Mechanistic modeling and simulation of water radiolysis for controling the chemistry in Generation-IV supercritical water-cooled reactors

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The radiolysis of water under operating conditions of both current pressurized water-cooled reactors (~250-330 oC, 10 MPa) and 4th-generation SCW reactors in the future (~430-625 oC, 25 MPa) remains largely unknown. Direct measurements at such high temperatures and pressures, and especially beyond the critical point of water (H2O: tc ~ 373.95 oC, pc ~ 22.06 MPa) are difficult, and computer simulations are an important route of investigation. The objective of this work is to obtain an enhanced understanding of the effects of radiation (fast neutrons, gamma-rays, recoil protons/Oq+ ions, q = 1-8) on aqueous systems in the reactor cores, in order to specify a chemistry control strategy that minimizes corrosion and degradation of reactor components resulting from the radiolytic formation of oxidizing species such as .OH, H2O2, O2, and O2.-/HO2.. Our long-term goal is to develop a benchmark SCW reactor radiolysis model using coupled molecular dynamics (MD) and Monte-Carlo (MC) simulations. This project is based on preliminary results (obtained in our laboratory) that suggest that the heterogeneous molecular structure of SCW (high-density, "liquid"-like regions surrounded by low-density, "gas"-like regions) might be a key factor to better understand water radiolysis near or above tc. It is proposed here to use MD simulations to generate molecularly detailed configurations of SCW at different 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 all radiolytic species for temperatures up to 625 oC. A number of tests of validity of the codes that are being developed will be carried out, including the influence of key parameters: LET, pH, added solutes (H2 and other .OH scavengers, dissolved O2), high dose rate, etc.). The resulting data will be key inputs into the development of a water chemistry control strategy for the Canadian SCW reactor, defining the oxidizing power of the coolant at various locations in the core. These data will also be of great value for the interpretation of the results of upcoming tests using in-core SCW loops in the Czech Republic.