Theses
Simulation of electronic thermal conductivity in Zr-doped uranium nitride
Martin Iskandar
Research Intership 2025 - Ecole Polytechnique, France
Abstract
Uranium mononitride (UN) is a promising nuclear fuel due to its high energy density and high thermal
conductivity. In this work, the impact of zirconium doping on the electronic thermal conductivity of UN using
ab initio calculations was investigated. Transport properties were obtained using Density Functional Theory
(DFT) combined with the Boltzmann transport theory of charge carriers, as implemented in the BoltzTraP2
code. To exploit the normalized results provided by the code, an approximation was introduced in which an
effective relaxation time for the (U1−x ,Zrx )N alloy is defined as a weighted average of the constant relaxation
times calculated for UN and ZrN. The results show that, despite the higher total thermal conductivity of
ZrN compared to UN, the introduction of Zr into the UN matrix does not lead to an increase in the electronic
contribution to thermal conductivity. On the contrary, it tends to decrease it, even at high Zr concentrations (up
to 50%). Moreover, the effects strongly depend on the spatial distribution of Zr atoms: a uniform distribution
and an aggregated distribution lead to markedly different behaviors, highlighting the importance of local disorder
and electron scattering at interfaces. Finally, the limitations of the constant relaxation time approximation and
the necessity of explicitly including electron–phonon scattering mechanisms and phonon transport are discussed.
This work represents a first step toward a detailed understanding of the thermal transport properties of (U,Zr)N
alloys and paves the way for more comprehensive studies including the phonon contribution and comparison
with experimental data.
Slow Strain Rate Testing of advanced metal alloys in liquid lead doped with tellurium
Federico Mencarelli
Master Thesis 2025
Abstract
As the development of Generation IV lead-cooled fast reactors (LFRs) progresses, ensuring the mechanical integrity of structural materials in harsh core environmentsis essential, especially under accident scenarios involving fission product release. One such concern is Liquid Metal Embrittlement (LME), a phenomenon wherea liquid metal causes a transition from ductile to brittle behavior in structural alloys. This thesis examines the susceptibility of the ferritic alumina-forming alloy Fe10Cr4Al (10-4) to LME when exposed to molten lead contaminated with tellurium (Te), a fission product of particular relevance. Slow Strain Rate Testing (SSRT) is conducted on 10-4 specimens across a range of temperatures, strain rates, and Te concentrations, using both unnotched and notched samples. For reference, 316L stainless steel is tested under similar conditions. Topographic and chemical analysis (SEM/EDS) on 10-4 specimens show that LME is most evident in notched samples, where stress concentration damages the oxide layer that forms on the steel surface, promoting liquid-solid contact. The strongest embrittlement occurs at intermediate Te concentrations (0.3 wt%), where brittle fracture features dominate. Ductility fully recovers at the highest Te content (1.2 wt%), due to the supersaturated solution that leads to the formation of Te-rich precipitates that limit wetting or reduce effective phase interaction. Moreover, LME effects are not observed in unnotched specimens at lower strain rates, suggesting that slower deformation helps preserve oxide integrity and enables the formation of protective precipitates that hinder penetration.
Modelling of nitride superlattice coatings for BWR claddings
Kamila Oppelová
Master Thesis 2024
Abstract
Since the Fukushima-Daiichi accident in 2011, mitigating the rapid reaction of zirconium alloyed cladding with steam during a loss of coolant accident at high temperatures has been a key research objective. Consequently, a new development branch dealing with so-called Accident Tolerant Fuels (ATF) has emerged. One approach within ATF is to apply a thin coating on the cladding, creating a protective layer. While chromium is poised to be used as a first-generation coating for pressurised water reactors, it proves inadequate in the more oxidising environments of boiling water reactors, where the formed oxide ends up dissolving. Based on autoclave tests, this master thesis focuses on designing chromium-niobium nitride (Cr,Nb)N using density functional theory to identify the optimal microstructure and to understand some of the underlying phenomena.Firstly, CrN and NbN are examined as separate materials in terms of bulk structures, the introduction of cationic and anionic vacancies, and cases of substitutional atoms. The main part of the thesis focuses on the microstructure of the coating, exploring whether a monolithic structure or a superlattice form is more favourable for (Cr,Nb)N. Changes in the structure depending on the number of superlattice layers were studied. Finally, to examine the oxidation effect, an oxygen atom was introduced into CrN and NbN cells.Even though it was found that, within the scope of this thesis, the most energetically favourable state for (Cr,Nb)N is to be in the form of a superlattice with 2 layers of CrN and 2 layers of NbN, there is still much to investigate.
Development of a controlled reduction process for U3O8
Matilda Dickman Ekvall
Master Thesis 2024
Abstract
This report investigates a controlled reduction process (CRP) of triuraniumoctoxide (U3O8) powder into a hyperstoichiometric uranium dioxide (UO2+x) powder that could be mixed with the fresh uranium dioxide (UO2) powder without decreasing its sinterability. Before the CRP could be performed, scrap UO2 pellets were oxidized into U3O8 powder which was then reduced. Five versions of the CRP process were performed with different parameters. The characteristics of the resulting CRP powders were investigated through X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). The powders were mixed with fresh UO2 powder, pressed and sintered to pellets. The density of the pellets was investigated, as was their surface through SEM. The results indicate that CRP powders with x below 0.25 had higher final pellet density, suggesting that there is a benefit of reducing the U3O8 powder to the fluorite crystal structure before mixing with the fresh UO2 powder. Further investigation and optimisation of the process is necessary, however its future implementation could lead to an increase in the weight fraction of recycled scrap material above the current 9 wt% maximum, among other benefits.