Radiation-chemical Processes Research Articles

Physicochemical characteristics and radiation-chemical processes in the system PuO2-sorbed water

The experimental data on formation of H2 and O2 during storage of PuO2 samples containing sorbed water were analyzed. It was shown that the rates of formation of these gases, all other factors being the same, are governed by the procedure of water sorption: in sorption from the liquid phase these rates are significantly higher than those in sorption from the gas phase. To elucidate the conditions of safe storage of PuO2, it is necessary to obtain systematic quantitative data on the kinetics of H2 and O2 formation in the system consisting of PuO2 and water sorbed from moist air. The optimized mathematical model of sorbed water radiolysis adequately describing the experimental data on the formation of H2 and O2 at room temperature in the system consisting of PuO2 and water sorbed from the liquid phase is presented. The rate constant of H2 and O2 recombination in the presence of PuO2 containing 2–3% water was found to be ≤ 1 × 10−5 mol−1 s−1. With the knowledge of the reaction rate constants, the model allows calculation of the amounts of H2 and O2 and the pressure in the storage vessel depending on the amount of sorbed water, radiation dose rate, and storage duration.

Radiation-chemical processes in polyorganosiloxanes

Radiation-induced changes in polymethylphenylsilsesquioxane block copolymers were studied. It was found that a pregel was formed at doses of 30–50 kGy and gel formation mainly occurred in polydimethylsiloxane (PDMS) blocks. The UV spectrophotometry of samples in a cyclohexane solution demonstrated that, at doses of 10–50 kGy, a noticeable broadening of absorption bands due to phenyl groups was observed only in oligophenylsilsesquioxane and not in the block copolymers. Hydrogen atoms formed in the radiolysis of PDMS did not reach polyphenylsilsesquioxane blocks.

Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique

A new pulse radiolysis system based on a femtosecond electron beam and a femtosecond laser light with oblique double-pulse injection was developed for studying ultrafast chemical kinetics and primary processes of radiation chemistry. The time resolution of 5.2 ps was obtained by measuring transient absorption kinetics of hydrated electrons in water. The optical density of hydrated electrons was measured as a function of the electron charge. The data indicate that the double-laser-pulse injection technique was a powerful tool for observing the transient absorptions with a good signal to noise ratio in pulse radiolysis.

Spectrophotometric and luminescent analysis of polytetrafluoroethylene treated by γ-irradiation near the melting point

Radiation-modified polytetrafluoroethylene (PTFE) that exhibited intense fluorescence in the visible spectral region was prepared. Radiation modification was performed with 60Co γ rays at 330°C, a dose of 0.2 MGy, and an atmospheric air pressure ranging from 10−3 to 100 mmHg. The fluorescence and electronic absorption spectra of film (95 µm) specimens irradiated under the given conditions were studied. An important role of molecular oxygen in radiation-chemical processes responsible for the generation and buildup of fluorescing entities was revealed. The spectral characteristics of the four most intense fluorescent centers were determined. It was concluded that polyene systems of π bonds-CF2-(CF=CF)n-CF2-(n=4−11) in the product are responsible for fluorescence. A scheme of radiation-chemical transformation resulting in π-conjugated structures was proposed.

Motivation and development of ultrafast laser-based accelerator techniques for chemical physics research

The products of radiation induced chemical reactions are determined by rapid primary processes such as energy transfer, thermalization and solvation. Ultrafast photoionization experiments on liquid water demonstrate that these initial events occur on time scales <5ps and involve a complicated interplay between electronic relaxation and vibrational energy redistribution. These experiments also show that the chemical processes originating from ionizing radiation are unique and cannot be reproduced by laser photons alone. Due to the lack of a suitable femtosecond source of ionizing radiation, knowledge of the primary processes in radiation chemistry remains poor. To address this issue a 20TW laser system has been constructed to obtain subpicosecond electron pulses with energies in the 1–10MeV range. In addition to the production of femtosecond electron pulses, future efforts will be directed towards using this laser for accelerating heavier particles such as protons and generating hard X-rays.

Development of compact picosecond pulse radiolysis system

A pulse radiolysis technique is known as a powerful method to study the primary processes in radiation chemistry. However, a large-sized accelerator and a huge cost should be required for the construction and operation of pulse radiolysis system. Very recently, the new compact picosecond pulse radiolysis system composed of the small size accelerator and laser system has been developed at RISE, Waseda University. To evaluate the performance of the system, the absorption measurement of hydrated electrons was carried out as a preliminary experiment. As a result, the time resolution was estimated to be approximately 26ps. It is analytically calculated to be approximately 30ps (FWHM) with the convolution among the beam size, the bunch length of electron beam, the laser spot size, and the laser pulse length. The experimental value is in good agreement with the calculated one. The system has been developed to get better S/N ratio and will be applied for the study of primary processes in radiation chemistry of polymer system.

Ultrafast processes in radiation chemistry

Ultrafast (<10 −11 s) physical processes play a pivotal role in radiation-induced chemical reactions. Due to the lack of subpicosecond pulse radiolysis tools knowledge of these events is lacking. We describe the development of laser-based techniques that are designed to provide a source of femtosecond electron and X-ray pulses for chemical physics research. Terawatt laser systems have succeeded in relativistically accelerating subpicosecond electron bunches to energies greater than 5 MeV. The electron pulses generated this way are suitable for pulse radiolysis.

