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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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