Effect of gadolinium nitrate concentration on the corrosion compatibility of structural materials in a proposed Indian tube type boiling reactor

Gadolinium (Gd³⁺) is added with nitric acid to moderator heavy water as a neutron poison in nuclear reactors to control reactivity and pH is maintained in the range of 5 to 5.5 to prevent Gd³⁺ precipitation. Usually ∼15 ppm of Gd³⁺ is used during actuation of secondary shutdown system and is subsequently removed on ion exchange up to a residual ∼2 ppm before start-up. In the moderator system of a proposed tube type boiling water reactor of Indian origin, a higher concentration (20-400ppm) of Gd(NO₃)₃ was proposed to be used in the emergency safety shutdown system. With higher concentration of Gd³⁺, the pH can go down and affect the radiolytic yields and thus affecting the integrity of the structural materials. Considering a long life of 100 years of operation for the proposed reactor, the concentration dependence of Gd³⁺ on the yields of molecular products like H₂ and H₂O₂ during radiolysis and corrosion compatibility of structural materials like (1) SS 304 LN (proposed structural material for this reactor) and (2) SS 410 (proposed to be used in the valves of the moderator system as an alternative to hard facing alloy, colmonoy) is of interest. The pH and conductivity of the system were observed to be in the range of 5.33-3.76 and 50-870 μS/cm for 20-400 ppm of Gd³⁺. From the electrochemical studies it was observed that the electrochemical potential increased to more positive potential with increase in Gd³⁺ concentrations. The yield of H₂ and H₂O₂ was also found to increase with increase in [Gd³⁺] concentrations. A detailed study on corrosion of the above said alloys at varying [Gd³⁺] concentrations, temperature, pH and simulated irradiation conditions and its effect on microstructure will be described in the paper. (author)