Cement As a Waste Form for Nuclear Fission Products: The Case of 90Sr and Its Daughters

One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of ⁹⁰Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that ⁹⁰Sr is stable when it substitutes the Ca²⁺ ions in C-S-H, and so is its daughter nucleus ⁹⁰Y after β-decay. Interestingly, ⁹⁰Zr, daughter of ⁹⁰Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for ⁹⁰Sr storage.

Engineering Simulations

Over recent years, ionic liquids have emerged as a class of novel fluids that have inspired the development of a number of new products and processes. The ability to design these materials with specific functionalities and properties means that they are highly relevant to the growing philosophy of chemical-product design. This is particularly appropriate in the context of a chemical industry that is becoming increasingly focussed on small-volume, high-value added products with relatively short times to market. To support such product and process development, a number of tools can be utilised. A key requirement is that the tool can predict the physical properties and activity coefficients of multi-component mixtures and, if required, model the process in which the materials will be used. Multi-scale simulations that span density functional theory (DFT) to process-engineering computations can address the relevant time and length scales and have increased in usage with the availability of cheap and powerful computers. Herein we will discuss the area of engineering calculations relating to the design of ionic liquid processes, that is, the computational tools that bridge this gap and allow for process simulation tools to utilise and assist in the design of ionic liquids. It will be shown that, at present, it is possible to use available tools to estimate many important properties of ionic liquids and mixtures containing them with a sufficient level of accuracy for preliminary design and selection.

Insight into CO Activation over Cu(100) under Electrochemical Conditions

The reduction of CO₂ on copper electrodes has attracted great attentions in the last decades, since it provides a sustainable approach for energy restore. During the CO₂ reduction process, the electron transfer to CO_ads is experimentally suggested to be the crucial step. In this work, we examine two possible pathways in CO activation, i.e. to generate COH_ads and CHO_ads, respectively, by performing the state-of-the-art constrained ab initio molecular dynamics simulations on the charged Cu(100) electrode under aqueous conditions, which is close to the realistic electrochemical condition. The free energy profile in the formation of COH_ads via the coupled proton and electron transfer is plotted. Furthermore, by Bader charge analyses, a linear relationship between C-O bond distance and the negative charge in CO fragment is unveiled. The formation of CHO_ads is identified to be a surface catalytic reaction, which requires the adsorption of H atom on the surface first. By comparing these two pathways, we demonstrate that kinetically the formation of COH_ads is more favored than that of CHO_ads, while CHO_ads is thermodynamically more stable. This work reveals that CO activation via COH_ads intermediate is an important pathway in electrocatalysis, which could provide some insights into CO₂ electroreduction over Cu electrodes.

High-order-harmonic generation in benzene with linearly and circularly polarised laser pulses

High-order-harmonic generation in benzene is studied using a mixed quantum-classical approach in which the electrons are described using time-dependent density functional theory while the ions move classically. The interaction with both linearly and circularly polarised infra-red ($\lambda = 800$ nm) laser pulses of duration 10 cycles (26.7 fs) is considered. The effect of allowing the ions to move is investigated as is the effect of including self-interaction corrections to the exchange-correlation functional. Our results for circularly polarised pulses are compared with previous calculations in which the ions were kept fixed and self-interaction corrections were not included while our results for linearly polarised pulses are compared with both previous calculations and experiment. We find that even for the short duration pulses considered here, the ionic motion greatly influences the harmonic spectra. While ionization and ionic displacements are greatest when linearly polarised pulses are used, the response to circularly polarised pulses is almost comparable, in agreement with previous experimental results.