Low linear energy transfer radiolysis of supercritical water at 400 °C: in situ generation of ultrafast, transient, density-dependent "acid spikes".

There is growing interest in the radiation chemistry of supercritical water (SCW), as its use as a coolant in a nuclear reactor (Generation IV) is the logical evolution of the current (Generation III or less) water-cooled reactors. However, current knowledge about the potential effects of water radiolysis in a Gen-IV supercritical water-cooled reactor (SCWR) is incomplete. In this work, Monte Carlo track chemistry simulations of the low linear energy transfer (LET) radiolysis of SCW (H2O) at 400 °C are used in combination with a spherical "spur" model to study the effect of water density on the in situ radiolytic formation of H3O+ ions and the corresponding abrupt, transient, highly acidic pH response ("acid spikes") that is observed immediately after irradiation. The magnitude and duration of this acidic pH effect depend on the water density in the considered range of 0.15-0.6 g cm-3. It is strongest at times less than a few tens of picoseconds with the pH remaining nearly constant at ∼1.6 and 1.9 for the highest ("liquid-like") and lowest ("gas-like") density, respectively. At longer times, the pH gradually increases for all densities and finally reaches a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Our results show that the lower the density of the water, the longer the time required to reach this constant value, ranging from ∼50 ns at 0.6 g cm-3 (pH ∼ 5.6) to ∼1 µs at 0.15 g cm-3 (pH ∼ 8.5). The generation of these highly acidic pH fluctuations around the "native" radiation tracks, though local and transient, raises questions about the potential implications of this effect in proposed Gen-IV SCW-cooled reactors regarding corrosion and degradation of materials.

Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3 -) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium ß-particles at room temperature.

The effect of the azide ion N3 - on the yield of molecular hydrogen in water irradiated with 60Co γ-rays (∼1 MeV Compton electrons) and tritium ß-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N3 - is shown to interfere with the formation of H2 through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H2 yield in the diffusing radiation tracks. Chemical changes are observed in the H2 scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species (i.e., the structure of the electron tracks). In the "short-track" geometry of the higher "linear energy transfer" (LET) tritium ß-electrons (mean LET ∼5.9 eV nm-1), radicals are formed locally in much higher initial concentration than in the isolated "spurs" of the energetic Compton electrons (LET ∼0.3 eV nm-1) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical-radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H2 for tritium ß-electrons compared to 60Co γ-rays. These changes in the scavengeability of H2 in passing from tritium ß-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium ß-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N3 - concentrations (>1 M), our H2 yield results for 60Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons (i.e., the short-lived "dry" electrons prior to hydration) by N3 -. These processes tend to reduce significantly the yields of H2, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET 3H ß-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N3 -.

Calculation of the yields for the primary species formed from the radiolysis of liquid water by fast neutrons at temperatures between 25-350°C.

Monte Carlo simulations were used to calculate the yields for the primary species (e(-)aq, H(•), H2, (•)OH and H2O2) formed from the radiolysis of neutral liquid water by mono-energetic 2 MeV neutrons at temperatures between 25-350°C. The 2 MeV neutron was taken as representative of a fast neutron flux in a reactor. For light water, the moderation of these neutrons generated elastically scattered recoil protons of ∼1.264, 0.465, 0.171 and 0.063 MeV, which at 25°C, had linear energy transfers (LETs) of ∼22, 43, 69 and 76 keV/µm, respectively. Neglecting the radiation effects due to oxygen ion recoils and assuming that the most significant contribution to the radiolysis came from these first four recoil protons, the fast neutron yields could be estimated as the sum of the yields for these protons after allowance was made for the appropriate weightings according to their energy. Yields were calculated at 10(-7), 10(-6) and 10(-5) s after the ionization event at all temperatures, in accordance with the time range associated with the scavenging capacities generally used for fast neutron radiolysis experiments. The results of the simulations agreed reasonably well with the experimental data, taking into account the relatively large uncertainties in the experimental measurements, the relatively small number of reported radiolysis yields, and the simplifications included in the model. Compared with data obtained for low-LET radiation ((60)Co γ rays or fast electrons), our computed yields for fast neutron radiation showed essentially similar temperature dependences over the range of temperatures studied, but with lower values for yields of free radicals and higher values for molecular yields. This general trend is a reflection of the high-LET character of fast neutrons. Although the results of the simulations were consistent with the experiment, more experimental data are required to better describe the dependence of radiolytic yields on temperature and to test more thoroughly our modeling calculations.

Effect of temperature on the low-linear energy transfer radiolysis of the ceric-cerous sulfate dosimeter: a Monte Carlo simulation study.

