Radiolytic Processes Research Articles

Effect of nanostructured surface of radiation-oxidized aluminum on radiolytic processes in n-hexane

The effect of a nanostructured surface of radiation-oxidized aluminum on radiolytic processes in n-hexane was investigated. It was ascertained that the decreased thickness of oxide films on Al surface by approximately two orders of magnitude (from 600 to 8 nm) led to an increase in the rate of molecular hydrogen formation by a factor of ∼7 (from 1.1 to 7.6 × 1015 molecule g−1 s−1); meanwhile, the radiation chemical yield of H2 changed from 4.3 to 8.2 molecules/100 eV. The results obtained were interpreted in terms of the difference in the degree of imperfection of the surfaces of radiation-generated Al plates and enhancement of radiation-generated centers, which was evidenced by AFM images and RTL measurements.

Enhancement of carboxylic acid degradation with sulfate radical generated by persulfate activation

The aim of this work was to investigate the generation of sulfate radical for the removal of two carboxylic acids in aqueous solution: acetic and citric acids. From photochemical and radiolytic processes, kinetics of the degradation of these two carboxylic acids was studied as a function of the pH of the solution. It was shown that the maximum of acetic acid degradation occurred at pH 5. Above this pH, competitive reactions with the carbon mineralized inhibit the reaction of with the solute. In the case of citric acid, pH has only a little effect on the kinetic of citric acid degradation. The determination of mineralization yields shows several differences depending on carboxylic acids and pH. The degradation of both carboxylic acids was also studied in the radiolysis process whether with or without persulfate addition. A comparison of the processes of sulfate radical production is presented.

Experimental monitoring of ozone production in a PET cyclotron facility

Ozone produced from radiolytic processes was investigated as a possible health hazard in the working environment at the University Hospital "S.Orsola--Malpighi" PET facility. Intense radiation fields can generate ozone, known to be the most toxic gas produced by ionizing radiation around a particle accelerator. To evaluate ozone concentration in air, two different measurement campaigns were conducted with passive diffusion detectors. Comparison of the results with the concentration limits recommended by American Conference of Governmental Industrial Hygienists (ACGIH) demonstrated that ozone poses no health hazard to workers around a biomedical cyclotron.

Using 211At as internal alpha radiolysis source allowing for simple detection of radiolysis products

The determination of radiolysis products is an important field both for the basic understanding of the radiolysis process and for process development. The latter case mainly dealing with processes for handling radioactive wastes. There are several kinds of radiolysis processes that originate from the different kinds of radiation. Gamma rays and high-energy beta has a high ability to penetrate barriers while alpha irradiation in principle has to be performed with the radiation emitting nuclide inside the actual sample. This can be a problem since most laboratories able to identify radiolysis products cannot handle alpha contaminated samples. In this paper we suggest the use of 211At as internal alpha emitting radionuclide. Due to its short half-life and decay to more or less stable daughters the radiolysis products may be examined using normal equipment without causing contamination.

Effect of irradiation on preliminarily radiation-oxidation-treated stainless steel

Radiolytic processes of decomposition of water on specimens of stainless steel subjected to preliminary radiation-oxidation treatment are studied.

Ionization chemistry in H2O-dominated atmospheres of icy moons

The processes of the formation and dynamics of tenuous gaseous envelopes of icy moons in giant-planet systems are considered. Tenuous exospheres with relatively dense surface layers are likely to form around more massive icy satellites, such as, for example, the Galilean satellites Europa and Ganymede in the Jovian system. Escaping exospheres are formed in the case of low-mass icy moons, as happens for the icy satellite Enceladus in the Saturnian system. The main parent component of such gaseous envelopes is water vapor, which enters into the atmosphere as a result of thermal degassing processes, nonthermal radiolysis, and other active processes and phenomena on the icy surface of a satellite. A numerical kinetic model has been developed to study on a molecular level the processes of the formation, chemical evolution, and dynamics of tenuous gaseous envelopes dominated mainly by H2O. The ionization processes in such tenuous gaseous envelopes are caused by solar ultraviolet (UV) radiation and solar-wind and/or magnetospheric plasma. The primary processes when ultraviolet solar photons and plasma electrons affect the tenuous gas of the H2O-dominated atmosphere are responsible for the chemical diversity of the gaseous envelopes of icy moons. Ionization chemistry, including ion-molecular reactions, dissociative recombination of molecular ions, and the reactions of the charge exchange with magnetospheric ions, is important for the formation of chemical diversity in gaseous envelopes of icy satellites. The model considered in the study was used to numerically simulate the formation and development of chemical diversity in the tenuous gaseous envelope of Enceladus. The numerical results were compared to the direct Cassini measurements during its close flyby near Enceladus.

