Water degradation in ITER Neutral Beam Test Facility cooling circuits

SPIDER is the 100 keV full-size Ion Source prototype of the ITER Neutral Beam Injectors and it is operating at Consorzio RFX in Padova, Italy. SPIDER components shall be actively cooled to remove up to 10 MW thermal power from in-vessel components and auxiliary systems using ultrapure water which is provided by an in-house purification system. Ultrapure water was chosen as a coolant in SPIDER since it serves as a dielectric fluid, at the same time operating with a low conductivity coolant helps minimising corrosion build up in metallic circuits. Since starting of operation in 2019, however, it was observed that water degraded more quickly than estimated by design in SPIDER power supplies cooling circuit, called PC01. Water degradation affected severely water conductivity that has to be maintained within tight ranges in SPIDER PC01 circuit (< 0.3 ?S/cm at 25 °C). To tackle this problem, PC01 circuit was sectioned in different configurations where circulation experiments were performed at constant temperature (T=25 °C) and flowrate (~75 m3/h) using high purity water (conductivity of ~ 0.06 ?S/cm at 20°C). These tests enabled to reveal which part of the circuit was affected by water degradation. Solid corrosion products were found and these were primarily made of zinc and iron oxides, characterised by scanning electron microscopy and X-ray diffraction analysis. Trace metal analysis on the water sampled from the circuit using Inductively coupled plasma mass spectrometry revealed the presence of zinc and copper in relevant quantities. The presence of non-compatible materials are considered the cause of the water degradation in this circuit. Solutions including the replacement of galvanised steel components with stainless steel ones were implemented. Water chemistry monitoring will continue with the aim of minimising corrosion phenomena in SPIDER cooling circuits that inevitably see the presence of several materials coupled together.