The stochastic modeling of the (60)Co γ/fast-electron radiolysis of the ceric-cerous chemical dosimeter has been performed as a function of temperature from 25-350°C. The system used is a dilute solution of ceric sulfate and cerous sulfate in aqueous 0.4 M sulfuric acid. In this system, H(•) (or HO2(•) in the presence of dissolved oxygen) and H2O2 produced by the radiolytic decomposition of water both reduce Ce(4+) ions to Ce(3+) ions, while (•)OH radicals oxidize the Ce(3+) present in the solution back to Ce(4+). The net Ce(3+) yield is given by G(Ce(3+)) = g(H(•)) + 2 g(H2O2) - g((•)OH), where the primary (or "escape") yields of H(•), H2O2 and (•)OH are represented by lower case g's. At room temperature, G(Ce(3+)) has been established to be 2.44 ± 0.8 molecules/100 eV. In this work, we investigated the effect of temperature on the yield of Ce(3+) and on the underlying chemical reaction kinetics using Monte Carlo track chemistry simulations. The simulations showed that G(Ce(3+)) is time dependent, a result of the differences in the lifetimes of the reactions that make up the radiolysis mechanism. Calculated G(Ce(3+)) values were found to decrease almost linearly with increasing temperature up to about 250°C, and are in excellent agreement with available experimental data. In particular, our calculations confirmed previous estimated values by Katsumura et al. (Radiat Phys Chem 1988; 32:259-63) showing that G(Ce(3+)) at ∼250°C is about one third of its value at room temperature. Above ∼250°C, our model predicted that G(Ce(3+)) would drop markedly with temperature until, instead of Ce(4+) reduction, Ce(3+) oxidation is observed. This drop is shown to occur as a result of the reaction of hydrogen atoms with water in the homogeneous chemical stage.

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.

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.

On the spur lifetime and its temperature dependence in the low linear energy transfer radiolysis of water.

In the spirit of the radiation chemical "spur model", the lifetime of a spur (τ(s)) is an important indicator of overlapping spurs and the establishment of homogeneity in the distribution of reactive species created by the action of low linear energy transfer (LET) radiation (such as fast electrons or γ irradiation). In fact, τ(s) gives the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution, thus defining the so-called primary (or "escape") radical and molecular yields of radiolysis, which are obviously basic to the quantitative understanding of any irradiated chemical system. In this work, τ(s) and its temperature dependence have been determined for the low-LET radiolysis of deaerated 0.4 M aqueous solutions of H(2)SO(4) and pure liquid water up to 350 °C using a simple model of energy deposition initially in spurs, followed by random diffusion of the species of the spur during track expansion until spur overlap is complete. Unlike our previous τ(s) calculations, based on irradiated Fricke dosimeter simulations, the current model is free from any effects due to the presence of oxygen or the use of scavengers. In acidic solutions, the spur lifetime values thus obtained are in very good agreement with our previous calculations (after making appropriate corrections, however, to account for the possibility of competition between oxygen and Fe(2+) ions for H˙ atoms in the Fricke dosimeter, an effect which was not included in our original simulations). In this way, we confirm the validity of our previous approach. As expected, in the case of pure, oxygen-free water, our calculated times required to reach complete spur overlap are essentially the same (within uncertainty limits) as those found in acidic solutions. This explicitly reflects the fact that the diffusion coefficients for the hydrated electron and the H˙ atom that are involved in the overall calculation of the lifetime of spurs in neutral or acidic media, respectively, are of similar magnitude over the 25-350 °C temperature range studied.

Time-dependent yield of the hydrated electron in subcritical and supercritical water studied by ultrafast pulse radiolysis and Monte-Carlo simulation.

Fast kinetics and time-dependent yields of the hydrated electron (e(-)(aq)) in pure water under conditions of high temperature and pressure up to the supercritical region were investigated by picosecond and nanosecond pulse radiolysis experiments. More significant decays at short times followed by plateau components at longer times were observed with increasing temperature, suggesting faster spur reaction processes. In supercritical water, it was also found that the e(-)(aq) yields strongly depend on the pressure (density). Comparison of these measurements with Monte-Carlo computer simulations allowed us to identify spur reactions of e(-)(aq) that occur predominantly at high temperatures and also to provide new key information on certain spur model parameters. In particular, the experimental time-dependent e(-)(aq) yields were best reproduced if the electron thermalization distance decreases with increasing temperature. This "shrinkage" of spur sizes at high temperatures was attributed to an increase in the scattering cross sections of subexcitation electrons, likely originating from a decrease in the degree of structural order of water molecules as the temperature is increased.

Density dependence of the "escape" yield of hydrated electrons in the low-LET radiolysis of supercritical water at 400 °C.

The "spur lifetime" (τ(s)) in the low-linear energy transfer (LET) radiolysis of supercritical water (SCW) at 400 °C has been determined as a function of water density by using a simple model of energy deposition initially in spurs, followed by the random diffusion (Brownian motion) of the species formed until spur expansion is complete. The values of τ(s) are found to decrease from ∼5.0 × 10(-6) to 5.0 × 10(-8) s over the density range from 0.15 to 0.6 g cm(-3). Using Monte-Carlo simulations, our calculated density dependence of the "escape" hydrated electron (e(aq)(-)) yield (i.e., at time τ(s)) reproduces fairly well Bartels and co-workers' scavenged e(aq)(-) yield data, suggesting that these data may have been measured at times close to τ(s).

