Temperature dependence of the Fricke dosimeter and spur expansion time in the low-LET high-temperature radiolysis of water up to 350 °C: a Monte-Carlo simulation study

Abstract

Monte-Carlo simulations of the radiolysis of the ferrous sulfate (Fricke) dosimeter with low-linear energy transfer (LET) radiation (such as ⁶⁰Co γ-rays or fast electrons) have been performed as a function of temperature from 25 to 350 °C. The predicted yields of Fe²⁺ oxidation are found to increase with increasing temperature up to ∼100–150 °C, and then tend to remain essentially constant at higher temperatures, in very good agreement with experiment. By using a simple method based on the direct application of the stoichiometric relationship that exists between the ferric ion yields so obtained G(Fe³⁺) and the sum {3 [g(e⁻_aq + H˙) + g(HO₂˙)] + g(˙OH) + 2 g(H₂O₂)}, where g(e⁻_aq + H˙), g(HO₂˙), g(˙OH), and g(H₂O₂) are the primary radical and molecular yields of the radiolysis of deaerated 0.4 M H₂SO₄ aqueous solutions, the lifetime (τ_s) of the spur and its temperature dependence have been determined. In the spirit of the spur model, τ_s is an important indicator for overlapping spurs, giving the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution. The calculations show that τ_s decreases by about an order of magnitude over the 25–350 °C temperature range, going from ∼4.2 × 10⁻⁷ s at 25 °C to ∼5.7 × 10⁻⁸ s at 350 °C. This decrease in τ_s with increasing temperature mainly originates from the quicker diffusion of the individual species involved. Moreover, the observed dependence of G(Fe³⁺) on temperature largely reflects the influence of temperature upon the primary free-radical product yields of the radiolysis, especially the yield of H˙ atoms. Above ∼200–250 °C, the more and more pronounced intervention of the reaction of H˙ atoms with water also contributes to the variation of G(Fe³⁺), which may decrease or increase slightly, depending on the choice made for the rate constant of this reaction. All calculations reported herein use the radiolysis database of Elliot (Atomic Energy of Canada Limited) and Bartels (University of Notre Dame) that contains all the best currently available information on the rate constants, reaction mechanisms, and g-values in the range 20 to 350 °C.