What does it mean for a causal structure to be ‘unknown’? Can we even talk about ‘repetitions’ of an experiment without prior knowledge of causal relations? And under what conditions can we say that a set of processes with arbitrary, possibly indefinite, causal structure are independent and identically distributed? Similar questions for classical probabilities, quantum states, and quantum channels are beautifully answered by so-called “de Finetti theorems”, which connect a simple and easy-to-justify condition—symmetry under exchange—with a very particular multipartite structure: a mixture of identical states/channels. Here we extend the result to processes with arbitrary causal structure, including indefinite causal order and multi-time, non-Markovian processes applicable to noisy quantum devices. The result also implies a new class of de Finetti theorems for quantum states subject to a large class of linear constraints, which can be of independent interest.

Proton irradiation with a primary ion energy of 2 MeV was used to simulate radiation damage in UN and (U,Zr)N fuel pellets. The pellets, nominally at room temperature, were irradiated to peak levels of 0.1,1,10 dpa and 100.0 dpa resulting in a peak hydrogen concentration of at most 90 at. %. Microstructure and mechanical properties of the samples were investigated and compared before and after irradiation. The irradiation induced an increase in hardness, whereas a decrease in Young's modulus was observed for both samples. Microstructural characterization revealed irradiation-induced cracking, initiated in the bulk of the material, where the peak damage was deposited, propagating towards the surface. Additionally, transmission electron microscopy was used to study irradiation defects. Dislocation loops and fringes were identified and observed to increase in density with increasing dose levels. The high density of irradiation defects and hydrogen implanted are proposed as the main cause of swelling and consequent sample cracking, leading simultaneously to increased hardening and a decrease in Young's modulus.

The temperature-dependent effective thermal conductivity of UN-X-UO2 (X = Mo, W) nuclear fuel composite was estimated. Following the experimental design, the thermal conductivity was calculated using Finite Element Modeling (FEM), and compared with analytical models for 10%, 30%, 50%, and 70% (in mass) uncoated/coated UN microspheres in a UO2 matrix. The FEM results show an increase in the fuel thermal conductivity as the mass fraction of the UN microspheres increases from 1.2 to 4.6 times the UO2 reference at 2,000 K. The results from analytical models agree with the thermal conductivity estimated by FEM. The results also show that Mo and W coatings have similar thermal behaviors, and the coating thickness influences the thermal conductivity of the composite. At higher weight fractions, the thermal conductivity of the fuel composite at room temperature is substantially influenced by the high thermal conductivity coatings approaching that of UN. Thereafter, the thermal conductivity from FEM was used in the fuel thermal performance evaluation during LWR normal operation to calculate the maximum centerline temperature. The results show a significant decrease in the fuel maximum centerline temperature ranging from -94 K for 10% UN to -414 K for 70% (in mass) UN compared to UO2 under the same operating conditions.

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.

Olyckstoleranta bränslen (ATF) är utformade för att motstå en allvarlig olycka i reaktorhärden längre än de standard UO₂-Zr system som används i lättvattenreaktorer (LWR) idag. Kompositbränslen som UN-UO₂ anses vara ett ATF koncept som kan förbättra den lägre oxidationsbeständigheten hos UN bränslet ur ett säkerhetsperspektiv för användning i LWR, samtidigt som UO₂-bränslets beteende och prestanda i reaktorn förbättras. Huvudsyftet med denna avhandling är att tillverka, karakterisera och utvärdera ett innovativt ATF-koncept för LWR: inkapslade UN-sfärer som tillsatser för standardbränslet UO₂. Flera utvecklingssteg tillämpades för att förstå inverkan av sintringsparametrarna på mikrostrukturen för UN-UO₂ bränslet, utvärdera potentiella beläggningskandidater för att kapsla in UN sfärerna med hjälp av olika beläggningsmetoder, bedöma kompositernas oxidationsbeständighet och uppskatta det termiska beteendet hos obelagda och inkapslade UN-UO₂ bränslen. Alla kompositer sintrades med starkströmsassisterad varmpressning (SPS) och karakteriserades av flera komplementära tekniker. Molybden och volfram visar sig vara bra materialkandidater för den skyddande beläggningen med hjälp av en kombination av modellering och experiment. Det visas att pulverlackeringsmetoderna bildar ett tjockt, tätt men ojämnt skikt på sfärerna, medan kemiska- och fysikaliska- ångavsättningsmetoder ger tunnare och mer enhetliga skikt. Finita elementmodellering indikerar att bränslets centertemperatur kan minskas med mer än 400 K när 70 wt% av inkapslade UN-sfärer används jämfört med referensen UO₂. Dessutom reduceras degraderingen av nitridfasen när den är inbäddad i en UO₂-matris och den kan också reduceras ännu mer genom närvaron av ett beläggningsskikt. Dessa resultat bidrar till ytterligare utveckling av metoder för att tillverka, karakterisera, och utvärdera olyckstoleranta bränslen för LWR.

