Effect of temperature gradients on void formation in modified 316 stainless steel cladding

Focusing on the effect of temperature gradients across the cladding wall on void formation, observation of void distribution was carried out by means of electron microscopic techniques in the direction of the wall thickness in the irradiated fuel claddings made of P, Ti-modified 316 steel. A deformation analysis was also conducted including irradiation creep and swelling deformation, using a finite element method. At the beginning of swelling a secondary stress occurred at the cladding surface due to non-uniform swelling arising from the temperature gradient. Swelling appears to accelerate by this secondary stress, and the swelling developed a non-monotonic distribution, namely the swelling in mid-wall region was lower than that in the surface regions. This non-monotonic swelling distribution disappears with increasing fluence, however. In the deformation analysis, it became clear that swelling is not continuously accelerated by the secondary stress, because irradiation creep deformation relaxes the stress.