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Inspections revealed stress corrosion cracks in low-pressure shrunk-on disks of nuclearturbines. Cracks initiate in blade attaching pin holes and propagate through the disk intwo different ways. First type of crack only propagates in radial direction and second onechanges its paths and develops in two different directions (branched crack). This paperdeals with the FEM approach used to understand the crack paths propagation and toevaluate the harmfulness of these cracks during over speed start-up. 3D and 2D crackeddisk models depending on the size and on the type of cracks have been developed. 2D FEmodels are validated on 3D FE models by comparing results. Loads due to the rotatingblades and to the shrinking effect are taken into account through a contact modelintroduced at the pin hole surfaces and at the contact surface between the disk and therotor. The stress intensity factors have been calculated and compared to the fracturetoughness. Results show that the crack harmfulness strongly depends on the angular sectorof the disk model used in calculations, which is equal to say that the disk lifetime dependson the number of crack affecting the disk. The branched crack path has been explainedthrough the analysis of the G parameter values obtained from the 2D calculations. A goodagreement between theoretically predicted and experimental crack path is noticed.