Radiolysis of water

The second reaction scheme investigated in this work is shown in Table 6.2, with the respective diffusion coefficients for all the species involved shown in Table 6.3. In this reaction scheme, reactive products are generated at steps (1) and (2) of the reaction scheme. For H+ + Ojlq, the reaction is not fully diffusion controlled because eff and it becomes necessary to impose a boundary velocity. A reaction distance of 5 A has been used by Pimblott et al. [6] based on the work done by Hart and Anbar [7], using a reaction boundary velocity of 4.2 m s .  

As the reaction scheme contains charged species with a Coulomb potential, generating an analytical recombination time is not possible since the backward diffusion equation for ions cannot be solved in closed form (as discussed in Chap. 4 of this work). However, for high-permittivity solvents such as water, an excellent approximation has been developed [6] which scales the encounter radius and initial separation 

Pimblott et al. [9] have demonstrated the importance of using the transformed distance scale for both the encounter distance and interparticle separation. If only is used, significant errors are shown to occur for reactants at short separation distances, greatly biasing the spur kinetics. 

For partially diffusion controlled reactions the reactivity of the surface must be taken into consideration, which can be readily achieved by using the radiation boundary conditions of the form 

The effective reaction distance can be used to link v to the bimolecular rate 

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Potential fusion appHcations other than electricity production have received some study. For example, radiation and high temperature heat from a fusion reactor could be used to produce hydrogen by the electrolysis or radiolysis of water, which could be employed in the synthesis of portable chemical fuels for transportation or industrial use. The transmutation of radioactive actinide wastes from fission reactors may also be feasible. This idea would utilize the neutrons from a fusion reactor to convert hazardous isotopes into more benign and easier-to-handle species. The practicaUty of these concepts requires further analysis. 

Hulls Handling. After the fuel has been dissolved, the residual pieces of zirconium cladding, referred to as hulls, are rinsed and removed from the dissolver vessel. The decay of activation products provides sufficient heat to ensure drying of the hulls and preclude hydrogen formation caused by the radiolysis of water. 

A mathematical model of the operating characteristics of a modem HLW storage tank has been developed (60). This model correlates experimental data for the rate of radiolytic destmction of nitric acid, the rate of hydrogen generation owing to radiolysis of water, and cooling coil heat transfer. These are all functions of nitric acid concentration and air-lift circulator operation. 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

It is pertinent that S20g accepts an electron generated by pulse radiolysis of water to give optically detectable S04 within 1.5 x 10 sec . ... 

In these solutions, the hydrated electrons from the radiolysis of water produce CO by their reaction with COj, and the OH radicals attack the alcohol to form (CHjIjCOH radicals (see also footnote on page 117). 

Radicals can react with bases via hydrogen atom abstraction or, more commonly, by addition to the pi bonds in the heterocyclic nucleobases (Scheme 8.1). These reactions have been extensively studied in the context of hydroxyl radical (HO ), which is generated by y-radiolysis of water. When DNA is exposed to the hydroxyl radical, approximately 80% of the reactions occur at the bases. Many base damage products arising from the reaction hydroxyl radical with DNA have been characterized (Fig. 8.2). ... 

Despite its obvious importance, the interpretation and even the measured yields in the radiolysis of water vapor were doubtful until the sixties. It was not because of lack of experimental data rather, it was because of difficulties of comparing measurements of different workers due to artifacts and sheer experimental problems (Anderson, 1968). The greatest discrepancy is in the reported hydrogen yields, which varied between the extremes by a factor of -104 (Dixon, 1970 Anderson, 1968). It is now agreed that G(H2) in water vapor at 1021 eV/g varies around 10-3. But, as pointed out by Dixon, the absolute yield of hydrogen in pure water vapor is not a very meaningful quantity, because a steady state is achieved and a consistent steady state concentration of H2 and 02 may be... 

Gf /(Gf + Gr) = (57 + 27)/(57 + 27 + 183) = 0.315 for tritium radiolysis of water, which compares favorably with Hart s (1952) experimental value of 0.30. However, in this calculation, the particle slowing down was ignored and the interspur distance was taken to what corresponds to the beginning of the track. 

