The new concept of Multi-Level Current Reinjection (MLCR) combines the advantages of DC ripple reinjection, multi-level conversion and soft-switching technique. Taking advantage of the soft-switching technique which uses zero current switching for the main bridge switches, thyristor based MLCR current source converter (CSC) is proposed. This concept adds self-commutation capability to thyristors and produces high quality line current waveforms. Various thyristor based MLCR CSC topologies have been simulated extensively using PSCAD/EMTDC in this thesis and their performance characteristics investigated. Questions have been raised about the ability to force the main thyristors off using the reinjection bridge in a real-world implementation, where there are inevitable stray capacitances and inductances which may influence the thyristor turn-off; and simulation switching models may not represent the switching characteristics fully or accurately. For this proof of concept, a small scale prototype has been built in the laboratory. The 3-level MLCR CSC, which increases the pulse number from 12 to 24, is chosen to verify the concept. The experimental investigation of the 3-level MLCR CSC, under steady-state conditions, verified the following: • The reinjection current allows the main bridge thyristors to be switched at negative firing angles. • This current reinjection technique allows self-commutation capability in a practical system despite the finite turn-off times of the thyristor. • This current reinjection technique improves the harmonic characteristics of the thyristor based converter. • It is observed that the deviation of the actual waveforms from the theoretical waveforms is mainly due to the snubber across the reinjection switch, and a trade-off in the choice of snubber components is required.

The application of multi-concentration group mathematical modelling to the leaching of radionuclide waste-forms which have continuous porous matrix such as cemented waste form is described. The modelling is illustrated analysing a hypothetical of some transport mechanisms such as molecular diffusion, ionic migration and convective flow for release of interest radionuclide from a solidified waste form which contains discrete particles of radioactive Sr-85 nuclides into the aqueous environment. The group parameters are derived from the classical electrochemistry concept of ion transport in dilute electrolytic solution. The numerical analysis is based on the Crank-Nicolson Implicit Methods which assures the stability of the solution at a practical time step. The results show that, for a short-time period of leaching in demineralized water the leaching behavior follows the predominating diffusion mechanism. After this point, the role of other processes apparent and continue until all radionuclides in the cement waste are leached out when compared to the Semi-Infinite Diffusion model which is based on pure diffusion mechanism. The multi-concentration group model can also be applied to long-term prediction of complicated release mechanisms of the radionuclides in the waste form of a particular disposal environment, unless the variables of interest such as the corrosion rate, the chemical reaction, erosion rate and etc. are determined by experiment or theoretical hypothesis. The appropriate differential equation then can be solved by the same general numerical approach. Also, the results of the modelling indicate which parameters should be measured experimentally in order to provide a quantitative test of the hypothetical release mechanism.

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Globally, sea levels have risen in the last century, and various projections suggest substantial increases in sea level due to climate change in this century. In Nigeria, there are no up-to-date sea level rise (SLR) assessments for the coast. Much of the Nigerian coast is low lying with the consequence that a 1 to 3 metres rise in sea level, which may result from eustatic or climate change, will have a catastrophic effect on the human activities in these regions. This study examines the consequences of continued sea level rise with a focus on erosion and inundation for the Nigerian coast and considers the coastal management practices of coastal partnerships (CPs). The Nigerian coast has been delineated according to distinct geomorphological units, which include the Barrier, Mud, Delta and Strand coasts. The Bruun model has been used to compute shoreline recession along the Nigerian coast with the exception of the Mud coast. A Geographic Information System (GIS) was used to develop inundation models and examine the impact scenarios that SLR will have on critical elements, which include land, population, economic activity (Gross Domestic Product), urban extent, agriculture and wetlands with the aid of high quality spatially disaggregated global data. A case study approach was used to assess the management practices of Pro-Natural International Nigeria; Niger Delta Wetland Centre, Niger Delta Development Commission; and Flood Erosion and Coastal Zone Management, Rivers State with the aid of a suite of systemic sustainability appraisal indices. Results indicate that shoreline recession will be mild along the coast while substantial loss due to inundation of the critical elements is expected for all the scenarios considered. The sustainability assessment indicates that the CPs did not meet the required standard for sustainability, however there was evidence of constructive management in some of them. This study has been able to provide up-to-date baseline data concerning the vulnerability of the coast to SLR for the four coastal systems in Nigeria. The coastal sustainability assessment, which is the first ever in Nigeria, reflects the need for corrective measures in the management practices of the CPs to achieve a sustainable coast in the light of coastal hazards.