Starting in the 1950s, significant efforts have been devoted to developing metallic nuclear fuels for use in sodium fast reactors. During irradiation, one performance limitation for this type of fuel ...is fuel cladding chemical interaction (FCCI), which occurs after the fuel swells and contacts the cladding. This can cause formation of interaction zones on the cladding inner surface that have low mechanical strength and that can contain relatively low-melting phases. Significant in-pile and out-of-pile research has been performed to improve understanding of how FCCI affects fuel performance and also to identify ways to mitigate and possibly eliminate this phenomenon. In celebration of the diamond anniversary for the Journal of Nuclear Materials, this paper reviews and highlights key aspects of the metallic fuel FCCI research performed over the last 60 years, and discusses some of the most current research being performed today.
Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and focused ion beam (FIB) milling were performed on an irradiated U–10Mo monolithic fuel to understand its irradiation ...microstructure. This is the first reported TEM work of an irradiated fuel sample prepared using a FIB. Advantages and disadvantages of using the FIB to create TEM samples from this irradiated fuel will be presented along with some results from the work. Sample preparation techniques used to create SEM and FIB samples from the brittle irradiated monolithic sample will also be discussed.
Home‐field advantage (HFA) is a commonly used sports analogy, which has seen recent growth within the ecosystem ecology literature. It is most often invoked in litter transplant studies, where local ...adaptation (HFA) explains higher decomposition rates of leaf litter on ‘home’ soil communities. In exploring the mechanisms driving functional differences among soil decomposer communities, a consistent quantitative framework is lacking. We review methods for calculating HFA, propose a consolidated regression approach and demonstrate why HFA must be calculated along with a new ‘ability’ metric if we are to test definitively the competing hypotheses that soil decomposer communities are functionally equivalent versus dissimilar. We demonstrate that qualitative interpretations of HFA differ when the ability of a decomposer community is calculated simultaneously with HFA. For example, communities may differ in their ability to degrade litter in the absence of HFA, or apparent HFA may instead be caused by differing abilities, changing our ecological interpretation of the factors generating functional differences among decomposer communities. Synthesis: We propose a single, statistical approach to help evaluate how and why soil decomposer communities differ in functional abilities. Our approach should help formalize mechanistic interpretations of why soil community composition commonly influences litter decomposition rates.
This paper will discuss the results of transmission electron microscopy (TEM) analysis performed on two samples taken from the low-flux and high-flux sides of the same fuel plate with U–7Mo fuel ...particles dispersed in Al–2Si matrix. The corresponding local fission density of the fuel particles and the peak fuel-plate centerline temperature between the low-flux and high-flux samples are 3.32×1027f/m3 and 90°C, and 6.31×1027f/m3 and 120°C, respectively. The results showed the presence of a bubble superlattice within the U–7Mo grains that accommodated fission gases (e.g., Xe). The presence of this structure helps the U–7Mo exhibit a stable swelling behavior during irradiation. The change in bubble distribution at the high fission density suggests that the bubble superlattice is stable as the U–7Mo matrix remains crystalline. It appears that there is a threshold Si content in the fuel particle above which the U–Mo turns to amorphous under irradiation. The threshold Si content is approximately 8at.% and 4at.% for low-flux and high-flux conditions, respectively.
The plate-type dispersion fuels, with the atomized U(Mo) fuel particles dispersed in the Al or Al alloy matrix, are being developed for use in research and test reactors worldwide. It is found that ...the irradiation performance of a plate-type dispersion fuel depends on the radiation stability of the various phases in a fuel plate. Transmission electron microscopy was performed on a sample (peak fuel mid-plane temperature ∼109°C and fission density ∼4.5×1027fm−3) taken from an irradiated U–7Mo dispersion fuel plate with Al–2Si alloy matrix to investigate the role of Si addition in the matrix on the radiation stability of the phase(s) in the U–7Mo fuel/matrix interaction layer. A similar interaction layer that forms in irradiated U–7Mo dispersion fuels with pure Al matrix has been found to exhibit poor irradiation stability, likely as a result of poor fission gas retention. The interaction layer for both U–7Mo/Al–2Si and U–7Mo/Al fuels is observed to be amorphous. However, unlike the latter, the amorphous layer for the former was found to effectively retain fission gases in areas with high Si concentration. When the Si concentration becomes relatively low, the fission gas bubbles agglomerate into fewer large pores. Within the U–7Mo fuel particles, a bubble superlattice ordered as fcc structure and oriented parallel to the bcc metal lattice was observed where the average bubble size and the superlattice constant are 3.5nm and 11.5nm, respectively. The estimated fission gas inventory in the bubble superlattice correlates well with the fission density in the fuel.
