A B S T R A C T Steam generators are important components of a nuclear reactor. They are subjected to system pressure and force controlled bending moments. Leak before break (LBB) criteria based on a fracture mechanics approach is used to ensure the reliability of the primary heat transport system of nuclear reactors. Experimental studies were recently completed for justifying LBB for the cold end of the shell-nozzle junction of the steam generator of the 500 MWe Fast breeder reactor (FBR) under construction at Kalpakkam. The internal diameter of the main tube is 1120 mm and that of the nozzle is 386.4 mm. The thickness of the main tube is 24 mm while that of the nozzle is 18 mm. Fatigue tests were conducted on two carbon steel and two modified 9Cr1Mo steel shell-nozzle junction specimens at room temperature under constant internal pressure and cyclic in-plane bending moment. Fracture tests were conducted on these components until collapse after the fatigue test. FEM analysis was also performed to identify critical locations and factor of safety for fatigue loading and collapse. The objective of the studies was to obtain the factor of safety for crack initiation, through-wall and critical crack size, and collapse. The studies demonstrated adequate factor of safety for all the above LBB criteria.