To implement this approach, the ammonia dissociation has been studied, an apparatus for the controlled evaporation of UF6 has been designed and built, and two different injection nozzleshave been tested in different setup configurations. Eventually, the complete prototype has been tested altogether in a synthesis experiment at 800°C, and the products thus obtained have been converted into UN at 1100°C. Numerous auxiliary experiments have been performed using UF4as a reactant, as it is easier to handle and the results thus obtained can be largely extended to UF6. Lastly, a UF4 synthesis experiment has been performed, as educationally helpful to further dig into some chemistry features of this material, and a UN pellet has been sintered with Spark Plasma Sintering (SPS).

Simulation of radiation damage in uranium nitride

Ida Andersson Neretnieks, Jonas Planck

Bachelor Thesis 2024

Investigation of Activation in Novel Materials and Radiation Exposure to Personnel for the SUNRISE Lead-cooled Fast Reactor.pdf (pdf 7.4 MB)

Abstract

During the development of new small modular gen IV nuclear reactors, promising nuclear fuels such as uraniumnitride (UN) are investigated. Radiation damage and its effects on the fuel material have often been investigatedexperimentally through controlled irradiation methods. The aim of this thesis is to modify and develop existingMolecular Dynamics simulation methods for use in UN irradiation simulations and investigate its viability for suchuse. The present work primarily used the CRA to simulate irradiation damage and conducted Wigner-Seitz analysisto find defects. Pressure data for different probability values converged to approximately 150 kPa at a dose of 1 dpaand simulations indicated volumetric swelling of around 6% to 7%, suggesting that microstructural swelling due todefect accumulation could explain the experimental observations of cracking. Cluster analysis reveals that interstitialclusters increase to a peak, after which their number decreases towards a steady state, while DXA analysis returned ahandful of dislocation lines at 1d pa for both the uranium sublattice as well as for the nitrogen sublattice.

Investigation of Activation in Novel Materials and Radiation Exposure to Personnel for the SUNRISE Lead-cooled Fast Reactor

Eloi Pallarès Abril

Master Thesis 2023

Abstract

This master’s thesis conducts investigations on the activation and radiation exposure of crucial components of the SUNRISE-LFR. The primary focus centres on neutron activation in the reactor’s internal structures, reactor vessel and steam generators, with particular attention to novel materials such as FeCrAl and AFA stainless steel. Through rigorous simulations and analysis, we have identified niobium-94 as the dominant isotope responsible for the specific external gamma and x-ray dose rate in materials composed of Fe-10Cr-4Al-RE (FeCrAl), such as the internal structures and the main body of the steam generator. In the case of the reactor vessel, iron-55 has been found to determine the cooling time, while manganese-54 dominates in materials comprising Alumina-Forming-Austenitic (AFA). It is noteworthy that, despite most materials showing a specific activity greater than the clearance limit, their specific external gamma and x-ray dose rates are extremely low. Moreover, we explore the neutron and high-energy photon flux outside the reactor, revealing higher levels along the lateral reactor portion compared to the top. Specifically, the observed neutron flux reaches approximately 10e8 neutrons·cm−2·s−1 for the lateral part of the reactor and 10e6 neutrons · cm−2 · s−1 on top of the reactor. Meanwhile, no high energy photons have been detected outside the reactor. Overall, this master’s thesis provides valuable insights into reactor safety, structural component handling, and strategies for implementing radiation protection in the SUNRISE-LFR. The outcomes serve as a foundation for future research endeavours, guiding advancements in nuclear reactor design and promoting sustainable electricity generation with enhanced safety standards.

First principles investigation of the thermal conductivity of Zr, ZrC, and ZrN

Daniel Karlsson

Master Thesis 2023

Abstract

The thermal conductivity and electrical resistivity of Zr, ZrC, and ZrN were calculated using first-principles density functional theory (DFT) and the Boltzmann transport equation. The electron-phonon scattering was modeled via the self-energy relaxation time approximation (SERTA), and the phonon-phonon scattering via the analogous single-mode relaxation time approximation (SMRTA). The results obtained from Abinit's electron-phonon coupling code EPH is in good agreement with experimental reference data for Zr and ZrN. Notably, the calculated electrical resistivity of ZrC was found to be significantly lower than the available reference data, likely due to deviations from a perfect Zr/C stoichiometric ratio in the experimental samples. Additionally, it was observed that the calculated lattice thermal conductivity was overestimated at low temperatures, possibly attributed to the neglect of electron-phonon scattering that otherwise appears in metallic systems.

