Development of a temperature-dependent chemical simulation code based on PHITS for water radiolysis from 0 to 350 °C

Water radiolysis plays an important role in radiation effects on materials, including DNA damage in the human body and corrosion processes in nuclear reactors. To quantitatively evaluate radiolytic molecular yields (G-values), several Monte Carlo simulation codes for analyzing chemical species kinetics have been developed worldwide. However, conventional chemical simulation codes are generally limited to room temperature (roughly equivalent to the human body), which differs from the temperatures encountered in nuclear reactor environments. Therefore, incorporating temperature dependence into chemical simulations is essential for evaluating G-values under high-temperature conditions. In this study, we developed a chemical simulation code (PHITS-Chem) based on the general-purpose Monte Carlo code, Particle and Heavy Ion Transport code System (PHITS), applicable to the 0-350 °C temperature range. The present PHITS-Chem code explicitly accounts for the temperature dependences of diffusion coefficients and chemical reaction rate constants. The present code was benchmarked against reported experimental and theoretical G-values for low-LET (~ 0.2 keV/µm), moderate-LET (~ 11.9 keV/µm), and high-LET (~ 63.4 keV/µm) radiations, showing good agreement with the literature. The validated temperature range spans from 0 to 350 °C, covering conditions relevant to the human body, cryosphere, and light water reactors. To further improve the predictive capability and extend the applicability of the model, additional verification and updates will be required in future studies. The renewed PHITS-Chem thus enables high-precision estimation of radiolytic chemical species kinetics across a broad temperature range, which would be valuable for assessing in-core material degradation and mitigating severe accidents in nuclear reactors.