Power Transient Analysis of Experimental Devices for Jules Horowitz Material Testing Reactor (JHR)

The objective of this thesis is the power transient analysis concerning experimental devices placed within the reflector of Jules Horowitz Reactor (JHR). Since JHR material testing facility is designed to achieve 100 MW core thermal power, a large reflector hosts fissile material samples that are irradiated up to total relevant power of 3 MW. MADISON devices are expected to attain 130 kW, conversely ADELINE nominal power is of some 60 kW. In addition, MOLFI test samples are envisaged to reach 360 kW for what concerns LEU configuration and up to 650 kW according to HEU frame. Safety issues concern shutdown transients and need particular verifications about thermal power decreasing of these fissile samples with respect to core kinetics, as far as single device reactivity determination is concerned. Calculation model is conceived and applied in order to properly account for different nuclear heating processes and relative time-dependent features of device transients. An innovative methodology is carried out since flux shape modification during control rod insertions is investigated regarding the impact on device power through core-reflector coupling coefficients. In fact, previous methods considering only nominal core-reflector parameters are then improved. Moreover, delayed emissions effect is evaluated about spatial impact on devices of a diffuse in-core delayed neutron source. Delayed gammas transport related to fission products concentration is taken into account through evolution calculations of different fuel compositions in equilibrium cycle. Provided accurate device reactivity control, power transients are then computed for every sample according to envisaged shutdown procedures. Results obtained in this study are aimed at design feedback and reactor management optimization by JHR project team. Moreover, Safety Report is intended to utilize present analysis for improved device characterization.

Automotive diesel engine transient operation: modeling, optimization and control

Traditionally, the study of internal combustion engines operation has focused on the steady-state performance. However, the daily driving schedule of automotive engines is inherently related to unsteady conditions. There are various operating conditions experienced by (diesel) engines that can be classified as transient. Besides the variation of the engine operating point, in terms of engine speed and torque, also the warm up phase can be considered as a transient condition. Chapter 2 has to do with this thermal transient condition; more precisely the main issue is the performance of a Selective Catalytic Reduction (SCR) system during cold start and warm up phases of the engine. The proposal of the underlying work is to investigate and identify optimal exhaust line heating strategies, to provide a fast activation of the catalytic reactions on SCR. Chapters 3 and 4 focus the attention on the dynamic behavior of the engine, when considering typical driving conditions. The common approach to dynamic optimization involves the solution of a single optimal-control problem. However, this approach requires the availability of models that are valid throughout the whole engine operating range and actuator ranges. In addition, the result of the optimization is meaningful only if the model is very accurate. Chapter 3 proposes a methodology to circumvent those demanding requirements: an iteration between transient measurements to refine a purpose-built model and a dynamic optimization which is constrained to the model validity region. Moreover all numerical methods required to implement this procedure are presented. Chapter 4 proposes an approach to derive a transient feedforward control system in an automated way. It relies on optimal control theory to solve a dynamic optimization problem for fast transients. From the optimal solutions, the relevant information is extracted and stored in maps spanned by the engine speed and the torque gradient.

Kinetics of photogenerated carriers in nanostructured semiconductor heterojunctions for solar water splitting

This thesis aims to investigate the fundamental processes governing the performance of different types of photoelectrodes used in photoelectrochemical (PEC) applications, such as unbiased water splitting for hydrogen production. Unraveling the transport and recombination phenomena in nanostructured and surface-modified heterojunctions at a semiconductor/electrolyte interface is not trivial. To approach this task, the work presented here first focus on a hydrogen-terminated p-silicon photocathode in acetonitrile, considered as a standard reference for PEC studies. Steady-state and time-resolved excitation at long wavelength provided clear evidence of the formation of an inversion layer and revealed that the most optimal photovoltage and the longest electron-hole pair lifetime occurs when the reduction potential for the species in solution lies within the unfilled conduction band states. Understanding more complex systems is not as straight-forward and a complete characterization that combine time- and frequency-resolved techniques is needed. Intensity modulated photocurrent spectroscopy and transient absorption spectroscopy are used here on WO3/BiVO4 heterojunctions. By selectively probing the two layers of the heterojunction, the occurrence of interfacial recombination was identified. Then, the addition of Co-Fe based overlayers resulted in passivation of surface states and charge storage at the overlayer active sites, providing higher charge separation efficiency and suppression of recombination in time scales that go from picoseconds to seconds. Finally, the charge carrier kinetics of several different Cu(In,Ga)Se2 (CIGS)-based architectures used for water reduction was investigated. The efficiency of a CIGS photocathode is severely limited by charge transfer at the electrode/electrolyte interface compared to the same absorber layer used as a photovoltaic cell. A NiMo binary alloy deposited on the photocathode surface showed a remarkable enhancement in the transfer rate of electrons in solution. An external CIGS photovoltaic module assisting a NiMo dark cathode displayed optimal absorption and charge separation properties and a highly performing interface with the solution.