Long-term microstructural evolution of tungsten under heat and neutron loads

In nuclear fusion reactors, tungsten will be exposed to high neutron loads at high temper atures (>900 C). The evolution and degradation of the mechanical properties under these conditions is uncertain and therefore constitutes a major risk. Here, the microstructural evolution of tungsten under combined heat and neutron loads is studied, using a multi-scale approach incorporating clusters dynamics and a mean-field recrystallization model. The mean-field recrystallization model contains both nucleation in the bulk and at the grain boundaries. The cluster dynamics model includes the incorporation of loops in the dynamics of the dislocation network as a mechanism. The effects of bulk nucleation on the microstructural evolution are explored. The simulations predict a cyclically occuring neutron-induced recrystallization at all studied temperatures. Furthermore, the evolution of the irradiation hardening during neutron-induced recrystallization is assessed from the simulated microstructures.Comment: Submitted to peer-reviewed journa

Controlled irradiation hardening of tungsten by cyclic recrystallization

The economical lifetime of the divertor is a key concern for realizing nuclear fusion reactors that may solve the world's energy problem. A main risk is thermo-mechanical failure of the plasma-facing tungsten monoblocks, as a consequence of irradiation hardening induced by neutron displacement cascades. Lifetime extensions that could be carried out without prolonged maintenance periods are desired. In this work, the effects of potential treatments for extending the lifetime of an operational reactor are explored. The proposed treatments make use of cyclic recrystallization processes that can occur in neutron-irradiated tungsten. Evolution of the microstructure under non-isothermal conditions is investigated, employing a multi-scale model that includes a physically-based mean-field recrystallization model and a cluster dynamics model for neutron irradiation effects. The model takes into account microstructural properties such as grain size and displacement-induced defect concentrations. The evolution of a hardness indicator under neutron irradiation was studied. The results reveal that, for the given microstructure and under the assumed model behaviour, periodical extra heating can have a significant positive influence on controlling the irradiation hardening. For example, at 800 C, if extra annealing at 1200 C was applied after every 100 hrs for the duration of 1 hr, then the hardness indicator reduces from maximum 140 to below 70.Comment:

Exosome mimetics: a novel class of drug delivery systems

The identification of extracellular phospholipid vesicles as conveyors of cellular information has created excitement in the field of drug delivery. Biological therapeutics, including short interfering RNA and recombinant proteins, are prone to degradation, have limited ability to cross biological membranes, and may elicit immune responses. Therefore, delivery systems for such drugs are under intensive investigation. Exploiting extracellular vesicles as carriers for biological therapeutics is a promising strategy to overcome these issues and to achieve efficient delivery to the cytosol of target cells. Exosomes are a well studied class of extracellular vesicles known to carry proteins and nucleic acids, making them especially suitable for such strategies. However, the considerable complexity and the related high chance of off-target effects of these carriers are major barriers for translation to the clinic. Given that it is well possible that not all components of exosomes are required for their proper functioning, an alternative strategy would be to mimic these vesicles synthetically. By assembly of liposomes harboring only crucial components of natural exosomes, functional exosome mimetics may be created. The low complexity and use of well characterized components strongly increase the pharmaceutical acceptability of such systems. However, exosomal components that would be required for the assembly of functional exosome mimetics remain to be identified. This review provides insights into the composition and functional properties of exosomes, and focuses on components which could be used to enhance the drug delivery properties of exosome mimetics

Body mass index and dental caries in children aged 5 to 8 years attending a dental paediatric referral practice in the Netherlands

Background Obesity and dental caries are widely-recognised problems that affect general health. The prevention of both dental caries and obesity have proven very difficult: children and their parents may need professional support to achieve behaviour change. To find out whether both dental caries and overweight in childhood can be targeted using a common risk factor approach, it is necessary to establish whether the two diseases are indeed linked. The aim of the present study was therefore to use anthropometric data obtained professionally to investigate the association between Body Mass Index and dental caries experience in children aged 5-8 years receiving treatment in a referral centre for paediatric dental care in the Netherlands. Methods Children’s dmft and dmfs scores were calculated using dental records and sociodemographic data were also extracted from these records. Dentists were trained to measure standing height and weight in a standardised way. Body Mass Index was calculated by dividing kilograms by height squared (kg/m2). Extended International (International Obesity Task Force) body mass index cut-offs were used to define ‘no overweight’ and ‘overweight’ (with the latter category including obesity). Results No statistically significant differences were found between the mean dmft or dmfs scores of the two groups (overweight and non-overweight), even after correction for the effect of the potential confounders sex, socio-economic status and ethnicity. The percentage of caries-active children in the non-overweight group was almost the same as in the overweight group. No statistically significant differences were found. Conclusions We hypothesised to find a positive association between body mass index and dental caries experience in children aged 5-8 years attending our practice. However, this study did not find a relationship of this kind. A common risk factor approach for the prevention of caries and overweight is therefore not supported by our study

