Effect of very high dose rates on the radiolysis of supercritical water at 400 °C and 25 MPa

Monte Carlo multi-track chemistry simulations were used in combination with a cylindrical, “instantaneous pulse” irradiation model to study the effect of high dose rates on the early, transient yields (G values) of the “primary products” (e-aq, H•, H2, •OH, H2O2, H3O+, OH-,…) of the radiolysis of supercritical water (SCW) at 400 °C and 25 MPa pressure. Our simulation model consisted of randomly irradiating SCW with single pulses of N incident 300-MeV protons, which mimic the low linear energy transfer of 60Co γ/fast electron irradiations. The effect of dose rate was studied by varying N. Generally, high dose rates were found to favor radical-radical reactions, which increases the proportion of the molecular products at the expense of the radical products. However, as an exception, G(H•) increases with increasing dose rate in the track stage of radiolysis, predominantly due to the reaction of hydrated electrons with hydronium ions (H3O+). In addition, the generation of acidic spikes due to proton transfer reactions in the physicochemical stage was also examined. Interestingly, an early, transient, very acidic (pH ~ 3.5) response was observed at high radiation dose rates across the entire irradiated volume. The present work raises the question of whether the potential oxidizing species •OH and H2O2 and these highly acidic pH spikes at high dose rates could promote a corrosive environment under proposed Generation-IV SCW-cooled reactor or small modular reactor operating conditions that would lead to progressive degradation of materials.