Development of a radiolysis model for Generation-IV supercritical water-cooled reactors

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The radiation-induced chemistry of water under the operating conditions of both current pressurized water-cooled reactor systems (~320 °C, 10 MPa) and supercritical water (SCW) reactors in the future (~430-625 °C, 25 MPa) remains largely unknown. Direct measurements at such high temperatures and pressures, and especially beyond the critical point of water (H2O: 374 °C, 22.1 MPa), are difficult, and computer simulations are an important route of investigation. Our objective is to develop an enhanced, reliable understanding of the effects of radiation (energetic neutrons, gamma-rays, recoil protons and Oq+ ions with q = 1-8) on aqueous systems in the reactor cores, in order to develop a chemistry control strategy that minimizes corrosion and degradation of reactor components resulting from the radiolytic formation of oxidizing products such as .OH, H2O2, O2, and HO2./O2.-. Our long-term goal is to develop a benchmark SCW reactor radiolysis model using combined molecular dynamics (MD) and Monte Carlo (MC) computer simulations. This project is based on preliminary results (obtained in our laboratory) that suggest that the heterogeneous ("liquid"-like clusters surrounded by "gas"-like regions) molecular structure of SCW might be a key to better understand water radiolysis above 374 °C. It is proposed here to use MD calculations to generate molecularly detailed configurations of SCW at selected temperatures over a wide range of densities from very dilute vapor to dense SCW, in combination with MC simulations (i) to describe the radiolysis of SCW under these specific configurations and (ii) to quantitatively determine the yields of radiolytic species for temperatures up to 625 °C. Exciting developments that will also be made with our model calculations include the effects of pH, added solutes (such as H2 and other .OH scavengers, dissolved O2) and their concentrations, dose rate, as well as the effects of multiple ionization and fragmentation of water molecules under high-LET irradiation. This project will benefit from our long experience in the radiation chemistry of water and in the use of stochastic MC techniques applied to liquid water radiolysis under a variety of experimental conditions.