Hydrogen oxidation in H 2/O 2/N 2 gas mixture by pulsed DBD at atmospheric pressure

The removal of hydrogen in the off-gas from fuel cells in a safe manner is desirable. In the present research, a model gas mixture, 2.0 vol% H 2 and 19.5 vol% O 2 balanced with nitrogen, was directly treated with a pulsed dielectric barrier discharge (DBD). The DBD plasma reactor had coaxial electrode geometry with quartz glass tubes. The gas mixture was allowed to flow into one end of the coaxial quartz glass tubes, and the DBD plasma was generated in a gap at atmospheric pressure. The applied voltage (peak-to-peak value) was varied from 17.5 to 37.5 kV, with repetition rates from 2.1 to 49.6 kHz. The discharge power in the DBD plasma ranged from 50 to 1230 W. Effects of the applied voltage, its repetition rate, and the exhaust gas temperature on the hydrogen conversion rate were investigated. The hydrogen conversion rate generally increased with an increase in the exhaust gas temperature and discharge power. The most important characteristic of this procedure is that hydrogen was oxidized easily below the ignition temperature in normal combustion. A conversion rate as high as 99% was attained at a discharge power of 1200 W and an exhaust gas temperature of 80 °C. To understand the reaction mechanism of hydrogen oxidation in the DBD plasma, a reaction kinetic study was carried out. The chemistry was resolved into separate contributions from radiolysis processes and gas phase reactions. H and O radicals generated by electron-impact reactions in the DBD plasma play an important role in promoting hydrogen oxidation in the gas phase reactions. Three important characteristics of hydrogen oxidation in DBD plasma, the advantage of the DBD plasma, effect of gas temperature, and effect of discharge power, are discussed based on simulation results.

Behavior of NAFION Perfluorosulfonate Ionomer Membranes in Presence of Tritiated Water

The behavior of NAFION membrane was analyzed in presence of high activity tritiated water. The fundamental radiolytical processes have been analyzed by simulation, using quantum mechanical methods. NAFION and PTFE samples were immersed in water and exposed to gamma radiation fields. The samples were characterized by FTIR spectrometry and fluoride emissions. Self radiolytical processes were analyzed by storage of NAFION in high activity triatiated water. The induced modification analyses were carried out using FTIR and fluoride emissions characterization. The experimental results were correlated with quantum-chemical simulations.

Impact of oxygen concentration on yields of DNA damages caused by ionizing radiation

Local hypoxia-induced radiation resistance is one of the major problems in current radiation therapy of solid tumors. Concentration of cellular oxygen is not uniform within tumor volume and changes during the course of radiotherapy. The aim of this work is to evaluate the influence of oxygen concentration on the water radiolysis process and subsequently on the yields of primary DNA damages caused by ionizing radiation. Monte Carlo based computer modeling approach was used for this purpose. Detailed track structures provided by TRIOL code and molecular structure of 100 bp DNA oligomer generated using Amber 8 molecular dynamics package were used as input for stochastic model RADAMOL. Diffusion and chemical reactions of radicals produced in the water radiolysis process were followed in a step-by-step simulation. Both unscavengeable and scavengeable DNA damage was taken into account. It was shown that concentration of dilluted oxygen in water significantly influences yields of eaq-, Ḣ and O- and consequently modifies yields of primary DNA damage. However, this itself cannot explain the oxygen radiosensitizing effect in living cells. The influence of oxygen on DNA damage chemical repair and fixation should be included in further research.

A kinetic study of UO2 dissolution and H2O2 stability in the presence of groundwater ions

Summary Current reactors spent nuclear UO2 fuel, that will probably enter geologic formations for disposal, is assumed to get into contact with groundwater of a repository earliest after several thousands of years. After this time, its alpha activity will be still high enough to induce a process of water radiolysis that leads to production of oxidants and, probably, to the oxidative dissolution of the material, that could start a mobilisation of radiotoxic nuclides. The main oxidant produced during this process would be hydrogen peroxide (H2O2), which is thermodynamically unstable and can decompose during reactions with certain groundwater ions. Information on the stability of H2O2 is therefore important for assessing the behaviour of the material in the repository. We investigated the stability of H2O2 and the correlated dissolution of U from UO2 as a function of the concentration of H2O2 and of the groundwater ions carbonate, sulphate, and silicate. Decomposition of H2O2 was measured in the presence and absence of UO2(cr). We monitored the concentrations of H2O2, Udiss, groundwater ions, O2diss, and the pH in aqueous solution under Ar atmosphere. The lowest stabilities (in average) for H2O2 were found in carbonate solutions. When comparing the effect of carbonate systems containing UO2(cr) to carbonate solutions without a solid phase, we found that in some of the homogeneous tests H2O2 was consumed faster. This effect was not observed with the other groundwater ions. Also, U dissolution rates were significantly lower in the carbonate experiments than in those with sulphate or silicate. The results suggest that a radical-controlled mechanism (similar to the Halpern–Smith-mechanism of dissolved U(IV) oxidation) takes place at the UO2 surface during dissolution. The hypothesis is supported by ESR measurements on hydroxyl radical scavenging by the carbonate ion.