Platzman (1967) estimated that in the radiolysis of water the positive ion is left, on average, with an excitation energy of -8 eV this estimate was later lowered to 4 eV by Pimblott and Mozumder (1991). In any case, the chemical consequences of such excess energy of the positive ion is unknown, and it will be assumed that, at least in the condensed phase, the positive ion is ther-malized locally. 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

OH radicals react very fast (almost in a diffusion-controlled rate) with simple alkenes (k = 7.0 x 109 for 1-butene or cyclopentene and 8.8 x 109 M 1 s 1 for cyclohexene) and there is almost no change for 1,3- or 1,4-cyclohexadiene. Cycloheptatriene reacts very fast with all the three radicals formed in the radiolysis of water k = 6 x 109 with eaq, 8 x 109 with H atoms and 1 x 101CI M 1 s 1 with hydroxyl radicals13. 

Thus, the generation of these radicals leads to the hydroxylation of S-H. The reactive hydroxyl radicals can be produced by the radiolysis of water or... 

Metal-texaphyrin complexes such as 55 selectively accumulate in tumor cells (240) (see Section III). Complex 55 readily undergoes aone-electron reduction (Ei/2 = 0.08 V vs NHE), forming a free radical which is capable of damaging DNA. Because of the high electron affinity of 55, it may prolong the lifetime of HO- radicals formed by radiolysis of water. Complex 55 is now in phase II clinical trials for the treatment of brain tumors and lung, head, neck, and pancreatic cancer. 

The second-order rate constant for oxidation of Fe(CN)g by OH" radicals, produced by low-intensity-pulse radiolysis of water, varies with pH as in the accompanying table. Determine the pA for acid dissociation of the OH" radical in aqueous solution. (This is difficult to obtain by any other method.)... 

Carbon dioxide itself can accept e. during radiolysis of water, giving rise to the carbon dioxide anion-radical. This anion-radical can add to carboradicals. Thus, aliphatic alcohols react with the radiolytically generated hydroxyl radicals, rupturing their C—H bonds RCHjOH + OH HjO + RCH OH. These radicals accept the radiolytically generated COj" forming a-hydroxycarboxylic acids RCHjOH + CO2 RCH(OH)COO (Morkovnik and Okhlobystin 1979). 

Radiolysis of water is, at least for today, inherently limited due to the use of radioactive materials by which the product stream could be contaminated by radioactive species. Consequently while... 

Daniels M, Wigg E (1966) Oxygen as a primary species in radiolysis of water. Science 153 1533-1534... 

Sunaryo GR, Katsumura Y, Ishigure K (1995) Radiolysis of water at elevated temperatures-III. Simulation of radiolytic products at 25 and 250°C under the irradiation with y rays and fast neutrons. Rad Phys Chem 45 703-714... 

Katsumura Y, Sunaryo G, Hiroishi D, Ishiqure K (1998) Fast neutron radiolysis of water at elevated temperatures relevant to water chemistry. Prog Nucl Energy 32 113-121... 

Sawasaki T, Tanabe T, Yoshida T, Ishida R (2003) Application of gamma radiolysis of water for H2 production. J Radioanal Nucl Chem 255 271-274... 

Petrik NG, Alexandrov AB, Vail AI (2001) Interfacial energy transfer during gamma radiolysis of water on the surface of Zr02 and some other oxides. J Phys Chem B 105 5935-5944... 

LaVerne JA, Tandon L (2002) H2 produced in the radiolysis of water on Ce02 and Zr02. J Phys Chem B 106 380-386... 

It is now generally agreed that for low LET radiation, such as x-rays, y-rays, and fast electrons, the radiolysis of water can be represented to a good approximation by reaction (6) ... 

The properties of the primary radicals produced by the radiolysis of water are collected in... 

As we have seen in Sections 1 and 4, the principal primary products of the radiolysis of water are powerful oxidizing and reducing radicals in approximately equal yields. For water radiolysis to be a useful tool in general chemistry, it is desirable to convert the primary radicals to a single kind of secondary radical to achieve either totally oxidizing or reducing conditions. Moreover, there is the possibility of designing the system to have the required redox properties by suitable selection of the secondary radicals. Some useful systems that meet these requirements are described below. 

Figure 6 The production of HO2 in the proton radiolysis of water as a function of initial ion energy [75]. The slope of the solid line (0.030) is the track average yield while the slope of the dotted line (0.023) is the track segment yield for 10-MeV protons.

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