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Research Papers

# Radiolysis of Supercritical Water at 400°C: A Sensitivity Study of the Density Dependence of the Yield of Hydrated Electrons on the ($eaq−+eaq−$) Reaction Rate Constant

[+] Author and Article Information
Sunuchakan Sanguanmith

Department of Nuclear Medicine and Radiobiology,
Université de Sherbrooke,
3001, 12e Avenue Nord, Sherbrooke, QC J1H 5N4, Canada
e-mail: sunuchakan.sanguanmith@USherbrooke.ca

Jintana Meesungnoen

Department of Nuclear Medicine and Radiobiology,
Université de Sherbrooke,
3001, 12e Avenue Nord, Sherbrooke, QC J1H 5N4, Canada
e-mail: jintana.meesungnoen@USherbrooke.ca

David A. Guzonas

Canadian Nuclear Laboratories, Reactor Chemistry and Corrosion,
20 Forest Avenue, Deep River, ON K0J 1P0, Canada
e-mail: david.guzonas@cnl.ca

Craig R. Stuart

Canadian Nuclear Laboratories, Reactor Chemistry and Corrosion,
20 Forest Avenue, Deep River, ON K0J 1P0, Canada
e-mail: craig.stuart@cnl.ca

Jean-Paul Jay-Gerin

Department of Nuclear Medicine and Radiobiology,
Université de Sherbrooke,
3001, 12e Avenue Nord, Sherbrooke, QC J1H 5N4, Canada
e-mail: jean-paul.jay-gerin@USherbrooke.ca

1Corresponding author.

Manuscript received March 28, 2015; final manuscript received July 1, 2015; published online February 29, 2016. Assoc. Editor: Thomas Schulenberg.

ASME J of Nuclear Rad Sci 2(2), 021014 (Feb 29, 2016) (5 pages) Paper No: NERS-15-1035; doi: 10.1115/1.4031013 History: Received March 28, 2015; Accepted July 07, 2015

## Abstract

The temperature dependence of the rate constant ($k$) of the bimolecular reaction of two hydrated electrons ($eaq−$) measured in alkaline water exhibits an abrupt drop between 150°C and 200°C; above 250°C, it is too small to be measured reliably. Although this result is well established, the applicability of this sudden drop in $k(eaq−+eaq−)$) above $∼150°C$ to neutral or slightly acidic solution, as recommended by some authors, still remains uncertain. In fact, the recent work suggested that in near-neutral water the abrupt change in $k$ above $∼150°C$ does not occur and that $k$ should increase, rather than decrease, at temperatures greater than 150°C with roughly the same Arrhenius dependence of the data below 150°C. In view of this uncertainty of $k$, Monte Carlo simulations were used in this study to examine the sensitivity of the density dependence of the yield of $eaq−$ in the low–linear energy transfer (LET) radiolysis of supercritical water ($H2O$) at 400°C on variations in the temperature dependence of $k$. Two different values of the $eaq−$ self-reaction rate constant at 400°C were used: one was based on the temperature dependence of $k$ above 150°C as measured in alkaline water ($4.2×108 M−1 s−1$), and the other was based on an Arrhenius extrapolation of the values below 150°C ($2.5×1011 M−1 s−1$). In both cases, the density dependences of our calculated $eaq−$ yields at $∼60 ps$ and 1 ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for $D2O$) for the entire water density range studied ($∼0.15–0.6 g/cm3$). Only a small effect of $k$ on the variation of $G(eaq−)$) as a function of density at 60 ps and 1 ns could be observed. In conclusion, our present calculations did not allow us to unambiguously confirm (or deny) the applicability of the predicted sudden drop of $k(eaq−+eaq−)$ at $∼150°C$ in near-neutral water.

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## Figures

Fig. 1

Rate constant for the self-reaction of two hydrated electrons as a function of temperature. The solid line (denoted ka) shows the (eaq−+eaq−) reaction rate constant that was obtained by using an Arrhenius extrapolation procedure above 150°C as proposed by Elliot [18], Stuart et al. [19], and Hatomoto et al. [26]. The dashed line (denoted kb) shows the (eaq−+eaq−) reaction rate constant that was measured by Bartels and coworkers [20,27] (▪) under alkaline conditions. Note that kb was assumed to remain constant between 275°C and 400°C.

Fig. 2

Density dependence of G(eaq−) (in molecule per 100 eV) in SCW at 400°C measured directly by picosecond pulse radiolysis experiments (in D2O) [28,29] at ∼60  ps (○) and 1 ns (▪) (estimated uncertainty of ±10%). The solid and dashed lines show our Monte Carlo simulated results in supercritical H2O when k(eaq−+eaq−)=2.5×1011 M−1 s−1 was used at ∼60  ps and 1 ns, respectively. The dash-dot and dash-double-dotted lines show our corresponding calculated eaq− yields when k(eaq−+eaq−)=4.2×108  M−1 s−1 was used at ∼60  ps and 1 ns, respectively. For conversion into SI units (mol/J), 1 molecule per 100  eV≈0.10364  μmol/J.

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