Experimental studies of radiation damage in uranium nitride

Maria Giamouridou

Master Thesis 2023

Abstract

The effect of proton (H+) irradiation on uranium mononitride (UN) and UN compositefuel with 10 at.% ZrN (UN10at%ZrN) was examined. Protons of 2 MeV with fluences of1E17, 1E18, 1E19 and 1E20 ions/cm2 were accelerated towards the fabricated samples in orderto investigate the evolution of the micro-structure. Stopping and Range of Ions in Matter(SRIM) calculations were performed to determine the displacements per atom associatedwith the depth of the highest damage, for each fluence.X-Ray diffraction (XRD) was used in both samples to identify the chemical composition ofeach pellet, which revealed the low presence of oxygen. Based on scanning electron microscopy(SEM), deterioration of the samples surface was observed, as the proton fluence increased.The applied stress due to the irradiation, led to the cracking of the pellets at the highestfluences. Blisters and craters appear to surround the cracked region, which might originatefrom the significant levels of hydrogen implantation within the samples.From Electron backscatter diffraction (EBSD) analysis, the grain size of the UN10at%ZrNcomposite was found to be smaller than in UN, due to the nano-particle nature of the ZrNpowder. The latter technique was also used to observe the elevated irradiated regions, whichwere further investigated by atomic force microscopy (AFM). Nano-indentation detectedirradiation hardening for both samples in the irradiated regions. Focused ion beam (FIB)milling was applied to remove lamellas from the cracked regions in both UN and compositesamples in order to be analyzed by transmission electron microscopy (TEM). The latter mightreveals the formation of dislocation loops in the irradiated areas.

Effect of Proton Irradiation on the Mechanical Properties of Fe-10Cr-4Al in Liquid Lead

Gabriela Lapinska

Master Thesis 2023

Abstract

Among the structural materials under consideration for future lead-cooled fastreactors, special attention is paid to ferritic Fe-10Cr-4Al due to its superior corrosionand erosion protective properties, as well as its insensitivity to liquid metalembrittlement in liquid lead. This thesis gives an inside look into the radiation damageproperties of the alloy and the possible embrittlement scenarios. The samples wereirradiated with 5.5 MeV protons and then tested with a slow strain rate testing rig at375oC and 450oC. The results showed that for Fe-10Cr-4Al irradiated to a peak doseof 0.14 dpa, the total elongation to failure was reduced by 3-5%, compared to theunirradiated samples. Moreover, the mechanical properties (yield strength, ultimatetensile strength, and fracture elongation) of the irradiated samples depend stronglyon temperature. The scanning electron microscopy images show no signs of liquidmetal embrittlement. However, the brittle structures at the edges of the samples couldindicate the existence of hydrogen embrittlement.

Development of Encapsulated UN-UO₂ Accident Tolerant Fuel

Diogo Ribeiro Costa

Doctoral Thesis 2023

Abstract

Accident tolerant fuels (ATFs) are designed to endure a severe accident in the reactor core longer than the standard UO₂-Zr alloy systems used in light water reactors (LWRs). Composite fuels such as UN-UO₂ are being considered as an ATF concept to address the lower oxidation resistance of the UN fuel from a safety perspective for use in LWRs, whilst improving the in-reactor behaviour of the UO₂ fuel. The main objective of this thesis is to fabricate, characterise, and evaluate an innovative ATF concept for LWRs: encapsulated UN spheres as additives for the standard UO₂ fuel. Several development steps were applied to understand the influence of the sintering parameters on the UN-UO₂ fuel microstructure, evaluate potential coating candidates to encapsulate the UN spheres by different coating methodologies, assess the oxidation resistance of the composites, and estimate the thermal behaviours of uncoated and encapsulated UN-UO₂ fuels. All composites were sintered by the spark plasma sintering method and characterised by many complementary microstructural techniques. Molybdenum and tungsten are shown, using a combination of modelling and experiments, to be good material candidates for the protective coating. It is shown that the powder coating methods form a thick, dense, and non-uniform coating layer onto spheres, while the chemical and vapour deposition methods provide thinner and more uniform layers. Finite element modelling indicates that the fuel centreline temperature may be reduced by more than 400 K when 70 wt% of encapsulated spheres are used as compared to the reference UO₂. Moreover, the severity of the degradation of the nitride phase is reduced when embedded in a UO₂ matrix and may also be reduced even more by the presence of a coating layer. These results contribute to further developments in methodologies for fabricating, characterising, and evaluating accident tolerant fuels within LWRs.

