We have monitored in situ the lattice defect evolution induced by proton irradiation in 20Cr-25Ni Nb-stabilised stainless steel, used as fuel cladding material in advanced gas-cooled reactors. At 420°C, the damaged microstructure is mainly characterised by black spots and faulted a_0/3 111 Frank loops. Defect saturation is reached at only 0.1dpa. In contrast, at 460°C and 500°C proton bombardment induces the formation of a mixture of a_0/3 111 Frank loops and
perfect a_0/2 110 loops. These perfect loops evolve into dislocation lines that form a dense network. This transition coincides with the saturation in the dislocation loop size and number density at 0.8dpa (460°C) and 0.2dpa (500°C),
respectively. The presence of a high density of dislocation loops and lines at those two temperatures causes a vacancy supersaturation in the matrix, leading to the formation of voids and stacking fault tetrahedra.