Transmission electron microscopy characterization of irradiated U-7 wt%Mo dispersion fuel were performed on various U-Mo fuel samples to understand the effect of irradiation parameters (fission ...density, fission rate, and temperature) on the self-organized fission-gas-bubble superlattice that forms in the irradiated U-Mo fuel. The bubble superlattice was seen to form a face centered cubic structure coherent with the host U-7 wt%Mo body-centered cubic structure. At a fission density between 3.0 and 4.5 x 10 super(21) fiss/cm super(3), the superlattice bubbles appear to have reached a saturation size with additional fission gas associated with increasing bumup predominately accumulating along grain boundaries. At a fission density of ~4.5 x 10 super(21) fiss/cm super(3), the U-7 wt%Mo microstructure starts to undergo grain subdivision and can no longer support the ordered bubble superlattice. The sub-divided fuel grains are less than 500 nm in diameter with what appears to be micron-size fission-gas bubbles present on the grain boundaries. Solid fission products typically decorate the inside surface of the micron-sized fission-gas bubbles. Residual superlattice bubbles are seen in areas where fuel grains remain micron sized. Potential mechanisms of the formation and collapse of the bubble superlattice are discussed.
Summary Background Rheumatic heart disease accounts for up to 250 000 premature deaths every year worldwide and can be regarded as a physical manifestation of poverty and social inequality. We aimed ...to estimate the prevalence of rheumatic heart disease in endemic countries as assessed by different screening modalities and as a function of age. Methods We searched Medline, Embase, the Latin American and Caribbean System on Health Sciences Information, African Journals Online, and the Cochrane Database of Systematic Reviews for population-based studies published between Jan 1, 1993, and June 30, 2014, that reported on prevalence of rheumatic heart disease among children and adolescents (≥5 years to <18 years). We assessed prevalence of clinically silent and clinically manifest rheumatic heart disease in random effects meta-analyses according to screening modality and geographical region. We assessed the association between social inequality and rheumatic heart disease with the Gini coefficient. We used Poisson regression to analyse the effect of age on prevalence of rheumatic heart disease and estimated the incidence of rheumatic heart disease from prevalence data. Findings We included 37 populations in the systematic review and meta-analysis. The pooled prevalence of rheumatic heart disease detected by cardiac auscultation was 2·9 per 1000 people (95% CI 1·7–5·0) and by echocardiography it was 12·9 per 1000 people (8·9–18·6), with substantial heterogeneity between individual reports for both screening modalities ( I2 =99·0% and 94·9%, respectively). We noted an association between social inequality expressed by the Gini coefficient and prevalence of rheumatic heart disease (p=0·0002). The prevalence of clinically silent rheumatic heart disease (21·1 per 1000 people, 95% CI 14·1–31·4) was about seven to eight times higher than that of clinically manifest disease (2·7 per 1000 people, 1·6–4·4). Prevalence progressively increased with advancing age, from 4·7 per 1000 people (95% CI 0·0–11·2) at age 5 years to 21·0 per 1000 people (6·8–35·1) at 16 years. The estimated incidence was 1·6 per 1000 people (0·8–2·3) and remained constant across age categories (range 2·5, 95% CI 1·3–3·7 in 5-year-old children to 1·7, 0·0–5·1 in 15-year-old adolescents). We noted no sex-related differences in prevalence (p=0·829). Interpretation We found a high prevalence of rheumatic heart disease in endemic countries. Although a reduction in social inequalities represents the cornerstone of community-based prevention, the importance of early detection of silent rheumatic heart disease remains to be further assessed. Funding UBS Optimus Foundation.
Electron-electron interactions can render an otherwise conducting material insulating, with the insulator-metal phase transition in correlated-electron materials being the canonical macroscopic ...manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator-metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator-metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
For higher U-loading in low-enriched U–10
wt.%Mo fuels, monolithic fuel plate clad in AA6061 is being developed as a part of Reduced Enrichment for Research and Test Reactor (RERTR) program. This ...paper reports the first characterization results from a monolithic U–10
wt.%Mo fuel plate with a Zr diffusion barrier that was fabricated as part of a plate fabrication campaign for irradiation testing in the Advanced Test Reactor (ATR). Both scanning and transmission electron microscopy (SEM and TEM) were employed for analysis. At the interface between the Zr barrier and U–10
wt.%Mo, going from Zr to U(Mo), UZr
2, γ-UZr, Zr solid-solution and Mo
2Zr phases were observed. The interface between AA6061 cladding and Zr barrier plate consisted of four layers, going from Al to Zr, (Al,
Si)
2Zr, (Al,
Si)Zr
3 (Al,
Si)
3Zr, and AlSi
4Zr
5. Irradiation behavior of these intermetallic phases is discussed based on their constituents. Characterization of as-fabricated phase constituents and microstructure would help understand the irradiation behavior of these fuel plates, interpret post-irradiation examination, and optimize the processing parameters of monolithic fuel system.