Tidal energy systems and specifically tidal barrages are renewable energy systems, that use the potential energy of tides to produce electricity. Actually, there are few tidal power stations operating, although recent studies show the potentiality of installing new units in a recent future due to the constant improvement of the technology related. As in other conventional hydropower systems, the runner is one of the critical components of the unit as it has to withstand an extreme wide range of relative head and two main operating modes (ebb and flood). Therefore, fatigue problems and undesirable failures are more likely to occur. Based on the analysis of a crack found in one of the blades of a prototype, this paper analyzes and discusses under which operating conditions the runner is more likely to suffer from fatigue problems. CFD and FEM simulation models have been used to determine the stress hotspots, which approximate very well the point where the real crack was initiated. Based on a probabilistic approach of an existing initial defect or flaw, the reliability of the blades, or the probability of not growing a crack and therefore last an infinite life, has been calculated for a wide range of operating conditions. While fatigue problems have been deeply discussed and analyzed in conventional turbines, the flow excitation characteristics in these units exhibit important differences and therefore the discussion and results provided in this paper could be useful for future designs and fatigue analyses of tidal turbines.