The demanding operating environments of advanced nuclear reactors require the development of new nuclear materials that can withstand their increased physical, chemical, thermal, and ...radiation-related challenges. High-entropy alloys (HEAs) have shown often-impressive mechanical, thermomechanical, and corrosion-resistant properties, and offer a massive, unexplored compositional space which allows for the targeted development of application-specific materials. Furthermore, although still in a nascent stage, research has shown that HEAs may exhibit unique irradiation tolerance, including reduced defect production and resistance to irradiation-induced swelling and hardening. The mechanisms behind this increased tolerance are not yet well-understood, although the HEA-specific attributes of a complex energy landscape, reduced thermal conductivity, and shift in defect migration energies and pathways provide promising explanations. This work assesses the current and future challenges faced by structural nuclear materials, identifying the specific applications in which HEAs may provide a competitive advantage compared to industry-standard materials with the aid of Ashby material selection maps. Considerations are provided for the design of future nuclear HEAs, including calculations of nuclear-relevant properties to assist in the initial down-selection of elements depending on application requirements (e.g., low neutron capture for in-core applications), narrowing the existing compositional space of HEAs to a manageable scope.
Nuclear power can solve the energy trilemma of supplying baseload, clean and affordable power. However, a review of nuclear power plant (NPP) builds show mixed results, with delays in Finland and in ...the US offset by successes in China, South Korea and the UAE. In the West, financing for new builds has been difficult in the face of a deregulated energy market, billion-dollar upfront investments, long build times and in the case of the US historically low gas prices. We explore how the nuclear industry is innovating in facing these challenges through a review of nuclear power developments in the past, present and future. Early developments in nuclear power in the 1950s resulted in a variety of designs, out of which the pressurised water reactor (PWR) became dominant for its compactness and overall economy. Over the next 10 years, several PWR-based small modular reactor (SMR) designs are expected to come online within an eight-year timeframe. Their modular construction and fabrication in a controlled factory setting aims to shorten build times from 8 to 3 years. However, the lack of established regulatory approval pathways may be a time-limiting challenge that needs to be overcome by the first fleet of SMRs. The passive safety and a smaller fuel loading of SMRs will allow them to be deployed at more potential sites, including brownfield replacements of old coal-fired power plants or power unconventional, remote or islanded grids. Some SMRs are also designed to load follow which will allow them to work harmoniously with intermittent renewables sources with the promise of an affordable, truly carbon-neutral grid. In the longer term, advanced nuclear reactors in the form of sodium cooled, molten salt cooled, and high temperature gas cooled reactors hold the promise of providing efficient electricity production, industrial heat for heavy industry as well as the generation of hydrogen for synthetic fuel.
According to the properties determined for the ternary end‐members, MAX phases in the quaternary Cr–Ti–Al–C system could be of interest as protective coatings for nuclear fuel cladding in the case of ...severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH2, Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti3AlC2 and the mutual solubility of Ti2AlC and Cr2AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr2/3Ti1/3)3AlC2 MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr–Ti–Al–C system, most samples contained substantial amounts of TiCx and Al–Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high‐temperature oxidation resistance and so warranting further investigation. In addition to (Cr0.95Ti0.05)2AlC, composite samples containing substantial quantities of Al8Cr5 and AlCr2 formed a stable and passivating Al2O3 scale, whereas the other samples were fully oxidized.
Hydrogen embrittlement reduces the durability of the structural steels required for the hydrogen economy. Understanding how hydrogen interacts with the materials plays a crucial role in managing the ...embrittlement problems. Theoretical models have indicated that carbon vacancies in metal carbide precipitates are effective hydrogen traps in steels. Increasing the number of carbon vacancies in individual metal carbides is important since the overall hydrogen trapping capacity can be leveraged by introducing abundant metal carbides in steels. To verify this concept, we compare a reference steel containing titanium carbides (TiCs), which lack carbon vacancies, with an experimental steel added with molybdenum (Mo), which form Ti-Mo carbides comprising more carbon vacancies than TiCs. We employ theoretical and experimental techniques to examine the hydrogen trapping behavior of the carbides, demonstrating adding Mo alters the hydrogen trapping mechanism, enabling hydrogen to access carbon vacancy traps within the carbides, leading to an increase in trapping capacity.
Zirconium alloys are used in safety-critical roles in the nuclear industry and their degradation due to ingress of hydrogen in service is a concern. In this work experimental evidence, supported by ...density functional theory modelling, shows that the α-Zr matrix surrounding second phase particles acts as a trapping site for hydrogen, which has not been previously reported in zirconium. This is unaccounted for in current models of hydrogen behaviour in Zr alloys and as such could impact development of these models. Zircaloy-2 and Zircaloy-4 samples were corroded at 350 °C in simulated pressurised water reactor coolant before being isotopically spiked with
H
O in a second autoclave step. The distribution of
H, Fe and Cr was characterised using nanoscale secondary ion mass spectrometry (NanoSIMS) and high-resolution energy dispersive X-ray spectroscopy.
H
was found to be concentrated around second phase particles in the α-Zr lattice with peak hydrogen isotope ratios of
H/
H = 0.018-0.082. DFT modelling confirms that the hydrogen thermodynamically favours sitting in the surrounding zirconium matrix rather than within the second phase particles. Knowledge of this trapping mechanism will inform the development of current understanding of zirconium alloy degradation through-life.
