Chemical Species Influences under Flow Conditions Relevant to PWR and SCWR

The current chemistry control strategy in the primary coolant of pressurized water reactors (PWRs) and pressurized heavy water reactors involves operation under alkaline conditions, through the addition of lithium hydroxide, to minimize corrosion and corrosion product transport. The in-core generation of oxidizing species, such as H₂O₂, ·OH, and O₂ due to the radiolytic breakdown of water is suppressed by the addition of hydrogen gas to the coolant. It is not clear whether this water chemistry strategy will provide adequate chemistry control above the critical point of water, in the operation of the supercritical water-cooled reactor (SCWR). Application of LiOH for the pH control of the coolant at PWR and SCWR operating conditions is examined. The pH values of dilute aqueous LiOH solutions (10^-6 - 10^-2 mol/kg) are calculated based on recently reported Molecular Dynamics (MD) simulation data for the association constant of LiOH at supercritical water conditions. The results indicate that at the operating pressure of SCWR (25 MPa), addition of LiOH to the coolant will provide adequate pH control for water densities above 0.17 g/cm³ (T < 400 deg. C). At densities below 0.17 g/cm³ pH control becomes progressively more difficult as dielectric constant of water decreases. Corrosion testing was performed on stainless steel tubing at 350, 400, 500 and 650 deg. C for different pH values under conditions relevant to the operation of the SCWR (25 MPa). The loss of metal to the coolant was analysed using the voltammetry method, and the concentration of H₂ was monitored using a Dissolved Gas Analyser (DGA). In this paper we discuss these data and their implications for the development of the SCWR chemistry control strategy. (authors)