Diffusion of fission products and radiation damage in SiC

A major problem with most of the present nuclear reactors is their safety in terms of the release of radioactivity into the environment during accidents. In some of the future nuclear reactor designs, i.e. Generation IV reactors, the fuel is in the form of coated spherical particles, i.e. TRISO (acronym for Triple Coated Isotropic) particles. The main function of these coating layers is to act as diffusion barriers for radioactive fission products, thereby keeping these fission products within the fuel particles, even under accident conditions. The most important coating layer is composed of polycrystalline 3C-SiC. This paper reviews the diffusion of the important fission products (silver, caesium, iodine and strontium) in SiC. Because radiation damage can induce and enhance diffusion, the paper also briefly reviews damage created by energetic neutrons and ions at elevated temperatures, i.e. the temperatures at which the modern reactors will operate, and the annealing of the damage. The interaction between SiC and some fission products (such as Pd and I) is also briefly discussed. As shown, one of the key advantages of SiC is its radiation hardness at elevated temperatures, i.e. SiC is not amorphized by neutron or bombardment at substrate temperatures above 350°C. Based on the diffusion coefficients of the fission products considered, the review shows that at the normal operating temperatures of these new reactors (i.e. less than 950°C) the SiC coating layer is a good diffusion barrier for these fission products. However, at higher temperatures the design of the coated particles needs to be adapted, possibly by adding a thin layer of ZrC.http://iopscience.iop.org/hb201

Deconvolution models for determining the real surface composition of InP (1 0 0) after bombardment with 5 keV Ar ions at different angles

Low energy ion bombardment can induce compositional changes in the surfaces of compound materials. A fundamental problem is to determine which of the two main mechanisms caused the compositional change, viz. preferential sputtering or bombardment-induced segregation. This paper describes a method, using Auger electron spectroscopy (AES) taken at different angles, to determine the real (top) surface concentrations for an InP (1 0 0) surface after 5 keV Ar+ bombardment at varying impact angles. This bombardment results in an altered near-surface layer. This altered surface layer is amorphised and has a non-stoichiometric surface composition. AES intensity measures the average concentration over the information depth. In this paper, two deconvolution models were used to determine concentration vs depth distributions from the AES intensities. These two models were then used to calculate a surface concentration for each case. Using a deconvolution model in which chemical effects and segregation dominate, the calculated surface concentration was larger than 1, indicating an unphysical surface concentration. Applying a ballistic deconvolution model in the quantification equation, the surface concentration values determined, agree within 5% to the values obtained from TRIDYN simulations. From this follows that argon ion bombardment-induced compositional changes in InP are mainly due to preferential sputtering and ion beam mixing and (to a lesser extent) bombardment-induced diffusion.http://www.elsevier.com/locate/nimb2020-12-01hj2019Physic

Are high elevation crag lizards sensitive to climate change?

Crag lizards are restricted to montane areas where biota are strongly exposed to the effects of climate change. We investigated the factors shaping the distribution of Drakensberg crag lizards (Pseudocordylus melanotus melanotus; Cordylidae) by quantifying their elevational ranges, availability of shelter and prey as well as the thermal environment at three elevations. We recorded expected body temperatures using copper models of the lizards at each elevation in the field and in the laboratory, estimating the duration for which lizards must shelter from high temperatures. This correlated strongly with field observations of lizard activity during winter, spring and summer. Our models predicted that at lower elevations, lizards would shelter for longer periods each day, resulting in a marked reduction in time available for essential activities such as feeding and breeding, possibly explaining why these animals do not occur at lower elevations. The distribution and survival of these animals therefore appear to be impacted by the degree to which daytime temperatures limit their activity. Modelling future climate change scenarios at the high elevation site indicates that crag lizards would be resilient against small to moderate warming reflected by RCP4.5 climate change predictions, but populations are unlikely to persist under conditions predicted in the RCP8.5 scenario. This study is important as, unlike broader-scope studies that cannot quantify climate change impacts on individual species, we combine detailed field data with laboratory measurements and modelling of a single species to assess the ecological constraints to which crag lizards are exposed.The University of Pretoria and the National Research Foundation.https://onlinelibrary.wiley.com/journal/14429993hj2022Geography, Geoinformatics and MeteorologyPhysicsZoology and Entomolog

