Investigation of corrosion-erosion phenomena in primary cooling circuits of reactor components

The focus of this Industrial Doctorate project is to investigate the causes of water chemistry degradation and corrosion-erosion phenomena, particularly within components of the ITER Neutral Beam Test Facility (NBTF) that are cooled by ultrapure water. The NBTF, located at Consorzio RFX, Italy, hosts two experiments: MITICA, a 1 MeV full-scale prototype of the ITER Neutral Beam Injector (NBI), and SPIDER, the low-energy 100 keV ITER HNB full-size Ion Source. The investigation commenced by examining the main components of the primary cooling systems of SPIDER and MITICA, which are primarily made of copper alloys and stainless steel. Metal release tests were conducted to estimate the corrosion rate of these materials under various environmental conditions, including oxidizing and reducing environments and when these materials were coupled and joined, as is the case in NBTF, using a specific joining technique known as Vacuum Tight Threaded Junction (VTTJ). Metal releases tests results were compared with preliminary Stress Corrosion Cracking analyses of copper and stainless steel samples exposed to the same environmental conditions of the metal releases experiment ones. The second part of the analysis focused on investigating water degradation within some of the primary circuits of SPIDER and MITICA. The most significant degradation was observed in the primary circuit cooling the power supplies. Specific circulation tests were conducted at constant temperature and flow rate to estimate the increase in conductivity and measure the metallic cations dissolved in the water using the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) technique. Undesirable metals, such as zinc, were identified. The associations between the experimental observation and the material characterization carried out during this Industrial Doctorate project identified potential circuit improvements proposed and tested to mitigate water degradation and to prevent thinning of cooling channel walls. To address these issues, online sensors were recommended and subsequently installed in a SPIDER primary circuit. These sensors are designed to continuously monitor oxygen levels and pH. Their data will be utilized as soon as SPIDER resumes operation. Additionally, modifications were proposed and implemented in a MITICA primary circuit to minimize the ingress of air from elevated locations within the plant. These adjustments have proven effective in reducing water degradation. Additionally, online mixed bed resins were introduced to ensure a continuous purification of water in the primary loops, thereby maintaining low levels of water conductivity as a first complemental solution to the cyclic replacement of the degraded water