Temperature Dependence of the Primary Species Yields of Liquid Water Radiolysis by 0.8-MeV Fast Neutrons

The yields of species such as e-aq, H•, •OH, H2 and H2O2, formed from the radiolysis of neutral liquid water by the incidence of 0.8-MeV neutrons at temperatures between 25 and 350°C, were calculated by using Monte Carlo simulations. The slowing down of these neutrons through elastic scattering produced recoil protons elastically of ~0.5057, 0.186, and 0.0684 MeV which had linear energy transfers (LETs) of ~40, 67 and 76 keV/µm, respectively, at 25°C. The effects of neutron radiation can be predicted based on the contribution of those first three recoil protons by neglecting the radiation effects due to oxygen ion recoils. Then, the fast neutron yields could be estimated by summing the yields of contributing protons after corresponding weightings were used according to their energy. In this work, yields were calculated at 10-7 and 10-6 s after incidence of neutron radiation in water at the aforementioned temperature range. Overall, there is a reasonably good agreement between our calculated and existing experimental G-values for the entire temperature range. However, we proposed an hypothesis that the not very significant difference between experimental data and our calculated data is due to the different measuring time used in obtaining the experimental data as compared to the ones used in our calculation. Our computed yields for 0.8-MeV fast neutron radiation show an essentially similar temperature dependences over the range of temperature studied with 2-MeV fast neutron and low-LET radiation, but with lower values for yields of free radicals and higher values for molecular yields.Received: 04 October 2014; Revised: 23 March 2016; Accepted: 23 March 201

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

It has been a longstanding issue in the radiation chemistry of water that, even though H2 is a molecular product, its “escape” yield g(H2) increases with increasing temperature. A main source of H2 is the bimolecular reaction of two hydrated electrons (e-aq). The temperature dependence of the rate constant of this reaction (k1), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150°C. Recently, it has been suggested that this temperature dependence should be regarded as being independent of pH and used in high-temperature modeling of near-neutral water radiolysis. However, when this drop in the e-aq self-reaction rate constant is included in low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET) modeling calculations, g(H2) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g(H2) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k1 observed at ~150°C in alkaline water to near-neutral water is questionable and that further measurements of the rate constant in pure water are highly desirable.Received:13 June 2013; Revised: 27 August 2013; Accepted: 28 August 201

LOW-LINEAR ENERGY TRANSFER RADIOLYSIS OF SUPERCRITICAL WATER AT 400 °C: DENSITY DEPENDENCE OF THE G(•OH)

Monte Carlo simulations were used to predict the yield of primary specie •OH denoted as g(•OH) that is formed from the radiolysis of pure, deaerat- ed supercritical water (SCW) (H2O) at 400 °C in the range of water density between ~0.15 and 0.6 g/ cm3. It is known that •OH, is one of the oxidizing species that significantly can increase the possibil- ity of various corrosion and material degradation as well. The thorough radiolysis processes in SCW- cooled reactor is not established currently, and it is believed to be a challenge in developing chemis- try control strategies for future Supercritical Water Reactor (SCWR). Since SCWR technology is now still under the conceptual design, hence there is only limited information published on the yields of radiolysis under these conditions. In this work, g(•OH) was calculated at spur lifetime (τs/ minimum time needed before the species within spur distributed homogeneously into the bulk solu- tion), 10-7 and 10-6 sec after the ionization event at all densities. From this work, it is shown that the data measured by other researcher at lower density (0.35 g/cm3) is taken about near the spur lifetime. Finally, more experimental data are highly required in order to examine more thoroughly modeling calculation.

Temperature Dependence of the Primary Species Yields of Liquid Water Radiolysis by 0.8-MeV Fast Neutrons

The yields of species such as e-aq, H•, •OH, H2 and H2O2, formed from the radiolysis of neutral liquid water by the incidence of 0.8-MeV neutrons at temperatures between 25 and 350°C, were calculated by using Monte Carlo simulations. The slowing down of these neutrons through elastic scattering produced recoil protons elastically of ~0.5057, 0.186, and 0.0684 MeV which had linear energy transfers (LETs) of ~40, 67 and 76 keV/µm, respectively, at 25°C. The effects of neutron radiation can be predicted based on the contribution of those first three recoil protons by neglecting the radiation effects due to oxygen ion recoils. Then, the fast neutron yields could be estimated by summing the yields of contributing protons after corresponding weightings were used according to their energy. In this work, yields were calculated at 10-7 and 10-6 s after incidence of neutron radiation in water at the aforementioned temperature range. Overall, there is a reasonably good agreement between our calculated and existing experimental G-values for the entire temperature range. However, we proposed an hypothesis that the not very significant difference between experimental data and our calculated data is due to the different measuring time used in obtaining the experimental data as compared to the ones used in our calculation. Our computed yields for 0.8-MeV fast neutron radiation show an essentially similar temperature dependences over the range of temperature studied with 2-MeV fast neutron and low-LET radiation, but with lower values for yields of free radicals and higher values for molecular yields

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

It has been a longstanding issue in the radiation chemistry of water that, even though H2 is a molecular product, its “escape” yield g(H2) increases with increasing temperature. A main source of H2 is the bimolecular reaction of two hydrated electrons (eaq). The temperature dependence of the rate constant of this reaction (k1), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150°C. Recently, it has been suggested that this temperature dependence should be regarded as being independent of pH and used in high-temperature modeling of near-neutral water radiolysis. However, when this drop in the eaq self-reaction rate constant is included in low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET) modeling calculations, g(H2) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g(H2) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k1 observed at ~150°C in alkaline water to near-neutral water is questionable and that further measurements of the rate constant in pure water are highly desirable

Low-energy electron-induced DNA damage: effect of base sequence in oligonucleotide trimers

DNA damage induced by low-energy electrons (LEEs) has attracted considerable attention in recent years because LEEs represent a large percentage of the total energy deposited by ionizing radiation and because LEEs have been shown to damage DNA components. In this article, we have studied the effect of base sequences in a series of oligonucleotide trimers by the analysis of damage remaining within the nonvolatile condensed phase after LEE irradiation. The model compounds include TXT, where X represents one of the four normal bases of DNA (thymine (T), cytosine (C), adenine (A), and guanine (G)). Using HPLC-UV analysis, several known fragments were quantified from the release of nonmodified nucleobases (T and X) as well as from phosphodiester C-O bond cleavage (pT, pXT, Tp, and TXp). The total damage was estimated by the disappearance of the parent peaks in the chromatogram of nonirradiated and irradiated samples. When trimers were irradiated with LEE (10 eV), the total damage decreased 2-fold in the following order: TTT > TCT > TAT > TGT. The release of nonmodified nuclobases (giving from 17 to 24% of the total products) mainly occurred from the terminal sites of trimers (i.e., T) whereas the release of central nucleobases was minor (C) or not at all detected (A and G). In comparison, the formation of products arising from phosphodiester bond cleavage accounted for 9 to 20% of the total damage and it partitioned to the four possible sites of cleavage present in trimers. This study indicates that the initial LEE capture and subsequent bond breaking within the intermediate anion depend on the sequence and electron affinity of the bases, with the most damage attributed to the most electronegative base, T