Caesium | EPFL Graph Search

Caesium (IUPAC spelling; cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium has physical and chemical properties similar to those of rubidium and potassium. It is pyrophoric and reacts with water even at . It is the least electronegative element, with a value of 0.79 on the Pauling scale. It has only one stable isotope, caesium-133. Caesium is mined mostly from pollucite. Caesium-137, a fission product, is extracted from waste produced by nuclear reactors. It has the largest atomic radius of all elements whose radii have been measured or calculated, at about 260 picometers. The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a "getter

Characterisation of high-burnup LWR fuel rods through gamma tomography

Stefano Caruso

Current fuel management strategies for light water reactors (LWRs), in countries with high back-end costs, progressively extend the discharge burnup at the expense of increasing the 235U enrichment of the fresh UO2 fuel loaded. In this perspective, standard non-destructive assay (NDA) techniques, which are very attractive because they are fast, cheap, and preserve the fuel integrity, in contrast to destructive approaches, require further validation when burnup values become higher than 50 GWd/t. This doctoral work has been devoted to the development and optimisation of non-destructive assay (NDA) techniques based on gamma-ray emissions from irradiated fuel. It represents an important extension of the unique, high-burnup related database, generated in the framework of the LWR-PROTEUS Phase II experiments. A novel tomographic measurement station has been designed and developed for the investigation of irradiated fuel rod segments. A unique feature of the station is that it allows both gamma-ray transmission and emission computerised tomography to be performed on single fuel rods. Four burnt UO2 fuel rod segments of 400 mm length have been investigated, two with very high (52 GWd/t and 71 GWd/t) and two with ultra-high (91 GWd/t and 126 GWd/t) burnup. Several research areas have been addressed, as described below. The application of transmission tomography to spent fuel rods has been a major task, because of difficulties of implementation and the uniqueness of the experiments. The main achievements, in this context, have been the determination of fuel rod average material density (a linear relationship between density and burnup was established), fuel rod linear attenuation coefficient distribution (for use in emission tomography), and fuel rod material density distribution. The non-destructive technique of emission computerised tomography (CT) has been applied to the very high and ultra-high burnup fuel rod samples for determining their within-rod distributions of caesium and europium fission-product radionuclides. As indicated above, results provided by the transmission tomography measurements were employed in the emission tomography reconstruction phase, together with a calculated global efficiency matrix and input sinograms derived from the processing of measured projections. Different tomographic algorithms were tested and "tuned", on the basis of known test distributions, before being applied to the actual fuel rod measurements. Amongst the various possibilities, the Paraboloidal Surrogates Coordinate Ascent (PSCA) penalised likelihood method has been chosen for presentation of the final results, because it ensures high precision, especially in resolving the most difficult peripheral regions of the rods. The results of the emission tomography have indicated large central depressions in the caesium distributions, but of varying extent from sample to sample. Particularly interesting is the case of the 126 GWd/t sample, showing a very deep central depression (a factor of ∼2.5 for 137Cs, a factor of ∼3 for 134Cs). Differences in the relative activity distributions of 137Cs and 134Cs have, in fact, been observed for all the samples. The depression of 134Cs is more marked than that of 137Cs, probably due to the different origins of the two isotopes. In contrast, the europium shows an almost flat distribution. In order to support the tomographically measured caesium distributions, the results of destructive chemical techniques applied on samples from the same fuel rod (126 GWd/t sample) were examined and found to show reasonably good agreement with the tomography, thus confirming the depressed distributions at the centre of the rod. In addition to the tomographic reconstructions, the present research has also investigated the possibility to use single isotope activities, and/or isotopic concentration ratios from 134Cs, 137Cs, and 154Eu, as burnup indicators at very high and ultra-high burnups. The corresponding non-destructive measurements, performed using three different approaches, have been compared with chemical assays, as well as with reactor physics calculations (CASMO). Whereas the chemical results confirm the current gamma-spectroscopic measurements within uncertainties, the agreement between measurements and calculations is not satisfactory. It has been shown that certain indicators, well established for application at low and medium burnups, suffer a serious loss of reliability at high burnup. Nevertheless, the possibility of successfully employing other burnup monitors has been clearly highlighted. Considering that the overall effort required for the destructive chemical analysis of fuel samples is much greater, the current investigations have clearly demonstrated that non-destructive gamma spectrometry, in conjunction with corresponding transmission and emission tomography measurements, can indeed be considered a valid approach for characterising fuel burnup in the high/ultra-high range.

EPFL2007

First operations with caesium of the negative ion source SPIDER

Riccardo Agnello, Michele Fadone

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H- or D- ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m(-2) was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

2022

Molecular Engineering Towards High Efficiency Perovskite Solar Cells

Jiyoun Seo

High energy demands of our society stimulate the development of sustainable tech-nologies, such as those based on solar energy conversion or photovoltaics. Perovskite solar cells (PSCs) are an emerging photovoltaic technology that consist of the perov-skite film positioned between the electron transport and hole transport layer, while the photovoltaic properties are determined by each component of the structure, as well as the corresponding interfaces. Since 2009, power conversion efficiency of solar cells has already achieved values above 22%. This rapid progress in perovskite solar cell research led to the increased performance and operational stability, which nevertheless remains a challenge towards the commercialization of PSC technologies. In the course of this PhD thesis, new strategies in the design of perovskite solar cells were established based on molecular engineering by addressing (1) perovskite, (2) electron transport, as well as (3) hole transport layers, as detailed in the subsequent chapters. The research objectives focused on two key investigation domains, namely charge recombination in the perovskite grain boundaries or interfaces of perovskite / electron transport layers, and the stability of perovskite and hole transport materials. This approach resulted in the development of highly stable and efficient PSCs. First, with the aim of achieving high performance PSCs, the investigation of crystal growth mechanism was performed to discover the methods to control the grain size distribution within perovskite films that affects their performance by employing or-ganic agents. The study revealed the crystal growth mechanisms and resulted in the achievement of high efficiency of 19.5% in a planar perovskite solar cell architecture. Subsequently, cesium-based perovskites were studied as novel perovskite systems with excellent stability. The role of the cesium in the crystal formation of perovskites was primarily explored, facilitating the rapid room temperature crystallization that led to power conversion efficiency of 18.1% under room temperature. These results are very promising for low temperatue applcation demands, such as the roll-to-roll production and the development of flexible PSCs. In addition to the perovskite absorption layer, the roles of electron and hole transport layers are very important for the PSC performance. In this regard, while the mesoscopic titanium oxide (mp-TiO2) is predominantly used for electron transporting materials, studies of mp-TiO2 are scarce despite its common use in todayâs most effi-cient PSCs. Therefore, following the previous investigation of the perovskite materi-al, we further demonstrated for the first time that bimodal porous TiO2 nanoparticles and their surface doping with cesium halides further strengthens the interaction with the perovskite layer, resulting in very high fill factor of 80% and efficiencies exceed-ing 21%. Finally, a new dopant, Zn(TFSI)2, for organic hole transport materials was explored to enhance thermal stability by replacement of one of the dopant materials. This ap-proach revealed further improved stability with efficiency exceeding 22%. This work on the development of highly efficient and stable perovskite solar cells highlights the utility of molecular engineering and provides the basis for facilitating industrial applications in the near future.

EPFL2018