Water Degradation in ITER Neutral Beam Test Facility Cooling Circuits

Source for production of ion of deuterium extracted from rf plasma (SPIDER) is the 100 keV negative ion source prototype of the ITER neutral beam injector. The cooling plant is one of the SPIDER auxiliary systems where ultrapure water (UPW) is used as the cooling media, thanks to its advantageous properties such as good cooling performance and high resistivity. Water resistivity needs to be maintained above certain values during SPIDER operation to enable the electrical insulation of in-vessel components that operate at different voltages. Nonetheless, degradation of water resistivity was observed in some circuits during operation. Water resistivity degradation had a negative impact on the SPIDER experimental campaign since it limits cooling plant availability. To understand the cause for the observed water degradation, sampling points were installed to sample water during operation. Ad hoc experiments have been performed on the worst affected circuit, and water samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Circulation experiments were conducted at constant temperature, pressure, and flow rate to assess water degradation by monitoring water conductivity increase. ICP-MS analyses on water from the worst affected circuit revealed the presence of Cu in relevant quantities and Zn. Corrosion induced by the presence of a non-compatible component with SPIDER requirements was found. The identified component was made of a galvanized steel plate, carbon steel, and brass. This was found responsible for the contamination of the cooling loop and its water properties' degradation.