The engineering of high rate electrochemical energy storage devices can benefit from analysis techniques that can accurately attribute charge storage to individual energy storage mechanisms. A new ...time-domain analysis for potentiostatic intermittent titration technique (PITT) experiments that uses Laplace domain representations of impedance is presented for the characterisation of charge storage in electrochemical systems where charge can be stored via a combination of Faradaic processes and electric double layer storage. The derivation of this model is presented, along with a proof that the model collapses into the single electric double layer storage model or the Faradaic charge storage model under limiting conditions (infinitely restricted diffusion or zero capacitive storage, respectively). The parameter space of the model is explored, along with an evaluation of when the simpler, single process charge storage models can be used in place of the more complex two-mode storage model. The model is validated with an electric double layer capacitor, for which reasonable agreement is seen between fitted capacitance and its stated value, and using experimental data obtained from amorphous TiO2 nanotube arrays hierarchically grown on Ti mesh electrodes. PITT measurements using the proposed model, and electrochemical impedance spectroscopy (EIS) yielded similar fitting parameters, with the exception of CEDL, which is too small to estimate with PITT, and Rct/RΩ, which becomes inseparable when CEDL is small.
•PITT model for electrochemical energy storage with Faradaic and EDLC storage•Sample space of model explored for ratios of Faradaic and EDLC storage•Analysis of when single-process EDLC or Faradaic charge storage models can be used.•Validated with amorphous TNTA@Ti mesh experimental anode and EDLC supercapacitor
Zirconium-based alloys used for fuel cladding in nuclear fission reactors are susceptible to hydrogen embrittlement during operation, but we currently lack the necessary mechanistic understanding of ...how hydrogen behaves in the materials during service to properly address this issue. Imaging the distribution of hydrogen within material microstructures is key to creating or validating models that predict the behaviour and influence of hydrogen on material properties, but is experimentally difficult. Studying hydrogen in zirconium-alloys is further complicated by the fact that the most common routes for preparing specimens for Transmission Electron Microscopy and Atom Probe Tomography (APT) analysis, electropolishing and focused ion beam (FIB) milling, are known to induce hydride formation. This introduces uncertainty as to whether the hydrogen distribution in the analysed specimen is actually representative of the entire sample a priori. Recent work has shown that this effect can be mitigated by performing the final specimen thinning stages at cryogenic temperatures. In this paper we use cryo-FIB to prepare APT specimens of neutron-irradiated low Sn ZIRLO, showing that hydrogen is trapped within a β-Nb SPP and at Nb-rich nanoclusters formed by exposure to neutron irradiation. We then use density functional theory calculations to explain these experimental observations. These results highlight the importance of including niobium-rich features in models used to predict hydrogen pick-up in zirconium alloys during service and delayed hydride cracking during storage.
Specification of the T helper 17 (Th17) cell lineage requires a well-defined set of transcription factors, but how these integrate with posttranscriptional and epigenetic programs to regulate gene ...expression is poorly understood. Here we found defective Th17 cell cytokine expression in miR-155-deficient CD4+ T cells in vitro and in vivo. Mir155 was bound by Th17 cell transcription factors and was highly expressed during Th17 cell differentiation. miR-155-deficient Th17 and T regulatory (Treg) cells expressed increased amounts of Jarid2, a DNA-binding protein that recruits the Polycomb Repressive Complex 2 (PRC2) to chromatin. PRC2 binding to chromatin and H3K27 histone methylation was increased in miR-155-deficient cells, coinciding with failure to express Il22, Il10, Il9, and Atf3. Defects in Th17 cell cytokine expression and Treg cell homeostasis in the absence of Mir155 could be partially suppressed by Jarid2 deletion. Thus, miR-155 contributes to Th17 cell function by suppressing the inhibitory effects of Jarid2.
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•miR-155 is highly induced during mouse and human Th17 cell differentiation•Jarid2 and miR-155 are epistatic in Th17 and Treg cells•Jarid2 is required to recruit PRC2 to genomic sites in Th17 cells•Direct targets of PRC2 in Th17 cells include Il22, Il10, Il9, and Atf3
miR-155 is known to promote inflammatory Th17 cell responses, but the mechanism has been unclear. Escobar et al. find that miR-155 promotes cytokine expression in Th17 cells by repressing Jarid2 to relieve Polycomb-mediated gene silencing.
The charge-storage kinetics of amorphous TiO x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior ...storage capacities and rate capability to anatase TiO x nanotube electrodes but also improved areal capacities (324 μAh cm–2 at 50 μA cm–2 and 182 μAh cm–2 at 5 mA cm–2) and cycling stability (over 2000 cycles) over previously reported TiO x nanotube electrodes using planar current collectors. Amorphous TiO x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s–1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li+ in amorphous TiO x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li+ diffusion process (diffusion coefficients of 2 × 10–14 to 3 × 10–13 cm2 s–1) but also more facile interfacial charge-transfer kinetics than anatase TiO x .
Amorphous titanium niobium oxides (TNOs) with varying ratios of Ti and Nb (Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7) are presented as promising anode materials for Li ion batteries. The capacity of the TNO ...materials is seen to be equivalent to, or larger than, that of the binary oxides, with average volumetric capacities over the first 10 cycles of 717, 1,039 and 925 mAh cm−3 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of 0.2 A cm−3, compared to 720 mAh cm−3 and 425 mAh cm−3 for amorphous TiO2 and Nb2O5. Using densities estimated with X-ray reflectometry, these are equivalent to gravimetric capacities of 231, 335, 319 mAh g−1 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of ~70 mA g−1, compared to 257 mAh g−1 and 137 mAh g−1 for amorphous TiO2 and Nb2O5 at a current density ~80 mA g−1 and ~50 mA g−1, respectively. We discuss how rate capability varies with varying ratios of Ti and Nb and relate this to electrochemical parameters determined by the potentiostatic intermittent titration technique. Our findings reveal that the rate capability of the films is dominated by the diffusion resistance, RD, a composite parameter linked to the insertion rate and diffusion coefficient of Li, leading to a conclusion that the rate retention of the thin films is dominated by the density of insertion sites and the insertion reaction more generally.
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