Helium precipitation study in UO2 by Transmission Electron Microscopy

International audienc

Vegetation responses to climatic and geologic controls on water availability in southeastern Arizona

Vegetation distribution, composition and health in arid regions are largely dependent on water availability controlled by climate, local topography and geology. Despite a general understanding of climatic and geologic drivers in plant communities, trends in plant responses to water distribution and storage across areas under different local controls are poorly understood. Here we investigate the multi-decadal interactions between spatial heterogeneity of geologic controls and temporal variation of climate, and their impacts on water availability to vegetation and plant responses (via normalized difference vegetation index, NDVI) in a monsoon-driven arid region of southeastern Arizona. We find that grasslands display low NDVI and respond directly to monsoonal rainfall. In the uplands, vegetation on west-facing slopes and in canyons share similar NDVI averages and variability, suggesting that they both use water from surface-groundwater flow paths through fractured rocks. Along the San Pedro River, streamflow, groundwater, and NDVI in deciduous riparian woodlands are strongly responsive to monsoonal rainfall, but water availability stratifies between wet (perennial), intermediate, and dry reaches, underlain by different local geologic controls that affect water table elevation. These controls interact with the driving climate to affect water availability in the shallow alluvial aquifer of the riparian zone, a primary water source to the gallery phreatophytes. A recent shift toward a strengthened monsoon in the region has led to an increase in water availability for grasslands and for dry reaches of the San Pedro, while the benefit is more muted along wetter reaches, where the riparian forest shows signs of having reached its maturity, with diminished trends in NDVI. These results have implications for the future vulnerability of dryland vegetation to climate change, which may be either dampened or intensified by local controls such as geology

Finite size effects near the onset of the oscillatory instability

A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects

High resolution spatiotemporal patterns of flow at the landscape scale in montane non‐perennial streams

Intermittent and ephemeral streams in dryland environments support diverse assemblages of aquatic and terrestrial life. Understanding when and where water flows provide insights into the availability of water, its response to external controlling factors, and potential sensitivity to climate change and a host of human activities. Knowledge regarding the timing of drying/wetting cycles can also be useful to map critical habitats for species and ecosystems that rely on these temporary water sources. However, identifying the locations and monitoring the timing of streamflow and channel sediment moisture remains a challenging endeavor. In this paper, we analyzed daily conductivity from 37 sensors distributed along 10 streams across an arid mountain front in Arizona (United States) to assess spatiotemporal patterns in flow permanence, defined as the timing and extent of water in streams. Conductivity sensors provide information on surface flow and sediment moisture, supporting a stream classification based on seasonal flow dynamics. Our results provide insight into flow responses to seasonal rainfall, highlighting stream reaches very reactive to rainfall versus those demonstrating more stable streamflow. The strength of stream responses to precipitation are explored in the context of surficial geology. In summary, conductivity data can be used to map potential stream habitat for water‐dependent species in both space and time, while also providing the basis upon which sensitivity to ongoing climate change can be evaluated

Resistance to amorphisation in Ca1-xLa2x/3TiO3 perovskites – a bulk ion-irradiation study

The changes induced from 1 MeV Kr+ and 5 MeV Au+ ion irradiation at room temperature have been utilised to determine the impact of cation vacancies on the radiation damage response of bulk Ca1-xLa2x/3TiO3 perovskite structured ceramics. Perovskite systems have long been considered as candidate waste forms for the disposition of actinide wastes, and doping with multi-valent elements such as Pu may lead to cation deficiency. Based on GAXRD and TEM analysis, two regions of resistance/susceptibility to amorphisation have been confirmed with reference to CaTiO3. Increased resistance to amorphisation has been observed for 0.1 ≤ x ≤ 0.4, with an increased susceptibility to amorphisation for x ≥ 0.5. It is proposed that these processes are induced by enhanced recovery from radiation damage for 0.1 ≤ x ≤ 0.4, and reduced tolerance for disorder/the increasingly covalent nature of the A-O bond for x ≥ 0.5. Lattice parameter analysis of the x = 0 and 0.5 samples showed a saturation in radiation damage induced volume swelling at 4.7 ± 0.1% and 1.8 ± 0.1%, respectively, while the saturation limit for the b parameter was lower than the respective a and c orthorhombic parameters. In the x = 0.2 and 0.4 samples, amorphisation was not observed, however the b parameter was found to swell to a lesser extent than the a and c parameters. Swelling was not observed for the ion irradiated x ≥ 0.6 samples

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

Thermal annealing of disorder in Pb-ion irradiated SrTiO_3

RADIOCHAttention was paid to thermal annealing of defects in the SrTiO3 lattice preliminary irradiated by Pb2+ ions which were used to simulate the recoil-nuclei during α-decay events in actinide waste forms. Two different types of defect depending upon Pb2+ irradiation fluence and annealing temperature were characterized with their activation energy and a prefactor related to the fraction recovered per second. Such thermal annealing data are necessary to predict the matrix behaviour under the storage conditions. Additional informations were obtained on the defect structure using Rutherford backscattering spectrometry in channeling condition. The measurement of the de-channeling yield along different axes and planes in the crystal has permitted to analyse partially the isolated defect structure