Spent fuel UO2 matrix alteration in aqueous media under oxidizing conditions

The effects of αβγ radiolysis of water on oxidation and dissolution of the UO2 matrix of spent fuel were investigated in aerated media. The prospect of long-term interim storage of spent fuel assemblies in pools requires prior examination of an incident scenario in which a cladding failure could eventually lead to contact between water and the fuel. Radiolytic oxidation of the matrix together with the formation of secondary phases subject to volume expansion could then no longer be disregarded, and could enhance the degradation of the failed rod. To determine the amplitude of the alteration various spent fuel leaching experiments conducted at different degrees of oxidizing conditions and of accessibility of species at the reaction interface are discussed. This work is based on modeling the radiolysis processes and on various possible interpretations of leaching test results. Properly interpreting these results implies full knowledge of the fuel sample history, and the development of a rigorous experimental approach to eliminate any artifacts (problems due to the residual inventory at the grain boundaries, or to inaccurate estimates of the actual surface area). The results indicate that despite highly variable oxidizing conditions (presence of an external γ irradiation source, addition of H2O2, aerated water) the spent fuel matrix alteration determined from the release of tracer elements such as Cs and Sr is always the same. Conversely, allowing for the alteration conditions (no solution renewal and no complexants), the uranium concentrations in solution do indeed depend on the oxidizing conditions as does the nature of the secondary phases liable to precipitate.

Degradation of epoxy resins under high energy electron beam irradiation: Radio-oxidation

The degradation of two epoxy systems exposed to electron beam irradiation in oxygen atmosphere has been studied and compared to our previous results obtained under helium atmosphere. A multi-scale approach to the degradation has been carried out using a variety of analytical methods including solid state NMR spectroscopy, 2D-WISE experiments, SEC measurements and infrared spectroscopy associated with chemical derivatisation treatments. It is shown that the degradation of both polymers is governed by radiolytic processes which involve chain scissions. The formation of oxidation products, mainly carboxylic acid chain ends, was shown at the surface of the irradiated film. Oxidative degradation is limited to the first 20 μm of the film due to the low oxygen diffusion coefficient in epoxy polymers combined with the high dose rates of irradiation.

A comprehensive model to describe radiolytic processes in cement medium

Basic mechanisms controlling the radiolysis in cementitious matrices are reviewed in the specific context of the gamma irradiation, in closed system without upper vapour space, at 25 °C, with a pore solution representative of a Portland cement paste. A general survey of data corresponding to each phenomenological area is given, particularly with a list of reactions for alkaline medium and a revised description of equilibria related to calcium hydroxide. Simulations as a function of dose rate, liquid saturation in the porosity or initial amount of H 2 are carried out with the CHEMSIMUL code. They show the existence of several ways in the radiolysis regulation involving calcium peroxide octahydrate precipitation, a law of mass action through the bulk general system, or the particular activation of an Allen’s type chain reaction. This latter seems faster in alkaline medium (pH > 13) where radicals H and OH are, respectively, replaced by e aq - and O − . Excepted when a strong reducing agent is initially present, O 2 is normally produced by radiolysis and CaO 2 · 8H 2O cannot be responsible of its disappearance.

Evaluation of fundamental processes in macromolecular structures radiolysis using quantum-chemical methods

The behavior of macromolecular structures in ionizing fields was analyzed by quantum-chemical methods. In this study, the primary radiolytical effect was analyzed using a two-step radiolytical mechanism: (a) ionization of molecule and spatial redistribution of atoms in order to reach a minimum value of energy, characteristic to the new quantum state and (b) neutralization of the molecule by capture of one electron and its rapid dissociation into free radicals. Chemical bonds suspected to break have minimum homolytic dissociation energies.

