Scientific Highlights LES
Yanting Qian has received the MSc/PhD competition award in the FISA2022-EURADWASTE'22 conference. She works on the retention of redox-sensitive Tc on Fe-bearing clay minerals. Clay minerals are the main component of the engineered barriers and the argillaceous host rocks considered in several deep geological repository concepts. Due to their natural content of structural Fe and adsorbed surface Fe, clay minerals can potentially slow down the migration of radioactive redox-sensitive element 99Tc by reducing TcO4- to lower and less soluble TcO2∙nH2O-like species. A few studies have shown that Fe-bearing minerals are able to catalyse this reduction/precipitation process, but the exact nature of the (surface) precipitates, the geochemical formation conditions as well as the mechanism of the redox reaction are not fully understood. This study focuses on the Tc sorption and electron transfer behind the redox reaction. By combining Tc K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, mediated oxidation and reduction experiments (MEO/MER), Mössbauer spectrometry, and (high-resolution) transmission electron microscopy, the contributions of the different FeII/FeIII ratios to the overall Tc retention process and to the surface products speciation can be identified. The results of this study are expected to improve the understanding of the Tc retention on FeII/FeIII bearing clay minerals, and thus contribute to a more reliable prediction of the Tc retention in radioactive waste repositories.
Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites.
Geological waste disposal, cement clay interaction
• A considerable reduction of HTO and 36Cl− was observed after 6 years interaction.
• The chloride flux showed a much stronger reduction compared to HTO.
• For HTO the relation between the De and the porosity in the clay part can be described using Archie's law.
• No complete clogging of the porosity was observed after 6 years interaction.
The credibility of long-term safety assessments of radioactive waste repositories may be greatly enhanced by a molecular level understanding of the sorption processes onto individual minerals present in the near- and far-fields. A study conducted at LES in collaboration with the Helmholtz Zentrum Dresden Rossendorf used extended X-ray absorption fine structure (EXAFS) and time-resolved laser fluorescence spectroscopies (TRLFS) to elucidate the uptake mechanism of trivalent lanthanides and actinides (Ln/AnIII) by the clay mineral montmorillonite.The excellent agreement between the thermodynamic model parameters obtained by fitting the macroscopic data, and the spectroscopically identified mechanisms, demonstrates the mature state of the 2SPNE SC/CE sorption model developed at LES for predicting and quantifying the retention of Ln/AnIII elements by montmorillonite-rich clay rocks.
An interdisciplinary study conducted at different PSI laboratories (LES, AHL, LRS, SYN) in collaboration with Studsvik AB (Sweden) demonstrates that selenium originating from fission in light water reactors is tightly bound in the crystal lattice of UO2. This finding has positive consequences for the safety assessment of high-level radioactive waste repository planned in Switzerland, as it implies (contrary to previous assumptions) that the safety-relevant radionuclide 79Se will be released at extremely low rates during aqueous corrosion of the waste in a deep-seated repository.
Drei Jahre in Folge hat Jenna Poonoosamy den Preis für die beste Präsentation am Doktorandentag im Bereich Nukleare Energie und Sicherheit (NES) am Paul Scherrer Institut PSI erhalten. Poonoosamy stammt ursprünglich von der Insel Mauritius im Indischen Ozean. Schon in der Schule interessierte sie sich vor allem für Chemie. «Die meisten meiner Freunde wollten in die Wirtschaft», erzählt sie. «Mich dagegen haben die Naturwissenschaften fasziniert.» Und so zog sie nach der Schule zum Chemie-Studium nach Paris. Und kam später für ihre Doktorarbeit ans PSI.
Obwohl die Schweiz aus der Kernenergie aussteigt, muss sie eine Lösung für das in den Kernkraftwerken, aber auch in Medizin, Industrie und Forschung entstandene, radioaktive Material finden. Daher stellt sie sich einer aussergewöhnlichen, verantwortungsvollen Aufgabe: Sie sucht einen Ort, an dem sie ihre radioaktiven Abfälle mehrere hunderttausend Jahre lang sicher lagern kann. So lange, bis sie von selbst die Radioaktivität natürlicher Gesteine erreicht haben.
When bridges, dam walls and other structures made of concrete are streaked with dark cracks after a few decades, the culprit is the so-called the concrete disease. Researchers from the Paul Scherrer Institute PSI and Empa have now solved the structure of the material produced in these cracks at atomic level - and have thereby discovered a previously unknown crystalline arrangement of the atoms.
In a deep geological repository, low and intermediate level radioactive waste from nuclear applications is solidified by cementitious materials for several thousand years. Researchers from the Paul Scherrer Institute and the Karlsruhe Institute of Technology have now demonstrated how cement limits the mobility of those radioactive substances. The new findings improve our understanding of the processes involved in this early phase of deep geological disposal.
A study by the Centre for Technology Assessment TA-Swiss, coordinated by the Paul Scherrer Institute, recommends further pursuing deep geothermal energy in Switzerland. The energy resources underground are vast, environmentally friendly to extract and available around the clock, the authors conclude. The earthquake risk and the cost of electricity production, which are still too high, however, remain challenges that society needs to weigh up against the advantages of deep geothermal energy.