High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2

Values are presented for thermal conductivity, specific heat, spectral and total hemispherical emissivity of ThO2 (a potential nuclear fuel material) in a temperature range representative of a nuclear accident - 2000 K to 3050 K. For the first time direct measurements of thermal conductivity have been carried out on ThO2 at such high temperatures, clearly showing the property does not decrease above 2000 K. This could be understood in terms of an electronic contribution (arising from defect induced donor/acceptor states) compensating the degradation of lattice thermal conductivity. The increase in total hemispherical emissivity and visible/near-infrared spectral emissivity is consistent with the formation of donor/acceptor states in the band gap of ThO2. The electronic population of these defect states increases with temperature and hence more incoming photons (in the visible and near-infrared wavelength range) can be absorbed. A solid state physics model is used to interpret the experimental results. Specific heat and thermal expansion coefficient increase at high temperatures due to the formation of defects, in particular oxygen Frenkel pairs. Prior to melting a gradual increase to a maximum value is predicted in both properties. These maxima mark the onset of saturation of oxygen interstitial sites

Fusing Multiple Land Cover Products Based on Locally Estimated Map-Reference Cover Type Transition Probabilities

There are a variety of land cover products generated from remote-sensing images. However, misclassification errors in individual products and inconsistency among them undermine their utilities for research and other applications. While it is worth developing advanced pattern classifiers and utilizing the images of finer spatial, temporal, and/or spectral resolution for increased classification accuracy, it is also sensible to increase map classification accuracy through effective map fusion by exploiting complementarity among multi-source products over a study area. This paper presents a novel fusion method that works by weighting multiple source products based on their map-reference cover type transition probabilities, which are predicted using random forest for individual map pixels. The proposed method was tested and compared with three alternatives: consensus-based weighting, random forest, and locally modified Dempster–Shafer evidential reasoning, in a case study, over Shaanxi province, China. For this case study, three types of land cover products (GlobeLand30, FROM-GLC, and GLC_FCS30) of two nominal years (2010 and 2020) were used as the base maps for fusion. Reference sample data for model training and testing were collected following a robust stratified random sampling design that allows for augmenting reference data flexibly. Accuracy assessments show that overall accuracies (OAs) of fused land cover maps have been improved (1~9% in OAs), with the proposed method outperforming other methods by 2~8% in OAs. The proposed method does not need to have the base products’ classification systems harmonized beforehand, thus being robust and highly recommendable for fusing land cover products

Improving the Sintering Behavior of Thorium Oxide

Globally, a great majority of nuclear fuel is composed of a fissile component within a 238UO2 matrix. This can take the form of natural or enriched UO2 or of a mixture of PuO2 with 238UO2. Such fuel is generally well described and has a high reliability. Nonetheless, there are several advantages to supplementing the fuel with rods on the basis of a ThO2 matrix. Thorium dioxide presents several advantages over uranium dioxide as a matrix. These include the higher melting point and chemical stability of thorium dioxide. The 232Th-233U breeding cycle is also superior to the 238U-239Pu cycle in the thermal spectrum. Together these allow the fuel to be kept in the reactor longer under safe conditions. The better chemical stability of ThO2 allows the spent fuel to be stored more safely both in the short and the long term. Yet despite these advantages, thorium dioxide is unlikely to be used as a nuclear fuel matrix in the short term. One of the barriers to commercial adoption is the lack of supply of thorium dioxide pellets which is both reliable and competitive with uranium dioxide on price. Here the same properties which are a boon during operation present barriers during manufacture. ThO2 presents additional costs over UO2 both in the powder precipitation and pellet sintering stage. The higher thermal stability of ThO2, which increases operation safety margins, introduces costs during sintering. It often pushes the sintering temperature needed to achieve the operator-specified density beyond the range of commercial furnaces. Though 232Th breeds better than 238U, its decay chain is less favorable, as it introduces costs during powder precipitation. The half-lives of the decay products are such that significant activity will build up during storage. This can lead to significant waste disposal costs if not adequately addressed. Of the decay products, radium (as 228Ra and 224Ra) is the one which must be dealt with. The other decay products are sufficiently short lived and can be simply allowed to decay away. Ideally radium would be removed alongside thorium into the product, where it would present no additional waste disposal costs. Of the two costs introducing effects, the poor sintering is much more limiting and needs to be addressed first. The common oxalate precipitation route was turned to first to help improve the sintering of ThO2 without introducing milling, which is an undesirable source of radioactive dust. Precipitation at reduced temperatures produced thorium oxalate platelets with a wide size distribution. When calcined, these produced a well-sintering ThO2 powder. If calcined at <800 °C, the powder also pressed to form stable green pellets. These two factors together make ThO2 from oxalate precipitation viable for the commercial nuclear fuel production. Besides the calcination temperature effects, changing the order of addition during precipitation also changed things. Adding oxalate to thorium produced oxalate platelets stacked into cubes which produced fragile green pellets even when calcined at low temperatures. In contrast, adding thorium to oxalate produced thinner platelets with a larger face. Green pellets from the calcined oxide formed stable green pellets at low calcination temperatures. To further expand on the effect of platelet shapes, several different oxalate shapes were produced: both large and small platelets in the shape of squares, rectangles and octagons which were intact or contained holes. The outer shape was shown to have little effect on the sintering. Instead, the sintering was influenced by the size of platelets and the presence or absence of holes. Small platelets of oxalates and/or oxalates containing holes sintered to much higher density, but produced considerably smaller sintered grain sizes. Despite the interesting sintering effects, oxalate precipitation did not precipitate radium sufficiently to remove radioactive waste disposal costs. Because of this, an alternate precipitation strategy needs to be attempted to reduce manufacturing costs further. The thermal decomposition of urea offered good control for homogeneously precipitating Th4+ with ammonia and Ra2+ with carbonate. The process produced very uniform, nanometric ThO2 and had good agglomeration control. It did not, however, present a significant improvement over oxalate precipitation in terms of radium removal. Because the radium removal of urea-based precipitation was insufficient, a move was made instead to a heterogeneous two-step precipitation process on the same ammonia and carbonate basis. The slight sacrifices in terms of uniformity is more than compensated by the good properties this confers. By splitting the ammonia and carbonate steps, the precipitation could take advantage of the good parts of each step. With good pH control, the ammonia step can confer good morphology and filterability on a nanometric ThO2 precipitate. Good filterability allows the product to be easily separated from the liquid phase by a simple, low investment-cost method. After the ThO2 precipitate is fully formed, adding carbonate binds radium to the precipitate very efficiently. The produced fresh product is a nanometric ThO2, with large quantities of weakly bound water and carbon dioxide. These beneficial processing characteristics of good filtering and efficient radium removal make the process good for a powder manufacturer. Upon calcination, the ThO2 coarsens gradually but retains some porosity in the agglomerates produced. This internal porosity gives the powder some softness, which allows it to press into stable green pellets. The coarsening reduces sinterability slightly, but the green pellets still sinter to closed porosity. Because it forms stable green pellets and sinters to closed porosity, ThO2 precipitated in two-steps with ammonia and carbonate is also suitable for the fuel manufacturer. On the basis of these findings, the two-step ammonia-carbonate precipitation process might outperform the simpler one-step oxalate precipitation route by reducing or avoiding radium contaminated liquid effluents. Further studies about the scalability of the two-step process will be needed to confirm its industrial viability.status: publishe

