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In the field of nuclear energy, ensuring the safety of reactors is a top priority. Traditional safety assessments rely on static models that do not fully capture the complexity of real-world accidents. To address this, a new method called Dynamic Probabilistic Safety Assessment (D-PSA) is being developed and applied to modern nuclear reactor designs. D-PSA enhances safety analysis by dynamically simulating how reactor systems interact and evolve during accident scenarios, providing more accurate risk assessments. This work focuses on applying D-PSA to CANDU reactors, a type of nuclear power plant used in Canada and around the world. CANDU reactors have unique features, such as using heavy water as a coolant and moderator, making safety analysis particularly complex. By applying D-PSA to potential CANDU reactor accidents, system behaviour can be modeled more realistically, including how operators and safety systems respond in real time. In parallel, the research also explores the validation of thermal-hydraulic codes—software used to simulate fluid flow and heat transfer in nuclear reactors. For this purpose, the thermal-hydraulic code ASYST4.1 is applied to assess the behavior of small modular reactors (SMRs), a new generation of nuclear reactors designed to be smaller, safer, and more flexible. By validating this code for SMRs, researchers can improve the accuracy of simulations in future studies. The combination of D-PSA with thermal-hydraulic code validation offers a powerful approach to improving nuclear safety. In future research, using both methods together will provide better insights into how reactors behave during accidents and help develop more robust safety measures. This work aims to advance nuclear safety methodologies for both existing reactors like CANDU and future designs like SMRs, ensuring safer and more reliable energy generation. / Thesis / Doctor of Philosophy (PhD) / Ensuring the safety of nuclear reactors is critical, and traditional methods often fall short in capturing real-world accident complexities. This research applies Dynamic Probabilistic Safety Assessment (D-PSA) to CANDU reactors, enhancing the accuracy of risk analysis by dynamically modeling how reactor systems respond during accidents. CANDU reactors, widely used in Canada, have unique features that make safety analysis challenging, and D-PSA provides more realistic insights into potential accident scenarios. Additionally, the study focuses on validating the thermal-hydraulic code ASYST4.1 for small modular reactors (SMRs), a new generation of smaller, safer nuclear reactors. Accurate modeling of fluid flow and heat transfer is crucial for SMR safety assessment. By combining D-PSA with thermal-hydraulic code validation, this research offers a more comprehensive approach to improving nuclear safety for both current reactors like CANDU and future designs like SMRs, ensuring more reliable and secure energy generation.

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One of the objectives of this study is to develop a methodology, together with a set of software programs that evaluate, in a power system, the risks of catastrophic failures caused by hidden failures in the hardware or software components of the protection system. The disturbance propagation mechanism is revealed by the analysis of the 1977 New York Blackout. The step-by-step process of estimating the relay hidden failure probability is presented. A Dynamic Event Tree for the risk-based analysis of system catastrophic failures is proposed. A reduced 179-bus WSCC sample system is studied and the simulation results obtained from California sub-system are analyzed. System weak links are identified in the case study. The issues relating to the load and generation uncertainties for the risk assessment of system vulnerabilities are addressed. A prototype system - the Hidden Failure Monitoring and Control System (HFMCS) - is proposed to mitigate the risk of power system catastrophic failures. Three main functional modules - Hidden Failure Monitoring, Hidden Failure Control and Misoperation Tracking Database - and their designs are presented. Hidden Failure Monitoring provides the basis that allows further control actions to be initiated. Hidden Failure Control is realized by using Adaptive Dependability/Security Protection, which can effectively stop possible relay involvement from triggering or propagating disturbance under stressed system conditions. As an integrated part of the HFMCS, a Misoperation Tracking Database is proposed to track the performance of automatic station equipment, hence providing automatic management of misoperation records for hidden failure analysis. / Ph. D.