SSRT of 10-4 FeCrAl in LBE and Pb to Characterize Liquid Metal Embrittlement Effects

Daniel Stein

Master Thesis 2022

Abstract

In this work the susceptibility of Fe-10Cr-4Al steel to liquid metal embitterment (LME)in low oxygen environment was investigated. slow strain rate testing (SSRT) wereconducted on 10-4 FeCrAl steel in a stagnant lead from 340-480◦C, lead-bismutheutectic (LBE) from 140-450◦C and lead-bismuth mixture at 375◦C with increasingbismuth content from 0.1wt%-40wt%. The results showed that in the stagnant leadenvironment the FeCrAl steel showed no sign of LME with all samples being subjectedto around 25% strain before final break. In LBE the samples were affected by LMEespecially at 350-400◦C. The total elongation to failure reduced in LBE from 25%to 13.1% and a ductility trough from 190-400◦C was observed. In the lead-bismuthmixture there was a reduction in ductility at 5wt% going from 25% to 20% totalelongation, at 15wt% going from 20% to 16% total elongation and at 30wt% going from 16% to 13% total elongation.

CFD Results Used in the Design Process of the SEFACE Facility

Nathaniel Torkelson

Master Thesis 2022

Transient Analysis on SUNRISE-LFR with an Updated Version of BELLA Plant Simulator.pdf (pdf 3.0 MB)

Abstract

This project uses CFD analysis to make design choices for a facility to test flow accelerated lead corrosion erosion of steel samples. Two conceptual designs are considered and compared through mechanical and physical criteria. The first design uses steel samples on stationary plates next to rotating discs. The second design has the steel samples on the rotating disc. The first design is considered unfeasible due to high pressure gradients in the system and a high power requirement from the motor. The second design removes the issue of high pressure gradients and can decrease the motor requirements. This design is selected for further analysis and discussion of manufacturing.

Transient Analysis on SUNRISE-LFR with an Updated Version of BELLA Plant Simulator

Alessandro Persico

Master Thesis 2021

Abstract

This thesis work is about the dynamical behavior of advanced nuclear systems, in particular lead-cooled fast reactors (LFRs). As they are currently under study and development, it is extremely important to provide accurate, though flexible and straightforward, computational tools to investigate their efficiency and safety under normal and off-normal conditions. This is done with reference to a demonstrator unit currently under design phase in Sweden, SUNRISE-LFR. Its primary circuit (reactor core, hot let, cold leg and cold pool) is modeled with a zero-dimensional plant simulator named BELLA, while a new model for the steam generator is built adopting the well-established moving boundary approach. The integrated code is adopted in MATLAB/Simulink® to perform safety analysis under four reference accidental scenarios: unprotected transient overpower (UTOP), unprotected loss of flow (ULOF), unprotected loss of heat sink (ULOHS) and station blackout (a combination of ULOF and ULOHS). The results obtained are physically reasonable and explainable in terms of causal relations between input and output variables. Moreover, they allow to critically discuss the design of SUNRISE-LFR with reference to the targeted safety margins to fuel melting and cladding creep rupture. In addition, they may constitute the basis and provide the means to further improve the codes, in particular working on the secondary side and on control strategies for such innovative nuclear systems.

Uncertainty & Sensitivity Analysis of Nuclear Fuel Using Transuranus & Dakota

Udyanth Vadiya

Master Thesis 2021