Kinetics of solid-state reactions between zirconium thin film and silicon carbide at elevated temperatures

Solid state reactions between a thin film (133 nm) of Zr and bulk single crystalline 6H-SiC substrates have been studied at temperatures between 600 °C and 850 °C for durations of 30, 60 and 120 min under high vacuum conditions. The deposited film and reaction zones were investigated by Rutherford backscattering spectrometry (RBS) and X-ray diffraction. The RBS spectra were simulated in order to obtain the deposited layer thickness, reaction zone compositions and reaction zone thickness. The as-deposited spectra fit well with those annealed at 600 °C, thus showing there were no reactions taking place. At temperatures of 700 °C and above, Zr reacted with the SiC substrate and formed a mixed layer of Zr carbide (ZrCx) and Zr silicides (ZrSi, Zr2Si and Zr5Si3). Annealing at 850 °C for 240 min revealed that all the deposited Zr had completely reacted. The interface reaction follows the parabolic growth law thereby indicating diffusion controlled reaction kinetics. The activation energy for the diffusion process obtained was 1.6 eV in the relatively narrow temperature range 700–850 °C.http://www.elsevier.com/locate/nimbhb201

Projected changes in tropical cyclone climatology and landfall in the southwest Indian Ocean region under enhanced anthropogenic forcing

The conformal-cubic atmospheric model, a variable-resolution global model, is applied at high spatial resolution to perform simulations of present-day and future climate over southern Africa and over the Southwest Indian Ocean. The model is forced with the bias-corrected sea-surface temperatures and sea-ice of six coupled global climate models that contributed to Assessment Report 4 of the Intergovernmental Panel on Climate Change. All six simulations are for the period 1961–2100, under the A2 emission scenario. Projections for the latter part of the 21st century indicate a decrease in the occurrence of tropical cyclones over the Southwest Indian Ocean adjacent to southern Africa, as well as a northward shift in the preferred landfall position of these systems over the southern African subcontinent. A concurrent increase in January to March rainfall is projected for northern Mozambique and southern Tanzania, with decreases projected further south over semi-arid areas such as the Limpopo River Basin where these systems make an important contribution as main cause of widespread heavy rainfall. It is shown that the projected changes in tropical cyclone attributes and regional rainfall occur in relation to changes in larger scale atmospheric temperature, pressure and wind profiles of the southern African region and adjacent oceans.The Water Research Commission (WRC Project No. K5/1847) in South Africa, as well as the Agricultural Research Council and the Council for Scientific and Industrial Research, South Africa.http://link.springer.com/journal/382hb201

Tropical systems from the southwest Indian Ocean making landfall over the Limpopo River Basin, southern Africa : a historical perspective

The study provides perspective on the contribution of landfalling tropical systems (cyclones, depressions, storms and lows) from the southwest Indian Ocean (SWIO) towards rainfall over the eastern interior of southern Africa, over the period 1948-2008. Although these systems contribute to < 10% of the annual rainfall occurring over the region, their relative contribution to local and widespread heavy rainfall events is shown to be highly significant. About 50% of widespread heavy rainfall events over northeastern South Africa are caused by landfalling tropical systems. Fourier analysis performed on the time series of rainfall occurring over notheastern South Africa in association with these systems reveals the existence of a quasi-18-year cycle. The cycle coincides with the well-known quasi-18-year Dyer-Tyson cycle in rainfall over the summer rainfall region of South Africa. These results suggest that atmospheric and surface conditions leading to wet phases of the Dyer-Tyson cycle also favour the landfall and subsequent westward movement of tropical systems from the SWIO over southern Africa – and their eventual contribution to rainfall over northeastern South Africa and southern Zimbabwe.Water Research Commission, South Africa (WRC Project No. K5/1847).http://www3.interscience.wiley.com/journal/4735/hom

The bi-decadal rainfall cycle, Southern Annular Mode and tropical cyclones over the Limpopo River Basin, southern Africa