Combustíveis tolerantes a acidentes (ATFs) são projetados para suportar um acidente severo no núcleo do reator por mais tempo que os sistemas combustíveis padrão composto por UO₂ e liga de Zr, atualmente usados emreatores de água leve (LWRs). Combustíveis compósitos do tipo UN-UO2 estão sendo considerados como um conceito ATF para solucionar a inferior resistência à oxidação do combustível UN, tendo em vista perspectivas desegurança para uso em LWRs, enquanto melhora o comportamento do combustível de UO₂ no reator. O objetivo principal desta tese é fabricar, caracterizar, e avaliar um conceito inovador de ATF para LWRs: esferas de UN encapsuladas como aditivos para o combustível padrão de UO₂. Várias etapas de desenvolvimento foram aplicadas para: entender a influência dos parâmetros de sinterização na microestrutura do combustível UN-UO₂, avaliar potenciais candidatos para encapsular as esferas de UN utilizando diferentes metodologias de revestimento, acessar a resistência à oxidação dos compósitos, e estimar o comportamento térmico dos combustíveis compósitos UN-UO₂ sem e com revestimentos. Todos os compósitos foram sinterizados pelo método de sinterização por descarga elétrica (SPS) e caracterizados utilizando diversas técnicas de caracterização microestrutural complementares. Molibdênio e tungstênio demonstraram ser bons materiais candidatos para o revestimento protetivo pela combinação de resultados de modelagem e experimentos. É demonstrado que o método de revestimento utilizando pó forma uma camada espessa, densa e não uniforme nas esferas, enquanto os métodos de deposição química e a vapor proporcionam camadas mais finas e uniformes. Modelagem por elementos finitos indica que a temperatura central do combustível pode ser reduzida em mais de 400 K quando 70 %m de esferas encapsuladas são utilizados, em comparação ao combustível referência UO₂. Além disso, a severidade da degradação da fase nitreto é reduzida quando integrada na matriz de UO₂, podendo ser reduzida ainda mais pela presença de uma camada de revestimento. Esses resultados contribuem para futuros desenvolvimentos em metodologias de fabricação, caracterização e avaliação de combustíveis tolerantes a acidentes em LWRs.

UN-UO₂ composites are considered an accident tolerant fuel (ATF) option for light water reactors (LWRs). However, the interactions between UN and UO2 and the low oxidation resistance of UN limit the application of such ATF composite concept in LWRs. A potential alternative to overcome these issues is encapsulating the UN fuel before sintering. Based on our recent studies, molybdenum and tungsten are selected to encapsulate UN spheres. In this article, different coating techniques, such as powder coating, chemical vapour deposition (CVD), and physical vapour deposition (PVD), were developed and applied to encapsulate surrogates and UN spheres. Encapsulated UN-UO₂ pellets fabricated by the spark plasma sintering (SPS) method (1773 K, 80 MPa) were characterised by complementary techniques and evaluated against their oxidation resistance in air up to 973 K. The results show inert, dense, and non-uniform Mo and W layers of about 28 μm and 32 μm, respectively, obtained by the powder coating method. PVD provided uniform and dense layers of Mo and W of approximately 1.0 μm and 4.0 μm, respectively, but with cracks at the interface with the surrogate spheres. PVD-Mo onto UN spheres shows a dense and well-adhered layer of about 0.5 μm but with W contamination from the previous coating. The PVD-W and CVD-W results and the oxidation experiments will be in the final version of this manuscript.

