In the last three decades, core design and operational requirements of German PWRs have evolved significantly in order to optimize plant operation and to react to changed market conditions. In the first phase, steady state operation was optimized to limit plant operation costs or increase availability and power output. The second phase of the evolution was triggered by the necessity of flexible ... [Show full abstract] plant operation in order to compensate fluctuating power generation by wind and solar power and by optimization of phase out cores. The changes of core design and operation parameters had among the direct, expected impacts on safety analyses and reactor physics also several indirect and unexpected ones. Examples for expected impacts are changes of reactivity coefficients or boron worth compared to safety analysis assumptions or changes of possible initial conditions of incidents. Unexpected impacts showed for example on rod bow, cladding corrosions or neutron noise. The tasks of TÜV NORD EnSys as technical expert organization is to identify all these impacts of core design and operational parameter evolution, evaluate whether these impacts compromise plant safety, evaluate models and correlations regarding unexpected effects, assess if intended countermeasures are effective and acceptable and evaluate whether the set and ranges of plant parameters used for core limitation and protection are still valid. The paper gives an overview over selected experiences and tasks performed by the Reactor Physics and Criticality Safety Group of TÜV NORD EnSys for the evolution of core designs and operational requirements in German PWRs.