Radiolytic Corrosion of238Pu-doped UO2Pellets in 5 M NaCl Solution

AbstractDeaerated 5 M NaCl solution is alpha-irradiated up to 700 days in the presence of UO2pellets. Experiments are conducted with238Pu-doped pellets in pure brine and with undoped UO2pellets in a238Pu containing brine. The long-lived radiolysis products H2, O2and ClO3-are formed in all cases proportional to the dose applied on the solution and with yields corresponding to 200, 80 and 7 nMol/(L*Gy).The U concentrations increase with time but do not exceed about 10−4Mol/L thus indicating Uranium to be solubility limited. Moreover, the rinse solutions of the vessels at the end of the experiments contain up to one order of magnitude more U than found in the solutions. The total amounts of mobilized U deviate by less than a factor 10 from each other regardless of the large ratio of surface dose rates (ratio up to 2300) or mean dose rates (ratio up to 100), applied while the reference experiment (UO2pellet in pure brine) yields only some 10−7Mol/L U which is over 3 orders of magnitude lower than observed in experiments with radiation present.The simulation of the radiation chemical processes using a kinetic reaction model can reasonably reproduce the findings of the experiments with respect to the formation of H2and O2and give the order of magnitude for the concentrations of chlorine species and of oxidized UO2.

Formation and Dissociation Dynamics of Molecular Superexcited States

Abstract This is a survey of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states. The Platzman’s concept of the superexcited states deduced from the theoretical analysis of the interaction of ionizing radiation with matter has been recently substantiated experimentally. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (and/or rotationally), doubly, or inner-core excited, and converge to each ion state. Non-Rydberg superexcited states are also observed. The dissociation dynamics of these states as well as the products are very different from those of the lower states excited below ionization thresholds. The neutral dissociation is unexpectedly more important than ionization. Molecular superexcited states are described as reaction intermediates in some fundamentally important collision processes, e.g., electron-ion or ion-ion recombination, Penning ionization, and electron attachment, and are described also as important species in the primary processes of radiation chemistry, VUV-photochemistry, and plasmas. Comments are presented on molecular superexcited states in the condensed phase. Finally, the future perspectives of the present studies are summarized.

Spectroscopy and dynamics of molecular superexcited states. Aspects of primary processes of radiation chemistry

A survey is given of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states. The Platzman’s idea of the superexcited states deduced from the theoretical analysis of the interaction of ionizing radiation with matter has been recently substantiated experimentally. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (or/and rotationally), doubly, or inner-core excited, and converge to each of ion states. Non-Rydberg superexcited states are also observed. The dissociation dynamics of these states as well as the products are highly different from those of the states excited below ionization thresholds. The neutral dissociation is unexpectedly important in comparison with ionization. Some remarks are presented of superexcited states in the condensed phase from the viewpoint to understand the primary processes in radiation chemistry.

A Calorimetric Method for Studying Radiation-Chemical Processes in Condensed Media

A calorimetric method is developed to study the thermokinetics of radiation-chemical processes (RCPs). The samples under study are heated by gamma radiation absorbed in the adiabatic mode. The developed method allows the minimum level of reliably detected thermal effects of RCPs to be decreased by more than an order of magnitude as compared with conventional calorimetric methods. The calorimetric system is also virtually free from the time lag typical of the majority of dynamic calorimeters. These advantages allow the kinetics of reactions with rather low radiation-chemical yields to be studied immediately during irradiation.

Radiation-Chemical Aspects of Radioactive Waste Management

Studies of radiation-chemical processes in model liquid radioactive waste are summarized. These processes are radiolytic transformations of various substances present in the waste (actinide ions (primarily, neptunium and plutonium ions), inorganic matter (for example, nitrate ions), and organic compounds (acetic acid, EDTA, etc.)) and radiolytic gas evolution. Attention was focused on the latter process. The volumes and compositions of gases formed in the γ-radiolysis of aqueous solutions that simulated intermediate-level and high-level liquid waste were determined. It was found that the nature and the radiation-chemical yields of released gases depend on the absorbed dose, the composition and concentration of solutions, the irradiation temperature, the presence of solid substances, etc. In a number of cases, data obtained with model solutions were compared to the results of studies of real liquid radioactive waste.

Radiolysis and corrosion of238Pu-doped UO2 pellets in chloride brine

Deaerated 5 M NaCl solution is irradiated in the presence of UO2 pellets with α-radiation from238Pu. Experiments are conducted with238Pu doped pellets and others with238Pu dissolved in the brine. The radiolysis products and yields of mobilized U and Pu from the oxidative dissolution of UO2 are determined. Results found for radiolysis products and for the oxidation/dissolution of pellets immersed in Pu containing brine are similar to results for Pu doped pellets, where the radiation chemical processes occur only in the liquid layer of some 10 σm thickness adjacent to the pellet. The yield of radiolysis products is comparable to earlier results, that of mobilized U from the pellets is < 1% of the total amount of oxidized species. Thus, the radiation chemical yield (G-value) for mobilized hexavalent U is < 0.01 ions/100 eV. In spite of the low radiation yield for the corrosion, the rate of UO2dissolution is higher than expected for the concentrations of long-lived oxidizing radiolysis compounds found in the solutions.

