Water Chemistry In Fusion Cooling Systems: Assessment Of Borated Water For The DTT Vacuum Vessel

A preliminary assessment for the Divertor Tokamak Test facility Vacuum Vessel (VV) water chemistry was performed using both experiments and simulations. The requirement to use 8000 ppm B in water enriched with 95% 10B as a neutron shield implies that water has a pH60C=3.6. Materials in contact with the borated water are stainless steel type, 316LN, resistant to general corrosion from borated water. Corrosion, however, is a complex phenomenon affected both by water chemistry and material characteristics. No water chemistry guidelines exist for nuclear fusion cooling circuits so the assessment was based on nuclear fission power plants operational experience. The need to add additives to minimize corrosion was assessed using metal release experiments and simulations with the use of computer codes devoted to the estimation of activated corrosion products (ACPs) production and transport. The release of ions induced by general corrosion was found to be more influenced by water chemistry (more releases were measured in the borated water environment than UPW) than microstructure (base metal vs welds). The possible addition of hydrogen in the DTT VV cooling circuit was also assessed, but without considering water radiolysis at this stage. It was found that at these low temperature (60-80°C) the addition of hydrogen seem not beneficial if the oxygen content can be maintained low.