The association between bi-decadal rainfall variability over southern Africa and the rainfall contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a potential means towards understanding decadal-scale variability over parts of the region. A multi-decadal period is considered, focusing on the anomalous tropospheric patterns that induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet sub-period was also characterized by a larger contribution to rainfall by tropical cyclones and depressions. The findings suggest that a broadening of the Hadley circulation underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered tropospheric steering flow, relative vorticity and moisture contents spatially during the subperiod of 8 years. These circulation modulations induced enhanced potential for tropical systems from the SWIO to cause precipitation over the Limpopo River Basin. The same patterns are also conducive to increasing rainfall over the larger subcontinent, therefore explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by tropical cyclonic systems from the SWIO. The regional circulation anomalies are also explained in hemispheric context, specifically in relation to the Southern Annular Mode, towards understanding variation over other parts of the Southern Hemisphere at this time scale.Water Research Commission (WRC Project No.K5/1847) in South Africa and the Agricultural Research Council (Project No. GW 50/053).http://link.springer.com/journal/382hb201

The effect of thermal annealing in a hydrogen atmosphere on tungsten deposited on 6H-SiC

Tungsten (W) film was deposited on a bulk single crystalline 6HeSiC substrate and annealed in H2 ambient at temperatures of 700 C, 800 C and 1000 C for 1 h. The resulting solid-state reactions, phase composition and surface morphology were investigated by Rutherford backscattering spectrometry (RBS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM) analysis techniques. These results are compared with the vacuum annealed results reported in our earlier work. As-deposited RBS results indicated the presence of W and O2 in the deposited thin film, the GIXRD showed the presence ofW,WO3, W5Si3 andWC. RBS results indicated the interaction betweenWand SiC was accompanied by the removal of oxygen at 700 C. The GIXRD analysis indicated the presence of W5Si3 and WC in the samples annealed at 700 C. At temperatures of 800 C and 1000 C, Wannealed in a H2 ambient further reacted with the SiC substrate and formed a mixed layer containing silicide phases and carbide phases, i.e.W5Si3, WSi2, WC and W2C. The SEM micrographs of the as-deposited samples indicated the W thin film had a uniform surface with small grains. Annealing at 800 C led to the agglomeration of W grains into clusters making the surface rough.National Research Foundation (NRF) (Grant number: 88661), South Africa.http://www.journals.elsevier.com/vacuum2017-07-30hb2016Physic

In-situ RBS studies of strontium implanted glassy carbon

The diffusion behaviour of strontium in glassy carbon was investigated using in-situ real time Rutherford backscattering spectrometry. The sample was annealed in vacuum from room temperature to 650 oC. Diffusion of the implanted strontium towards the bulk was observed after annealing at temperatures ranging from 450 oC – 560 oC. The diffusion depth was limited to the end-of-ion-range region where there were still some radiation damage present.No diffusion into the pristine glassy carbon was observed suggestion that diffusion of Sr in glassy carbon can only occur in regions with radiation damage. Annealing the sample at higher temperatures higher than 560 oC resulted in migration of the implanted strontium towards the surface of the glassy carbon substrate. The amount of the accumulated strontium at the surface increased as the annealing temperature is increased. The RBS spectra obtained after annealing the sample isothermally at 650 oC for 2 hours show that there was no further diffusion and accumulation of the strontium during this period.http://www.journals.elsevier.com/vacuum2017-04-30hb2017Physic

Migration behaviour of Europium implanted into single crystalline 6H-SiC

Migration behaviour of Europium (Eu) implanted into 6H-SiC was investigated using Rutherford backscattering spectroscopy (RBS), RBS in a channelling mode (RBS-C) and scanning electron microscopy (SEM). Eu ions of 360 keV were implanted into 6H-SiC at 600 °C to a fluence of 1 × 1016 cm−2. The implanted samples were sequentially annealed at temperatures ranging from 1000 to 1400 °C, in steps of 100 °C for 5 h. RBS-C showed that implantation of Eu into 6H-SiC at 600 °C retained crystallinity with some radiation damage. Annealing of radiation damage retained after implantation already took place after annealing at 1000 °C. This annealing of radiation damage progressed with increasing annealing temperature up to 1400 °C. A shift of Eu towards the surface took place after annealing at 1000 °C. This shift became more pronounced and was accompanied by loss of Eu from the surface at annealing temperatures >1000 °C. This shift was accompanied by broadening of Eu peak/Fickian diffusion after annealing at temperatures >1100 °C. The migration of Eu occurring concurrently with the annealing of radiation damage was explained by trapping and de-trapping of Eu by radiation damage.The National Research Foundation (NRF) (grant no: 94104) of South Africahttp://www.journals.elsevier.com/vacuum2018-07-30hj2017Physic