Structural and Chemical Modifications of AISI 316 SS FT Limiters Compared with the Disruption Simulation Damage

THE EFFECTS OF TOKAMAK DISRUPTION DISCHARGES AND REPETITIVE PULSE HEATING ON THE MOLTEN AND EVAPORATED LAYERS OF THE AISI 316 SS HAVE BEEN SIMULATED BY MEANS OF AN ELECTRON BEAM GUN WITH PULSE TIMES FROM 5 TO 20 MS AND POWER PEAK DENSITY RANGING BETWEEN 8 AND 40 KW CM-2. PHYSICAL CHANGES INDUCED BY THERMAL SHOCK ON THE LABORATORY SAMPLES HAVE BEEN COM- PARED WITH MODIFICATIONS OCCURRED AFTER 134 DISCHARGES ON THE FT MUSHROOM- LIMITER SAMPLES. ANALYSES WERE PERFORMED BY MEANS OF SEM-EDAX AND AUGER SPECTROSCOPY. AN APPROPIATE FACILITY ALLOWS TO CATCH THE METALLIC VAPOUR PRODUCED DURING SIMULATION TESTS. RESULTS INDICATE THAT MN AND CR CONCEN- TRATIONS DECREASE APPRECIABLY FROM THE MUSHROOM OF THE FT LIMITERS. THESE RESULTS ARE ALSO IN ACCORDANCE WITH THE INCREASING OF THE MN-LINE, INTENSITY IN THE PLASMA COLUMN, FOUND BY MEANS OF UV-SPECTROSCOPIC MEASU- REMENTS DURING THE TRANSIENT PHASE OF ALL 134-FT DISCHARGES. MICROSTRUC- TURAL CHANGES INDUCED BY HIGH RESOLIDIFICATION VELOCITIES OF THE MOLTEN LAYERS (ABOUT 105 C S-1) SUGGEST A PHASE TRANSFORMATION NEAR THE IRRADIATED SURFACE OF THE STAINLESS STEEL FROM THE AUSTENITIC TO THE MARTENSITIC PHASE WHICH IS MAGNETIC. RESULTS OF STRUCTURAL, CHEMICAL AND MAGNETIC ANALYSIS OF THE MOLTEN DROPLETS COMING FROM THE LIMITER AND CATCHED BY THE FT LINER WALLS CONFIRM THIS HYPOTHESIS. ALL PHENOMENA OCCURRING ON BOTH FT LIMITER PIECES AND SIMULATIVE-SAMPLES ARE VERY SIMILAR. MELTING BY THERMAL SHOCK, DISPLACEMENT OF THE MOLTEN MATERIAL, MICROSTRUCTURAL CHANGES, FISSURES, PREFERENTIAL EVAPORATION AND CHEMICAL COMPOSITION CHANGE ARE THE MAIN FEATURES OF THIS ANALOGY. FURTHERMORE, RELEASE OF SMALL LIQUID DROPLETS FROM THE LIMITER SURFACE AND REDEPOSITION OF THE REMOVED MATERIAL, CONFIRM THAT THE EROSION OF ALL FT LIMITERS IS CAUSED BY SYNER- GISTIC EFFECTS OF DIFFERENT MECHANISMS SUCH AS SPUTTERING, EVAPORATION, MELTING AND ARCING.NA-NOT AVAILABL