Results of water corrosion in static cell tests representing multi-metal assemblies in the hydraulic circuits of Tore Supra

Full text of publication follows: Tore supra (TS) has used from the beginning of operation in 1989 actively cooled plasma facing components. Since the operation and baking temperature of all in vessel components has been defined to be up to 230 deg. C at 40 bars, a special water chemistry of the cooling water plant was suggested in order to avoid eventual water leaks due to corrosion (general corrosion, galvanic corrosion, stress corrosion, etc.) at relative high temperatures and pressures in tubes, pipes, bellows, water boxes, coils, etc. From the beginning of TS operation, in vessel components (e.g. wall protection panels, limiters, ergodic divertor coils, neutralisers and diagnostics) represented a unique combination of metals in the hydraulic circuit mainly such as stainless steel, Inconel, CuCrZr, Nickel and Copper. These different materials were joined together by welding (St to St, Inconel to Inconel, CuCrZr to CuCrZr and CuCrZr to St-St via a Ni sleeve adapter), brazing (St-St to Cu and Cu-LSTP), friction (CuCrZr and Cu to St-St), explosion (CuCrZr to St-St) and memory metal junction (Cryo-fit to Cu - only test sample). Following experiences obtained with steam generator tubes of nuclear power plants, a cooling water quality of AVT (all volatile treatment) has been defined based on demineralized water with adjustment of the pH value to about 9.0/ 7.0 (25 deg. C/ 200 deg. C) by addiction of ammoniac, and hydrazine in order to absorb oxygen dissolved in water. At that time, a simplified water corrosion test program has been performed using static (no circulation) test cell samples made of above mentioned TS metal combinations. All test cell samples, prepared and filled with AVT water, were performed at 280 deg. C and 65 bars in an autoclave during 3000 hours. The test cell water temperature has been chosen to be sufficient above the TS component working temperature, in order to accelerate an eventual corrosion process. Generally all above mentioned metal combinations survived the test campaign without stress corrosion cracking, with the exception of the memory metal junction (creep in Cu) and the bellows made of St-St 316L and Inconel 625 while 316 Ti bellows survived. In contrary to the vacuum brazed Cu-LSTP to St-St samples, some of flame brazed Cu to St-St samples failed either in the braze joint or in the copper structure itself. For comparison, a spot weld of an inflated 316L panel sample, filled voluntary with a caustic solution of pH 11.5 (25 deg. C), failed after 90 h of testing (intergranular cracking at the spot weld), while an identical sample containing AVT water of pH 9.0 (25 deg. C) survived without damage. The results of these tests, performed during 1986 and 1997, have never been published and therefore are presented more in detail in this paper since corrosion in hydraulic circuits is also an issue of ITER. Up to day, the TS cooling water plant operates with an above mentioned water treatment and no water leaks have been detected on in-vessel components originating from water corrosion at high temperature and high pressure. (authors)