Abstract Initiation and slow propagation of cracks in a pressure vessel loaded by severe PTS-transients were studied. J values were calculated using linear-elastic and elastic-plastic material properties. The calculated J values were compared with the J–R curves of the irradiated Round compact tension (RCT)-test specimens at upper shelf temperature. Cladding and weld seam residual stresses were taken into account in the calculations. For short three-dimensional surface cracks, the influence of cladding depends upon the shape of the crack. In the case of a deep three-dimensional crack, very high J values exist in the cladding and at the cladding/base metal interface. For shallow surface cracks, the influence of cladding is not so significant at this interface. The effect of cladding and its residual stresses was also studied in the laboratory using small cladded compact tension (CT) specimens. The first specimens were made of carbon steel and cladded in two layers. Finite element calculations were performed in order to study the J-integral distribution in these CT specimens. Exact fatigue crack profile and cladding residual stresses were taken into account in these calculations. The shape of the calculated J-integral distribution resembled the observed profile of the crack propagation front.