The impact of China's Uighur policy on its international soft power

Provincie Sin-ťiang hraje klíčovou roli ve snahách Čínské vlády zajistit její neustálý rozvoj jak pro její domácí tak pro její mezinárodní politiku. Čínská vláda je odhodlaná zabezpečit Sin-ťiang všemi možnými způsoby, čemuž svědší její rozsáhlé utlačovéní Ujgurů, které Čína považuje za nebezpečnou menšinu ohrožující stabilitu v regionu. Zatímco mezinárodní komunita se dozvídá o zločinech proti Ujgurům, Čína využívá kombinaci měkké a tvrdé síly pro zmírnění negativního dopadu svých činů na její mezinárodní vjem. Zároveň uplatňuje mohutnost své ekonomiky pro odrazení významné mezinárodní odezvy. Tato práce zkoumá nástroje Čínské měkké síly a její odlišný přístup k jejich využití v rozvojových a rozvinutých zemích a jaký dopad události v Sin-ťiangu budou mít na schopnost Číny využívat měkké síly pro prosazování svých globálních ambicí.As China seeks to achieve continued growth and rapid development, it has deemed Xinjiang to be a crucial piece of both its international and domestic policy puzzle. It is willing to go to great lengths to secure the region, as seen in its large-scale repression of the Uyghurs, whom China views as a dangerous and restive minority threatening stability in the region. As the international community learns of the abuses carried out against the Uyghur people, China has deployed a combination of hard and soft power to shore up the negative impact on its global image while using its economic might to discourage any meaningful international response. The thesis explores China's soft power toolkit and its differing approach to soft power in developed and developing countries and what impact the developments in Xinjiang will have on China's ability to use soft power to further its global ambition

Simulated UO2 fuel containing CsI by Spark Plasma Sintering

Herein, an innovative preparation procedure has been deployed enabling, for the first time, the incorporation of volatile fission product simulant into highly dense nuclear fuel pellets. Highly volatile fission products were embedded in a dense UO2 matrix in the form of CsI by simply mixing starting materials and consolidation in a Spark Plasma Sintering step at 1000°C with a 5 minutes dwell time. CsI particles were evenly distributed throughout the pellet and were located at the grain boundaries. The sintering rate is dependent on the O/U ratio of the powder. Addition of CsI also acts as a sintering aid, reducing the temperature of maximum densification.JRC.E.4-Nuclear Fuel Safet

Spark Plasma Sintering at JRC-ITU: an overview of the ongoing activities

Spark Plasma Sintering (SPS) is an electric field assisted technique, currently being widely investigated in industrial and academic ceramic materials research communities. It allows fast and low temperature material densification and thus enables the synthesis of materials with unique microstructures and properties. Very recently JRC-ITU integrated a unique downscaled version of a SPS device in a glovebox, opening a new realm of opportunities for the synthesis of ceramics bearing transuranic elements. First results on the sintering of actinide dioxides will be presented along with a study on the incorporation of elements and compounds, hitherto impossible due to their high vapour pressure or low melting points. An outlook of the future developments, with a focus on safety of nuclear fuels, will be presented.JRC.E.4-Nuclear Fuel Safet

Alterations of thorium oxalate morphology by changing elementary precipitation conditions

Oxalates of actinide elements are widely used in research and industry mainly due to their low solubility in aqueous solution and easy conversion to oxide. Although thorium oxide is worldwide mostly produced by the oxalate precipitation and conversion route, the powder morphology obtained through this process is known to inhibit the packing and sintering step of the pellet production. The presented work investigates the effects of oxalate precipitation conditions on the final powder morphology. Among the precipitation conditions considered are: pH of the thorium feed solution, concentration, temperature and the order of addition (thorium solution in oxalic acid solution and vice versa) known as reverse/direct strike. Herein, we show that the morphology of the final oxalate depends significantly on the above mentioned precipitation parameters.status: publishe