Sodium pipe rupture tests representative of Fast Reactors accidents have been carried out on austenitic stainless steel surfaces. These tests improve our knowledge of small sodium leakage propagation in mineral wool insulation. Furthermore, they explain the new and unexpected aspects of the crevice corrosion phenomenon which has been observed on austenitic stainless steel external pipe surfaces. Experimental results show that the corrosion is limited to a peripheral annular zone, which extends out in concentric waves. The diameter of this corrosion zone is practically constant. Furthermore, the tests show that sodium insulation does not expand directly on the pipe surface. The sodium sprays through the mineral wool insulation, where chemical reactions bet ween silica fibers, occluded oxygen and water vapor occur at the same time. Simultaneously, there is a diffusion phenomenon of liquid sodium droplets on the mineral wool fibers. The study allows to prove the electro-chemical nature of the corrosion. The excess liquid sodium, spraying as droplets on the pipe surface, induces an anodic dissolution mechanism by differential aeration. This phenomenon explains the random microscopic and macroscopic aspects of material removal.
Keywords: Fast Reactors, sodium leaks, mineral wool insulation, pipe surfaces, pipe rupture tests, austenitic stainless steel, diffusion. electro-chemical corrosion, anodic dissolution