Rates of spectacle wear in early childhood in the Netherlands

BACKGROUND: Refractive errors are relatively common all around the world. In particular, early onset myopia is associated with a significant burden in later life. Little is known about refractive errors in preschool children. The aim of this study was to assess the prevalence of spectacle wear, visual acuity and refractive errors in young Dutch children. METHODS: We analyzed data of three prospective population-based studies: 99,660 3- to 5-year-olds undergoing vision screening at preventive child healthcare organizations, 6934 6-year-olds from the Generation R study, and 2974 7-year-olds from the RAMSES study. Visual acuity was measured with Landolt-C or LEA charts, spectacle wear was assessed, and refractive errors at age 6 and 7 were measured with cycloplegic refraction. RESULTS: The prevalence of spectacle wear ranged from 1.5 to 11.8% between 3 to 7 years with no significant gender differences. Among children with spectacle wear at 6 years (N = 583) and 7 years (N = 350) 29.8 and 34.6% had myopia respectively, of which 21.1 and 21.6% combined with astigmatism; 19.6 and 6.8% had hyperopia, 37.2 and 11.1% hyperopia and astigmatism, and 12.5 and 32.7% astigmatism only. CONCLUSIONS: Spectacle wear in European children starts early in preschool and increases to a relatively frequent visual aid at school age. Advocating early detection and monitoring of refraction errors is warranted in order to prevent visual morbidities later in life

Three mechanisms of hydrogen-induced dislocation pinning in tungsten

The high-flux deuterium plasma impinging on a divertor degrades the long-termthermo-mechanical performance of its tungsten plasma-facing components. A prime actor inthis is hydrogen embrittlement, a degradation phenomenon that involves the interactions between hydrogen and dislocations, the primary carriers of plasticity. Measuring such nanoscaleinteractions is still very challenging, which limits our understanding. Here, we demonstrate anexperimental approach that combines thermal desorption spectroscopy (TDS) andnanoindentation, allowing to investigate the effect of hydrogen on the dislocation mobility in tungsten. Dislocation mobility was found to be reduced after deuterium injection, which ismanifested as a ‘pop-in’ in the indentation stress-strain curve, with an average activation stressfor dislocation mobility that was more than doubled. All experimental results can be confidentlyexplained, in conjunction with experimental and numerical literature findings, by the simultaneous activation of three mechanisms responsible for dislocation pinning: (i) hydrogentrapping at pre-existing dislocations, (ii) hydrogen-induced vacancies, and (iii) stabilization ofvacancies by hydrogen, contributing respectively 38%, 52%, and 34% to the extra activationstress. These mechanisms are considered to be essential for the proper understanding and modeling of hydrogen embrittlement in tungsten

Aerodynamic and Aeroacoustic Performance of Small UAV Propellers in Static Conditions

The proliferation of small multi-rotor UAVs in commercial, recreational, and surveillance spheres has garnered significant interest in the noise produced by these vehicles. The current research aims to study the relationship between the aerodynamic performance and acoustic characteristics of small-scale UAV propellers. Three commercially available propellers for the DJI Phantom 2/3 UAV were selected for preliminary development and validation of an aeroacoustic experimental test setup and associated data reduction methods. Propeller thrust, torque, and power measurements were recorded at static conditions. Upon successful validation of the test bench, acoustic measurements were taken at the propeller disk’s upstream and in-plane locations. The power spectral density of these acoustic signals was estimated using the modified periodogram (Welch’s) method to identify frequency content and calculate sound pressure levels (SPLs) at each of the observation locations. Additionally, time-frequency analysis verified the periodogram results and identified possible sources of transient noise at static thrust. These methods found the nonrotor noise to be a major contributor to the SPL at higher frequencies and the propeller noise dominating the SPL spectra at the lower frequencies. Experimental thrust, torque, power, and sound pressure level (SPL) data were then compared for each propeller to identify relationships between aerodynamic performance and acoustic characteristics with variations in propeller geometry and blade loading

Viscous-Inviscid Interactions in a Boundary-Layer Flow Induced by a Vortex Array

In this paper we investigate the asymptotic validity of boundary layer theory. For a flow induced by a periodic row of point-vortices, we compare Prandtl's solution to Navier-Stokes solutions at different $Re$ numbers. We show how Prandtl's solution develops a finite time separation singularity. On the other hand Navier-Stokes solution is characterized by the presence of two kinds of viscous-inviscid interactions between the boundary layer and the outer flow. These interactions can be detected by the analysis of the enstrophy and of the pressure gradient on the wall. Moreover we apply the complex singularity tracking method to Prandtl and Navier-Stokes solutions and analyze the previous interactions from a different perspective

Referral patterns of children with poor growth in primary health care

Background. To promote early diagnosis and treatment of short stature, consensus meetings were held in the mid nineteen nineties in the Netherlands and the UK. This resulted in guidelines for referral. In this study we evaluate the referral pattern of short stature in primary health care using these guidelines, comparing it with cut-off values mentioned by the WHO. Methods. Three sets of referral rules were tested on the