In the frame of the Reactor Pressured Vessel integrity demonstration, a defect assessment has to be performed considering a Pressurized Thermal Shock. Several approaches are proposed in different reference codes and standards. Usually, the analyses are based on analytical methods. An important effort has been made recently by CEA to improve these tools in the frame of R&D activities funded by IRSN. For the assessment of an underclad defect in a vessel submitted to a pressurized thermal shock, the plasticity induced by cladding yielding is considered through the amplification β of the elastic stress intensity factor KI in the ferritic part of the vessel. The current solution in the French RSE-M code has been developed from a fitting of F.E. calculation results. A more physical solution is proposed in this paper. It takes into account two phenomena: the amplification of the elastic KI due to the plasticity in the cladding and a plastic zone correction in the ferritic part. The first amplification is determined assuming that the plasticity in the cladding could be represented as an imposed opening displacement on the crack lips.

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