Modeling of radiation spur processes in water at temperatures up to 300.degree.

Abstract

Deterministic diffusion-kinetic modeling has been performed to calculate the temperature dependence of the radiation chemical yields (G-values) of the radiolysis products of water. The FACSIMILE numerical method has been used to solve a set of coupled differential equations describing the diffusion and reactions of the species for the temperature range 20-300 "C. The low LET spherically symmetrical case of an "average" spur with 62.5 eV deposited in it has been considered. Modeling calculations have been compared for Gaussian, an exponential distribution, and a distribution with a central minimum assumed as the initial spatial distribution of the hydrated electron (eaq-). In all cases spatial distributions of the other radiolytic products started as Gaussian. To fit the experimental data it has been necessary to assume that the reaction eaq-+ eaq--H2

On the Temperature Dependence of the Rate Constant of the Bimolecular Reaction of Two Hydrated Electrons

“…Based on the above findings and in accordance with previous studies [24,26,[30][31][32][33], we believe that the applicability of the sudden drop in the (e − aq + e − aq ) reaction rate constant observed at ~150°C in alkaline water to neutral or slightly acidic (as the pH of water at 150-200°C is about 5.7-6 [2]) solution, as proposed by Bartels and coworkers [8,27], remain uncertain and should be examined further.…”

“…As predicted earlier, including the drop in k 1 above 150°C in deterministic diffusion-kinetic modeling calculations [30,31,34] and in Monte Carlo simulations [9,32,33] resulted in a sharp downward discontinuity in g(H 2 ), which is not observed experimentally. Figure 1 illustrates the simulation results of g(H 2 ) as a function of temperature as obtained recently by our group at the Université de Sherbrooke [9].…”

“…This approach assumes that such an abrupt change in k 1 does not occur and that reaction (R1) is diffusion controlled at temperatures greater than 150°C. This assumption was justified by the good agreement obtained between models and experiment [30][31][32][33].…”

Modelling spur chemistry for alkaline and acidic water at high temperatures

“…To address this problem, the extended diffusion-kinetic model [4][5][6] has been adapted to compute the primary radiation yields for the low-LET radiolysis of alkaline and acidic water [7]. The low-LET radiation tracks, due to irradiation by gamma rays or fast electrons, consist of localised well-separated regions of spherical symmetry that are commonly called spurs.…”

“…aq , H, which can either react within the spur to form ionic and molecular products or escape reaction and diffuse out into the bulk medium. According to the extended diffusion-kinetic model, an average spur also contains a small amount of molecular hydrogen, hydrogen peroxide and hydroxyl ions resulting from the dissociative attachment of subexcitation electrons, esub to the water molecule [4,5]. Since the positions of the reactants are initially correlated in space, spur chemistry is non-homogeneous.…”