This paper identifies the underlying physical mechanisms that frame the flexibility and operation of nuclear technologies, with analysis focused on the effects of two structural characteristics of nuclear fleets, i.e. nuclear schedule optimization and minimal power variations, on simplified French power system simulations. We develop a method to simulate nuclear schedule optimization to reflect how plant managers maximize plant availability during peak-demand periods. Using this schedule optimization, we compute each plant's minimum power level and its variation over time to evaluate the flexibility potential. Three nuclear flexibility hypotheses are considered: one where fleet schedule is considered constant, which is standard practice in the energy systems modeling literature, one with an optimized fleet schedule with constant minimum power, and one with an optimized fleet schedule with physics-induced minimum power variations. Sensitivity analysis highlights the links between the fleet's schedule optimization, minimal power variations, relative share of nuclear and renewables in the capacity mix, and results of simulation models. We find that the nuclear fleet's optimization and associated realistic minimum power variations gain in importance with increasing relative share of nuclear in the capacity mix. The schedule's importance holds as renewables' installed capacities heighten as the residual demand level decreases. This paper highlights the potential benefits of modeling nuclear schedule optimizations and the resulting minimum power evolutions. Both aspects are crucial for evaluating nuclear flexibility characteristics in simplified low-carbon electric systems that use a significant share of renewable energy.