In this study, the temperature-dependent effective thermal conductivity of the innovative UN-X-UO₂ (X=Mo, W) nuclear fuel composite has been estimated in the temperature range from room temperature to 2000 K. This composite fuel concept is considered as a promising accident tolerant fuel for light water reactors (LWRs). Following the previously reported experimental composite design, the composite fuel thermal conductivity was calculated using Finite Element modeling (FEM), and it is compared with analytical models of thermal conductivity for 10, 30, 50, and 70 wt.% uncoated/coated UN microspheres in a UO₂ matrix. The FEM results show an expected increase in the fuel thermal conductivity as the wt.% of the coated/uncoated UN microspheres increases – from 1.5 to 5.7 times the UO₂ reference at 2000 K. However, the analytical models show an overestimation of the fuel thermal conductivity as the wt.% increases. The results also show that Mo and W coatings have similar thermal behaviors and the coating thickness varying from 1-5 μm has an insignificant effect on the thermal behavior of the composite. However, at higher weight fractions, the thermal conductivity of the fuel composite at room temperature is substantially influenced by the high thermal conductivity coatings exceeding that of UN. Thereafter, the thermal conductivity profiles from FEM were used in the fuel thermal performance evaluation during LWR normal operation to calculate the maximum centerline temperature of the fuel composites. The results show a significant decrease in the fuel maximum centerline temperature ranging from −72 K for 10 wt.% UN to −438 K for 70 wt.% UN compared to the UO₂ under the same irradiation conditions, providing an enhanced safety margin and thermal and neutronic advantages.

Modelling of UO2 mechanical behavior requires detailed knowledge of the local stresses and strains during the fuel's operation in normal and accident conditions. Therefore, a crystal plasticity formulation is proposed for polycrystalline UO2. The model contains a dislocation-density-based formulation including three slip families and their interactions. The model is parametrized with single crystal and polycrystal experimental data using an optimization scheme. The model's capability to represent yield point, strain hardening behavior, temperature and strain rate dependencies are evaluated. Finally, different approaches to include porosity at the polycrystal are analyzed to assess the effect of porosity on homogenized macro-scopic stress-strain behavior, and stress/strain localization at the grain level.

Uranium nitride (UN) spheres embedded in uranium dioxide (UO₂) matrix is considered an innovative accident tolerant fuel (ATF). However, the interaction between UN and UO₂ restricts the applicability of such composite in light water reactors. A possibility to limit this interaction is to separate the two materials with a diffusion barrier that has a high melting point, high thermal conductivity, and reasonably low neutron cross-section. Recent density functional theory calculations and experimental results on interface interactions in UN-X-UO₂ systems (X = V, Nb, Ta, Cr, Mo, W) concluded that Mo and W are promising coating candidates. In this work, we develop and study different methods of coating ZrN spheres, used as a surrogate material for UN spheres: first, using Mo or W nanopowders (wet and binder); and second, using chemical vapour deposition (CVD) of W. ZrN-UO₂ composites containing 15 wt% of coated ZrN spheres were consolidated by spark plasma sintering (1773 K, 80 MPa) and characterised by SEM/FIB-EDS and EBSD. The results show dense Mo and W layers without interaction with UO₂. Wet and binder Mo methods provided coating layers of about 20 µm and 65 µm, respectively, while the binder and CVD of W methods layers of about 12 µm and 3 µm, respectively.

Uranium dioxide (UO₂) is widely used as a fuel in commercial nuclear light-water reactors (LWRs). Rigorous control of density, pore, and grain size of UO₂ pellets are important prerequisites for fuel performance. Solid lubricants, frequently used in pellets manufacturing, minimize structural defects on compaction such as cracks and end-capping, promoting grain growth during sintering. This work presents and discusses the effects of the aluminum distearate (ADS) addition on the sintering behavior and microstructure of UO₂ fuel pellets. UO₂ and UO₂-0.2wt% ADS pellets were sintered at 1760 °C for 5.7 h for comparison purposes. The results show that the densification rate increases using the solid lubricant, but the shrinkage is lowered by 0.7% due to low homogenization. The average grain size was increased by about 35% during sintering. Based on our results and a literature review, a mechanism for grain growth by aluminum addition is proposed.