Microstructure of ferritic-martensitic steel irradiated by krypton ions

The results of the microstructure development study of ferritic/martensitic steel of the 12Cr type are presented. The steel samples were irradiated at 350 °С by 300 keV 84Kr15+ ions up to a fluence of 1 ∙1021 m-2 in a vacuum of about 7 ⋅ 10-4 Pa. The experiment was carried out in relation to the storage of spent nuclear fuel for conditions of gaseous fission products introduction into the fuel cladding. It has been established that krypton implantation develops approximately the same microstructure regardless of the initial state of the sample (conventional heat treatment, namely – normalization followed by high tempering, or annealing at 350 °C for an 70 h). Gas porosity is formed in the steel, the features of its development are studied by the depth of the path of the introduced ions using cutting out of TEM objects perpendicular to the irradiated surface. The steel surface is strongly oxidized during irradiation, apparently due to the ballistic effect (“driving” of oxygen atoms into the sample by bombarded ions due to insufficiently vacuum in the target area), since the non-irradiated side of the sample did not oxidize when held for 70 h at 350 °С during irradiation of first side. Based on the experimental results obtained, it was concluded that during storage of spent nuclear fuel under standard conditions stresses should not arise in the cladding of spent fuel rods due to gas swelling.