Monte-Carlo simulation of the low-let radiolysis of liquid water over the range 25 to 350^oC

A re-examination of our Monte-Carlo modeling of the high-temperature radiolysis of liquid water by low linear energy transfer (LET ∼ 0.3 keV/μm) radiation was undertaken in an attempt to reconcile our computed g-values (primary or 'escape' yields) of the various radiolytic products (e^-_aq, ^·OH, H^·, H₂, and H₂O₂) with newly measured or recently re-assessed experimental data over the range from 25 up to 350^oC. In the calculations, we used the radiolysis database, including the rate constants, the diffusion coefficients of the radiation-induced species, the reaction mechanisms and the g-values, recently collected and summarized by Elliot (AECL) and Bartels (University of Notre Dame). Using a global-fit procedure, experimental data were found to be best reproduced when a discontinuity in the temperature dependence of certain physicochemical parameters was introduced at ~100-150^oC. The presence of such a discontinuity was hypothesized to be associated with a change in the liquid structure of water around these temperatures. In addition to the physicochemical factors intervening in the radiolysis, the importance of the reaction of H^· atoms with water in contributing to the unexplained yield of H₂ above 200^oC was also investigated. (author)