Water chemistry assessment in fusion cooling systems: borated water for the DTT vacuum vessel

The Divertor Tokamak Test (DTT) facility is an experimental fusion facility under design stage at ENEA Frascati, Italy dedicated to the optimisation of the divertor, a key component of thermonuclear fusion reactors exposed to extreme environments of temperature and irradiation. DTT is an actively cooled reactor that will exploit the alternation of ultrapure water and borated water in the Vacuum Vessel (VV). The use of borated water in fusion reactors, JT-60SA and DTT, using 95% 10B as H3BO3 at 40-80 °C has the primary function of shielding the superconducting coils by neutrons generated in the tokamak plasma during fusion reactions. The boric acid quantities needed in the DTT and JT-60SA fusion reactors are respectively 8000 ppm and 13400 ppm in B, which is well above the operational experience seen in any operating pressurized water reactor (PWR). Experimental work and modelling is undergoing to ensure best performance of the cooling circuit given the gap in knowledge between conventional PWRs water chemistry and the DTT VV set up. A first assessment for DTT VV water chemistry was performed using the chemistry module of an activated corrosion products (ACPs) assessment computer code. Water pH and SS316L alloying elements solubility were compared considering ultrapure water (UPW), borated water and borated water neutralised with LiOH. Stabilisation of 8000 ppm B with 30 ppm Li was considered a good option as pH=6-6.11 at 40-80 °C compared to a pH=4.17-4.23 without LiOH addition. Fe, Ni, Co, Cr and Mn solubility at 40-80 °C were 3 orders of magnitude larger in the borated water compared to the use of only UPW. The addition of 30 ppm Li is, however, beyond the safe margins currently established for PWRs. Slight improvement in terms of solubility was observed in the borated water scenario with the addition of 4 ppm Li compared to no addition of LiOH at 60 °C, so the use of additives is being reconsidered. Experiments to monitor water pH and metallic ions release in solution using Inductively coupled plasma mass spectrometry were also performed to confirm results obtained from the computer code.