Utilization of the ferrous sulfate (Fricke) dosimeter for evaluating the radioprotective potential of cystamine: experiment and Monte Carlo simulation.

Cystamine, an organic disulfide (RSSR), is among the best of the known radiation-protective compounds and has been used to protect normal tissues in clinical radiation therapy. Recently, it has also proved to be beneficial in the treatment of disorders of the central nervous system in animal models. However, the underlying mechanism of its action at the chemical level is not yet well understood. The present study aims at using the ferrous sulfate (Fricke) dosimeter to quantitatively evaluate, both experimentally and theoretically, the radioprotective potential of this compound. The well-known radiolysis of the Fricke dosimeter by (60)Co γ rays or fast electrons, based on the oxidation of ferrous ions to ferric ions by the oxidizing species (•)OH, HO(2)(•), and H(2)O(2) produced in the radiolytic decomposition of water, forms the basis for our method. The presence of cystamine in Fricke dosimeter solutions during irradiation prevents the radiolytic oxidation of Fe(2+) and leads to decreased ferric yields (or G values). The observed decrease in G(Fe(3+)) increases upon increasing the concentration of the disulfide compound over the range 0-0.1 M under both aerated and deaerated conditions. To help assess the basic radiation-protective mechanism of this compound, a full Monte Carlo computer code is developed to simulate in complete detail the radiation-induced chemistry of the studied Fricke/cystamine solutions. Benefiting from the fact that cystamine is reasonably well characterized in terms of radiation chemistry, this computer model proposes reaction mechanisms and incorporates specific reactions describing the radiolysis of cystamine in aerated and deaerated Fricke solutions that lead to the observable quantitative chemical yields. Results clearly indicate that the protective effect of cystamine originates from its radical-capturing ability, which allows this compound to act by competing with the ferrous ions for the various free radicals--especially (•)OH radicals and H(•) atoms--formed during irradiation of the surrounding water. Most interestingly, our simulation modeling also shows that the predominant pathway in the oxidation of cystamine by (•)OH radicals involves an electron-transfer mechanism, yielding RSSR(•+) and OH(-). A very good agreement is found between calculated G(Fe(3+)) values and experiment. This study concludes that Monte Carlo simulations represent a very efficient method for understanding indirect radiation damage at the molecular level.

Temperature dependence of the Fricke dosimeter and spur expansion time in the low-LET high-temperature radiolysis of water up to 350 °C: a Monte-Carlo simulation study.

Monte-Carlo simulations of the radiolysis of the ferrous sulfate (Fricke) dosimeter with low-linear energy transfer (LET) radiation (such as (60)Co γ-rays or fast electrons) have been performed as a function of temperature from 25 to 350 °C. The predicted yields of Fe(2+) oxidation are found to increase with increasing temperature up to ∼100-150 °C, and then tend to remain essentially constant at higher temperatures, in very good agreement with experiment. By using a simple method based on the direct application of the stoichiometric relationship that exists between the ferric ion yields so obtained G(Fe(3+)) and the sum {3 [g(e(-)(aq) + H˙) + g(HO(2)˙)] + g(˙OH) + 2 g(H(2)O(2))}, where g(e(-)(aq) + H˙), g(HO(2)˙), g(˙OH), and g(H(2)O(2)) are the primary radical and molecular yields of the radiolysis of deaerated 0.4 M H(2)SO(4) aqueous solutions, the lifetime (τ(s)) of the spur and its temperature dependence have been determined. In the spirit of the spur model, τ(s) is an important indicator for overlapping spurs, giving the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution. The calculations show that τ(s) decreases by about an order of magnitude over the 25-350 °C temperature range, going from ∼4.2 × 10(-7) s at 25 °C to ∼5.7 × 10(-8) s at 350 °C. This decrease in τ(s) with increasing temperature mainly originates from the quicker diffusion of the individual species involved. Moreover, the observed dependence of G(Fe(3+)) on temperature largely reflects the influence of temperature upon the primary free-radical product yields of the radiolysis, especially the yield of H˙ atoms. Above ∼200-250 °C, the more and more pronounced intervention of the reaction of H˙ atoms with water also contributes to the variation of G(Fe(3+)), which may decrease or increase slightly, depending on the choice made for the rate constant of this reaction. All calculations reported herein use the radiolysis database of Elliot (Atomic Energy of Canada Limited) and Bartels (University of Notre Dame) that contains all the best currently available information on the rate constants, reaction mechanisms, and g-values in the range 20 to 350 °C.