Calculation of the eigenfunctions of the two-group neutron diffusion equation and application to modal decomposition of BWR instabilities

In this thesis, numerical methods aiming at determining the eigenfunctions, their adjoint and the corresponding eigenvalues of the two-group neutron diffusion equations representing any heterogeneous system are investigated. First, the classical power iteration method is modified so that the calculation of modes higher than the fundamental mode is possible. Thereafter, the Explicitly-Restarted Arnoldi method, belonging to the class of Krylov subspace methods, is touched upon. Although the modified power iteration method is a computationally-expensive algorithm, its main advantage is its robustness, i.e. the method always converges to the desired eigenfunctions without any need from the user to set up any parameter in the algorithm. On the other hand, the Arnoldi method, which requires some parameters to be defined by the user, is a very efficient method for calculating eigenfunctions of large sparse system of equations with a minimum computational effort. These methods are thereafter used for off-line analysis of the stability of Boiling Water Reactors. Since several oscillation modes are usually excited (global and regional oscillations) when unstable conditions are encountered, the characterization of the stability of the reactor using for instance the Decay Ratio as a stability indicator might be difficult if the contribution from each of the modes are not separated from each other. Such a modal decomposition is applied to a stability test performed at the Swedish Ringhals-1 unit in September 2002, after the use of the Arnoldi method for pre-calculating the different eigenmodes of the neutron flux throughout the reactor. The modal decomposition clearly demonstrates the excitation of both the global and regional oscillations. Furthermore, such oscillations are found to be intermittent with a time-varying phase shift between the first and second azimuthal modes.

Electrochemical oxidation of Kraft lignin for the production of value-added chemicals on Ni and Co electrocatalysts

La vanillina è un’aldeide aromatica importante da un punto di vista industriale, in quanto viene ampiamente utilizzata dall’industria alimentare, cosmetica e farmaceutica. Attualmente, la vanillina da biomasse viene ottenuta attraverso l’ossidazione catalitica della lignina. Un’alternativa è rappresentata dall’ossidazione elettro-catalitica, un processo che sta riscuotendo un notevole interesse, perché permette di lavorare in condizioni blande. L’obiettivo di questo lavoro è stato quello di sintetizzare elettro-catalizzatori che favoriscano la depolimerizzazione della lignina Kraft per ottenere selettivamente vanillina. Sono state utilizzate schiume di Ni a cella aperta, tal quali e elettro-depositate con idrossidi di Ni-Co e Co. La formazione degli osso-idrossidi dei metalli, sulla superficie delle schiume, e la OER contribuiscono all’elettro-ossidazione della lignina, mentre la resa di vanillina dipende sia dal catalizzatore che dalle condizioni di reazione (potenziale applicato e tempo di reazione). La resa maggiore di vanillina è stata ottenuta applicando 0.6 V vs SCE con un tempo di reazione di un’ora e utilizzando la schiuma di Ni bare come catalizzatore. Indipendentemente dal tipo di catalizzatore usato, aumentando il tempo di reazione la resa di vanillina diminuisce, probabilmente a causa delle reazioni di ri-condensazione e ossidazione successiva dei prodotti che coinvolgono la vanillina stessa. La presenza di idrossidi di Ni-Co e Co sulla schiuma di Ni non ne migliora l’attività catalitica. La schiuma Co/Ni esibisce un’elevata carica accumulata e un’alta conversione, probabilmente dovuto alle reazioni parassite che sfavoriscono l’accumulo di vanillina. Le schiume Ni-Co/Ni invece, presentando sia una resa in vanillina intermedia tra le altre due ma associata ad una carica accumulata molto bassa. Un risultato incoraggiante per possibili sviluppi futuri.

Electrochemical oxidation of Kraft and Dealkaline lignin to produce value-added chemicals on Nickel electrocatalysts

One of the most important scientific and environmental issues is reducing global dependence on fossil sources and one of the solutions is to use biomass as feedstock. In particular, the use of lignocellulosic biomass to obtain molecules with considerable commercial importance is gaining more and more interest. Lignin, the most recalcitrant part of lignocellulosic biomass, is a valuable source of sustainable and renewable aromatic molecules, currently produced from petrochemical processes. Vanillin, one of the most important aromatic aldehydes on an industrial level, can be obtained through catalytic lignin oxidation. An alternative to the conventional catalytic oxidation process is the electro-catalytic process, which can be carried out at ambient temperature and pressure, using water as solvent, and it can be considered as a renewable energy storage. In this thesis, the electrocatalytic oxidation of Kraft and Dealkaline lignin in NaOH was investigated over Ni foam catalysts. The effect of the reaction parameters (i.e. time, applied potential, lignin concentration, NaOH concentration, and temperature) on the yields of vanillin and other valuable products was evaluated. After the screening of the reaction conditions, a systematic study of the contribution of the homogeneous reaction (lignin depolymerization due to the basic solvent) to the yield of the product was accomplished. Finally, considering the obtained results, an alternative reaction procedure was proposed.

Development of wearable electrochemical sensors for the determination of biological analytes

Wearable biosensors are attracting interest due to their potential to provide continuous, real-time physiological information via dynamic, non-invasive measurements of biochemical markers in biofluids, such as interstitial fluid (ISF). One notable example of their applications is for glycemic monitoring in diabetic patients, which is typically carried out either by direct measurement of blood glucose via finger pricking or by wearable sensors that can continuously monitor glucose in ISF by sampling it from below the skin with a microneedle. In this context, the development of a new and minimally invasive multisensing tattoo-based platform for the monitoring of glucose and other analytes in ISF extracted through reverse iontophoresis in proposed by the GLUCOMFORT project. This elaborate describes the in-vitro development of flexible electrochemical sensors based on inkjet-printed PEDOT:PSS and metal inks that are capable of determining glucose and chloride at biologically relevant concentrations, making them good candidates for application in the GLUCOMFORT platform. In order to make PEDOT:PSS sensitive to glucose at micromolar concentrations, a biocompatible functionalization based on immobilized glucose oxidase and electrodeposited platinum was developed. This functionalization was successfully applied to bulk and flexible amperometric devices, the design of which was also optimized. Using the same strategy, flexible organic electrochemical transistors (OECTs) for glucose sensing were also made and successfully tested. For the sensing of chloride ions, an organic charge-modulated field-effect transistor (OCMFET) featuring a silver/silver chloride modified floating gate electrode was developed and tested.

Thermodynamics of hydrogen absorption in Mg thin films and multilayers studied by electrochemical method

Electrochemical hydrogen loading is a technique used to produce and study the hydrogenation in metals starting from a liquid solution containing water. It is a possible alternative to another, well-established technique which loads hydrogen starting from its gas phase. In this work, the electrochemical method is used to understand the fundamental thermodynamics of hydrogen loading in constraint systems such as thin films on substrates, and possibly distinguish the role of interfaces, stresses and microstructure during the hydrogenation process. The systems under study are thin films of Pd, Mg/Pd, and Ti/Mg multilayers. Possible future technological applications may be in the field of hydrogen storage and hydrogen sensors. Towards the end, the experimental setup is modified by introducing an automatic relay. This change leads to improvements in the data analysis and in the attainable information on the kinetics of the systems.