High cycle thermal fatigue due to the mixing of hot andcold fluids may initiate cracking in pipes of safety relatedcircuits. A method has been developed to identify suchfluid mixing zones subjected to potential thermal fatiguedamage. This method is based on a loading model and amechanical model that depend on the main characteristicsof the mixing zone and on the material properties. It issupported by a large experimental program. This methodhas been applied to all the mixing zones of safety relatedcircuits of the EDF pressurised water reactors, allowingto identify sensitive zones and to apply an appropriateinspection program that ensures the control of the risk dueto this damage mechanism.