Low-linear energy transfer radiolysis of liquid water at elevated temperatures up to 350°C: Monte-Carlo simulations

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

“…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]. Indeed, above ~150°C the calculations predict a decrease in g(H 2 ) instead of the observed increase.…”

“…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.…”

“…To obtain acceptable fits of our calculated values of g(H 2 ) to the experimental data above 150°C, we had to adjust the temperature dependence of certain parameters involved in the early (<10 -12 s) "physicochemical stage" [35] of radiolysis, i.e., the thermalization distance of subexcitation-energy electrons (r th ), the DEA [19,36,37], and the branching ratios of the different excited water molecule decay channels [9]. Interestingly, g(H 2 ) was found to be the yield most sensitive to r th .…”

“…For water at neutral or near-neutral pH under low-LET radiation (such as 60 Co γ-rays and fast electrons), the primary (or "escape") yields (commonly denoted g-values) of the free radicals e − aq , H , and OH continuously increase when the temperature is increased, while the primary yield of H 2 O 2 decreases [8,9]. Although H 2 is a molecular product, g(H 2 ) increases monotonically with temperature, particularly above 200°C [8][9][10][11][12][13].…”

“…Although H 2 is a molecular product, g(H 2 ) increases monotonically with temperature, particularly above 200°C [8][9][10][11][12][13]. H 2 , whose formation is favored by fast neutron (high-LET recoil-ion) radiolysis [4], is an important component associated with the corrosion environment of the coolant system in NPPs.…”