Density dependence of the yield of hydrated electrons in the low-LET radiolysis of supercritical water at 400 °C: influence of the geminate recombination of subexcitation-energy electrons prior to thermalization

Abstract

Monte Carlo simulations were used to calculate the yield of hydrated electrons (eaq(-)) in the low-linear energy transfer radiolysis of supercritical water at 400 °C as a function of water density over the range of ~0.15 to 0.6 g cm(-3). Very good agreement was found between our calculations and picosecond pulse radiolysis experimental data at ~60 ps and 1 ns at high density (>0.35 g cm(-3)). At densities lower than ~0.35 g cm(-3), our eaq(-) yields were lower than the experimental data, especially at ~60 ps. However, if we incorporated into the simulations a prompt geminate electron-cation (H2O˙(+)) recombination (prior thermalization of the electron) that decreased as the density decreased, our computed eaq(-) yields at ~60 ps and 1 ns compared fairly well with the experimental data for the entire density range studied.

Self-radiolysis of tritiated water. 2. Density dependence of the yields of primary species formed in the radiolysis of supercritical water by tritium β-particles at 400 °C

“…At present, these 3 H b-particle G-values can only be estimated through computer simulation of the radiolysis. In the absence of any other information, we thus compare below our calculated G-values with data obtained for low-LET (g-rays from 60 Co or high-energy electrons) [20][21][22]62 and fast neutron 23 SCW radiolysis at 400 C. Fig. 3 (panel a) and 4 show our calculated primary (or "escape") yield for e À aq , denoted g(e À aq ), obtained for the low-LET radiolysis of SCW (H 2 O) at 400 C using the spur lifetimes (s s ) of Fig.…”

“…Stochastic modeling of the radiolysis of SCW by the low-energy b-electrons of tritium at 400 C was performed using an extended version of our Monte Carlo simulation code called IONLYS-IRT. 20,22,23,40 In brief, this program simulates, in a 3D geometrical environment, the initial production of the various radiolytic species and the subsequent track expansion during which these species diffuse randomly and react with one another or with dissolved solutes (if any) present at the time of irradiation. The IONLYS program models, on an event-by-event basis, all of the events of the physical and physicochemical stages of radiation action 41 up to $10 À12 s in the track development.…”

“…25. In the current version of the code, several updates and modications have been made. 21,22,48 In particular, we used the self-consistent radiolysis database (including rate constants, diffusion coefficients, and radiolytic yields) recently compiled by Elliot and Bartels. 49 This new, self-consistent database provides recommendations for the best values to be used in high-temperature models of light water radiolysis over the range of 20-350 C. Here, these data were simply extrapolated above their experimentally studied temperature range to obtain the rate constants of the various reactions involved in the radiolysis of SCW at 400 C, as well as the diffusion coefficients of the various intervening reactive species.…”

“…21,48 In addition, we included in the simulations a prompt geminate electron-cation (H 2 Oc + ) recombination (i.e., prior to the thermalization of the electron) that decreased in irradiated SCW at 400 C as the water density decreased from $0.6 to 0.15 g cm À3 . 22 The density (pressure) dependence of the self-diffusion coefficient of compressed SCW at 400 C has been taken from the measurements of Lamb et al 54 in the region from 0.1 to 0.7 g cm À3 . As for the diffusion coefficients of the radiolytic species cOH, Hc, H 2 O 2 , and H 2 , explicitly determined only at 25 C and essentially unknown at 400 C, we have estimated them here by assuming that they scale proportionally to the self-diffusion of water above room temperature.…”

“…Our aim is to predict the G-values for the primary species e À aq , Hc, H 2 , cOH, and H 2 O 2 resulting from this radiolysis as a function of water density over the range of $0.15 to 0.6 g cm À3 . Previous work has shown that the density of water has a major impact on the radiolysis yields of species in SCW under low-"linear energy transfer" (LET) [17][18][19][20][21][22] and fast neutron 10,23 irradiation. In the absence of any other information, our results will be compared with results in the literature that were obtained aer irradiation with 60 Co g-rays (or high-energy electrons) and with fast neutrons.…”