Radiolyis of polytetrafluoroethylene and polystyrene catalytic supports in presence of tritiated water

The behavior of hydrophobic catalyst supports in the isotopic exchange reactions H/D/T was analyzed by the determination of radiolysis processes in the presence of HTO. In this study, the behavior of polytetrafluoroethylene and styrenedivynilbenzene copolymer catalyst supports in the presence of HTO was analyzed. The primary and secondary radiolytic effects were evaluated using original computational methods. The radiolytical processes were analyzed by exposure of the catalytic supports immersed in water to gamma radiation, and also by analysis of chemical modification by FTIR spectroscopy, fluoride formation and sol/gel fraction determination.

Laser-muon spin spectroscopy in liquids—A technique to study the excited state chemistry of transients

This study introduces laser-muon spin spectroscopy in the liquid phase, which extends muonium chemistry in liquids to the realm of excited states and enables the detection of muoniated molecules by their spin evolution after laser excitation. This leads to new opportunities to study the Kinetic Isotope Effects (KIEs) of muonium/atomic hydrogen reactions and to probe transient chemistry in radiolysis processes involved in muonium formation, as well as muoniated intermediates in excited states.

Radiation-chemical transformations of humic acid in aqueous solutions

The transformations of kinetically stable aqueous humic acid solutions saturated with Ar, N2O, or air by the action of fast electrons at absorbed doses to 20 kGy were studied. In radiolytic and postradiation processes, humic acid was converted into insoluble compounds, whose precipitation accelerated with dose. The formation of insoluble compounds was enhanced in neutral and acidified solutions saturated with air and/or N2O. As low-molecular-mass carboxylic acids that resulted from the radiolytic fragmentation of humic acid were accumulated in solution, the phase transformation weakened. The irradiated solutions were easy to purify by chemical coagulation.

Cementation of ILW ion exchange resins: Impact of sulfate ions released by radiolysis on hydrated matrix

Some of the ion exchange resins used during treatment of spent nuclear fuels are intermediate level radioactive wastes which may be damaged by radiolysis process, releasing sulfate ions directly into the cement-based encapsulating material. This work consists in an experimental study of the resulting sulfate attack on the properties of the hydrated matrix: dimensional stability, mineralogy and microstructure of the samples, as well as variations in the chemical composition of the curing solution, were studied during six months. Three sites of delayed ettringite formation were detected: into the cement matrix near the surface exposed to solution, localized in the interfacial transition zone between cement matrix and resins, or progressively replacing the portlandite that initially fulfilled the cracks of anionic resins. During the experiment period, the ettringite precipitation and the expansion detected were moderate, and did not lead to cracking. The material involved was considered as having a good resistance to sulfate attack.

Trapping of radiolytic hydrogen by amorphous cobalt oxysulfide

Hydrogen production from the radiolysis of liquid and gaseous hydrocarbons was studied in the presence of several transition metal sulfides. Cobalt oxysulfide obtained by aqueous precipitation was the most efficient admixture to decrease radiolytic production of hydrogen by pure hydrocarbons or mixtures of saturated and unsaturated hydrocarbons. Cobalt oxysulfide was characterized by XRD, scanning and transmission microscopy, and IR spectroscopy. It seems to be amorphous compound with the impurities of lamellar Co(OH) 2 phase. The organic phases were analysed before and after irradiation by γ-rays or protons in order to elucidate the origin of the effect. It has been shown that the solid does not change the composition neither the amount of the organic radiolysis products, neither in liquid nor in gas phase experiments. Therefore the presence of solid does not influence any radiolysis processes in the organics. Amorphous cobalt oxysulfide acts essentially as a trap of hydrogen, being able to absorb considerable amounts of H 2 (up to 0.5 mol H 2/at. Co). The study of the solid–gas interaction showed that slow reaction of cobalt oxysulfide with hydrogen occurs at ambient conditions independently of the irradiation of the system.

Radiolysis of NaCl at high and low temperatures: development of size distribution ofbubbles and colloids

New experimental results are presented on low temperature irradiation(18 °C) of rock-salt samples which had been exposed to initial doses up to 320 GRad at100 °C. Differential scanning calorimetry (DSC) shows that the latent heat of melting (LHM) ofsodium colloids decreases during subsequent low-temperature irradiation, whereas thestored energy (SE) increases slowly, indicating that the process of radiolysis continues. Thedecrease of the LHM is due to dissolution of large colloids, because the intensities of themelting peaks decrease during the second stage irradiation at low temperature. The modelis formulated to describe the nucleation kinetics and the evolution of the sizedistribution of chlorine precipitates and sodium colloids in NaCl under high doseirradiation. It is shown that the mechanism of dissolution of large Na colloidsduring low temperature irradiation can be related to melting of sodium colloids.