High-dose-rate effects in the radiolysis of water at elevated temperatures

Monte Carlo track chemistry simulations were used to study the effects of high dose rates on the radical (e^-_aq, H· , and ·OH) and molecular (H₂ and H₂O₂) yields in the low linear energy transfer (LET) radiolysis of liquid water at elevated temperatures between 25 and 350 °C. Our simulation model consisted of randomly irradiating water with single pulses of N incident protons of 300 MeV (LET ~0.3 keV/lm), which penetrate at the same time perpendicular to this water within the surface of a circle. The effect of dose rate was studied by varying N. Our simulations showed that, at any given temperature, the radical products decrease with increasing dose rate and, at the same time, the molecular products increase, resulting from an increase in the inter-track, radical–radical reactions. Using the kinetics of the decay of hydrated electrons at 25 and 350 °C, we determined a critical time (t_c) for each value of N, which corresponds to the "onset" of dose-rate effects. For our irradiation model, t_c was inversely proportional to N for the two temperatures considered, with t c at 350 °C shifted by an order of magnitude to shorter times compared with its values at 25 °C. Finally, the data obtained from the simulations for N=2000 generally agreed with the observation that during the track stage of radiolysis, free radical yields increase, whereas molecular products decrease with increasing temperatures from 25 to 350 °C. The exceptions of e a^-_aq and H₂ to this general pattern are briefly discussed. (author)