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Current industrial trends to increase power production challenge the initial safety design limits of plants, which were evaluated generally using conservative tools and hypotheses. Advanced numerical tools and methods allow demonstrating that safety margins are still respected. These tools are modern fully validated thermal-hydraulic codes, coupled thermal-hydraulic/neutron-kinetic codes, and methodologies that use realistic hypotheses (rather than conservative ones) and estimate the uncertainty. The paper illustrates modern computational tools and approaches for improving plant operation and control and for supporting design modifications on existing nuclear power plants. A survey performed by Joint Research Centre's Institute for Energy is also presented and summarizes the feedback received by several countries of the European Union on the impact of modern Thermal-Hydraulics and Neutron-Kinetics tools on Operational Limits and Conditions and Operating Instructions and Procedures.