Water Chemistry in Fusion Cooling Systems: Borated Water for DTT Vacuum Vessel

The Divertor Tokamak Test (DTT) vacuum vessel (VV) is a toroidal chamber that ensures an enclosed vacuum environment for the plasma, a confinement barrier, and temperature control. Water flows in the double-shell D-shaped VV cross-sectional structure made from stainless steel to operate at a set temperature. Beyond temperature control, the water cooling circuit needs to act as a neutron shielding media to protect the structures installed outside of the VV, namely, the superconducting coils. The shielding function is achieved due to the addition of boric acid in the water. The requirement on the borated water for the DTT VV is to have 8000-ppm B solution highly enriched in ¹⁰B (95% ¹⁰B). Given the lack of water chemistry guidelines for fusion power plants, the water chemistry requirements from fission power plants were investigated. In this work, general corrosion of stainless steel, 316L type family, in concentrated borated water solutions was investigated experimentally using metal release test. Samples were exposed to ultrapure water (UPW) and 8000-ppm B borated water at 80 °C for one week to quantify the amount of ions released in solution. 316L general corrosion was studied considering the different water chemistries, UPW versus borated water, and steels microstructures. DTT VV presents many welded joints, so general corrosion of welding-induced microstructure was here investigated compared to 316L base microstructure. The release of ions from general corrosion was found to be more influenced by water chemistry than microstructure.