Radiation-Chemical Processes in the PuO2-Sorbed Water System

In this paper a mathematical model of the radiolysis of water sorbed onto PuO2 has been formulated and discussed The model is based on a postulate that kinetic characteristics of radiolysis formation products, in particular H2, O2 and H2O2, of sorbed water are dependent upon its content in PuO2. To explain the experimental findings, a reaction between H2O2 and PuO2 has been postulated ensuing the formation of water and overstoicheiometric PuO2. A minimum rate constant of the reaction rate (2x10-3 mole-1s-1) has been determined The mathematical model formulated adequately describes the experimental results on H2and O2 formation being occurred while PuO2 storage.

Degradation of Formic Acid in Different Oxidative Processes

Abstracts—The decomposition of formic acid by OH radicals produced upon UV irradiation of hydrogen peroxide or in a spark corona discharge between solid electrodes and water surface was studied. Experimental data for both processes are rationalized in terms of the same scheme of transformation of formic acid and various schemes of interaction of active species. The degradation yields in these processes are compared with those obtained in a radiation-chemical process. The yields with reference to the efficiency of the devices were shown to be close in value in all cases.

Contribution of Radiation Technology to Environmental Protection

The direct and indirect contributions of radiation technology to environmental protection are summarized. The indirect contribution consists in a considerably decreased or, in some cases, completely abandoned use of harmful substances (such as volatile organic solvents) in radiation-chemical processes, utilization of spent materials and products (in particular, butyl rubber), etc. The direct contribution includes the direct use of ionizing radiation for the purification and disinfection of polluted natural and drinking water, municipal and industrial wastewater, other liquid wastes, gas systems (for example, flue gases from thermal power stations), sewage sludge, and solid wastes (infected medical wastes, polluted soil, etc.); pest control (by radiation sterilization of male insects); etc. The corresponding examples based on recent studies are cited.

Ionizing-Radiation Ignition of a Hydrogen–Air Mixture

The behavior of a hydrogen–air mixture under the action of ionizing radiation was studied. A kinetic model for radiation-chemical processes in the H2–O2–N2 system was constructed, and the effects of the main parameters of radiolysis on shifting its flammability limits were analyzed. It was found that, under normal conditions (p = 0.1 MPa, T0 = 300 K), the ignition of the stoichiometric mixture began at a radiation intensity of about 0.1 kGy/s. The induction period of the chain H2 oxidation reaction shortened with increasing radiation dose rate and pressure.

Micellar electrophoretic capillary chromatographic analysis of the products produced in the radiolytic oxidation of toluene and phenol

In this paper, we consider the effect of CH 3 and OH substitution on a benzene ring in directing the site of OH radical attack in oxidative processes initiated by the gamma radiolysis of aqueous solutions of phenol and toluene. The analytical method used was micellar electrophoretic capillary chromatography. This approach permits the resolution of the radiolytic products resulting from the oxidation of the hydroxycyclohexadienyl radicals initially produced by addition of OH to the aromatic compounds and allows one to obtain a complete mechanistic picture of the radiation chemical processes.

Photoradiation processes in combined polymeric materials

The methods of ESR, optical and mass-spectroscopy have been used to study the formation and decay of active intermediates, radiation and photoradiation gas evolution in a series of combined polymeric materials. The investigated samples were films of polyimide (PI), one- or two-side coated PI–fluoroplast, poly(ethylene terephthalate) (PET), PET coated with polyacryl on both sides, etc. Different coatings and polymeric substrates have been established to produce a mutual effect on the radiation- and photo-induced transformations of the combined polymers at the stage of radical formation and gas evolution. Thus, γ-irradiation of PI–fluoroplast films at 300 K intensifies the radiation-chemical processes in the fluorinated coatings, namely, the radiation-chemical yield of macroradicals in the coatings is several times higher than that in fluoro-containing polymers. In the PET film coated with polyacryl the yield of macroradicals related to polyacrylic acid is significantly lower than the value generally cited in the publications. A mutual effect of the coatings and substrates on the radiation and photoradiation-induced gas evolution is even more noticeable. Thus, a gas mixture of the PI–fluoroplast film contains lower amounts of carbon oxide and fluorinated products and no hydrogen at all. The interphase processes, distortion of the boundary layer structure, electronic equilibrium, mutual effect of the evolved gases, etc. can account for the reasons of such non-additivity.

Control of 3D structure growth by the example of a biocell

This work, which relates to the area of biotechnology, touches upon the problem of decreasing nanoelectronic and nanooptic components and creating 3D controllable nanomechanic structures. The role of radiation chemical processes in controlling material transport, as well as the growth of membranes and fibers, is demonstrated with a biocell. The fibers are viewed as waveguides that direct control pulses. It is assumed that such waveguides make it possible to produce 3D neural networks.