Radiolysis of Liquid Water at Temperatures up to 300 °C:  A Monte Carlo Simulation Study

Abstract

Monte Carlo simulations were performed to calculate the temperature dependence of the primary yields (g-values) of the radical and molecular products of the radiolysis of pure, deaerated liquid water by low linear-energy-transfer (LET) radiation. The early energy deposition was approximated by considering short segments (∼100 μm) of 300-MeV proton tracks (corresponding to an average LET of ∼0.3 keV/μm). The subsequent nonhomogeneous chemical evolution of the reactive species formed in these tracks was simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of liquid water at ambient temperature under various conditions. Our calculated g-values for the radiolytic species: , OH, H, H2, and H2O2, are presented as a function of temperature over the range 25−300 °C. They show an increase in g( ), g(OH), and [g(H) + g(H2)] and a decrease in g(H2O2) with increasing temperature, in agreement with existing experimental data. The sensitivity of the results to the values of reaction rate constants and to the temperature dependence of electron thermalization distances (r th) was also investigated. It was found that the best agreement with experiment occurs when the distances of electron thermalization decrease with increasing temperature, a result that is at variance with the predictions of previous modeling studies. Such a decrease in r th as the temperature increases could be linked to an increase in the scattering cross sections of subexcitation electrons that would account for the corresponding decrease in the degree of structural order of water molecules. Our simulations also suggest that the variations of the g-values with temperature, and especially that of g(H2), are better described if we account for the screening of the Coulomb forces between the two in the bimolecular self-reaction of the hydrated electron. Finally, the time-dependent yields of and OH are presented as functions of temperature, in the range 10-12−10-6 s. It was found that the temporal variation of g( ) at elevated temperatures is sensitive to the temperature dependence of r th, suggesting that measurements of the decay of hydrated electrons as functions of time and temperature could, in turn, provide information on the thermalization of subexcitation electrons. The good overall accord of our calculated results with the experimental data available from the literature demonstrates that Monte Carlo simulation methods offer a most promising avenue at present to further develop our understanding of temperature effects in the radiolysis of liquid water.

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].…”

“…In fact, to compensate for the decrease of k 1 , a sharp decrease of r th above ~100-150°C had to be included in the simulations. This decrease in r th was supposed to be the signature of an increase in the scattering cross sections of subexcitation electrons probably reflecting a rapid deterioration in the degree of structural order of water (due to increased breaking of hydrogen bonds) at these temperatures (these subexcitation electrons are known to be very sensitive to the structural order of the surrounding medium, owing to their non-negligible delocalized character) [9,32]. Despite the lack of clear experimental evidence for such a change in the topology of intermolecular hydrogen bonding in water above 150°C, very good agreement was found under these conditions between simulated and experimental g(H 2 ), and the sharp downward discontinuity predicted at 150°C (Fig.…”

“…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].…”

Monte-Carlo Simulation of &gamma;-ray and Fast Neutron Radiolysis of Liquid Water and 0.4 <em>M</em> H₂SO₄ Solutions at Temperatures up to 325^oC

“…The detailed description of present MonteCarlo modeling for radiolysis of liquid water has been written elsewhere [1][2][3][4][5][6][7][8][9][10]. To reproduce the effect of 60 Co γ-ray radiolysis and fast electrons, 150 ions, ~150µm short track segments of 300MeV protons (where the average LET value obtained by the simulations is ~0.3 keV/µm at 25 ο C, which is similar to the LET of 60 Co γ-ray) [11] and end track length of 0.5~1.5µm for temperature range of 25~325 ο C were applied.…”