Radiolytic complications

The previous sections have discussed the radiolytic decomposition of pollutants in water and wastewater. Other applications are possible. Chelating agents were often used in various processes designed to separate radionuclides at nuclear laboratories. The presence of these compounds in the resulting nuclear wastes complicates treatment. The radiolytic degradation of these compounds is an area of current investigation [39]. 

The reduction of Am(V) by H0O2 has been studied in O.IM HCIO4 by Zaitsev et al. (99). Tlie rate is probably first order in each of the reactants, but it is complicated by some radiolytic decomposition of H2O2. The temperature dependence was determined, but the acid dependence was not. 

The reactions of polyhydric alcohols with the hydroxyl radical in aqueous solution have been extensively studied (e.g. in radiolytic and biomimetic systems), mainly because of their suitability as models for more complicated carbohydrate substrates [55] or enzymatic systems involving glycol-type radicals [56, 57]. Because there are no double bonds to which OH could add, only H-abstraction reactions are possible. Because the C-H bond energy is significantly lower than the 0-H bond energy, it is the carbons from which H are abstracted and not the alcohol function. In this type of reaction, a,yS-dihydroxyalkyl radicals are formed. The same radicals could, in principle, be produced by addition of "OH to enols, see Scheme 2, lower part. This shows the complementarity of H-abstraction and OH-addition and thereby the relevance of the former to one-electron oxidation of olefinic bonds (Scheme 2). 

The mechanismsof the acid effect has been extensively investigated (12-15, 21) whereas the current use of the polyfunctional monomers as enhancement additives in grafting is novel. The role of acid in these radiation grafting reactions is complicated and there is evidence that a number of pathways contribute to the overall enhancement effect. Thus mineral acid, at the levels used, should not affect the physical properties of the system such as swelling of the trunk polymer or precipitation of the grafted polystyrene chains. Instead evidence (12) indicates that the acid effect is due to a radiolytic increase in G(H) yields in the monomer-solvent system due to reactions similar to those depicted in Equations 1 and 2 for styrene-methanol. 

The nature of size distributions in etched track lengths is further complicated by the difficulty in determining the dimensions of unetched, or latent tracks. While there were some assumptions about the shape of a latent fission track (see Carlson 1990), their geometry has proven to be notoriously difficult to determine (Kobetich and Katz 1968). In a classic case of the act of observation modifying the observed property—radiolytic annealing was observed in apatite when samples were exposed to the electron beams... 

One of the most important parameters controlling iodine volatility is sump water pH not only will the I2 hydrolysis equilibrium and the iodine partition coefficient be affected by this parameter, but the product yields of radiolytic reactions and the extent of formation of organoiodine compounds as well. Because of the lack of practical experience, the sump water pH to be expected under severe accident conditions has to be calculated on the basis of assumed concentrations of potential sump water ingredients. In Table 7.17. (according to Beahm et al., 1992) an overview of substances to be expected in the sump water, which would effect a shift in solution pH either to lower or to higher values, is given. Besides these chemical substances, radiation may also affect sump water pH irradiation of trisodium phosphate solution (5.3 kGy/h) was reported to decrease the pH from an initial value of 9.0 to about 4.0 after 60 hours of irradiation (Beahm et al., 1992). It is obvious that in such a complicated system definition of the sump water pH to be expected in a real severe reactor accident is a difficult task. Nonetheless, a model for calculation has been developed by Weber et al. (1992). 

Finally, side reactions of the absorbent or the initiator may compete with and inhibit the chain reaction under investigation. The products of these side reactions interfere with the determination of the main reaction products. When radiolytic initiation is used the solvent is the main source of radicals and consequently these complications are minimized. 

The second complication encountered in radiolytic studies in which a radical derived from the solvent participates in the reactions, arises from the fact that the termination reaction of this radical and the initiation can result in the formation of the same product. Further clarification of this problem seems to be in order and it brings us back to the details of the initiation step. It has been found that formation of some of the radiolytic products cannot be suppressed by the addition of radical scavengers. These products are known as molecular products, which is misleading since not all these products are formed by a true molecular mechanism. There are various reasons for the inability of radical scavengers to suppress their formation, but they will not be discussed here. For the purpose of this work, the important consequence of the formation of molecular products lies in the fact that if a certain product is formed both by a molecular mechanism and in a radical termination reaction, then the yield of this product cannot, as such, be used for the estimation of the steady state concentration of radicals. For example, in cyclohexane a significant portion of the radiolytic yield of cyclohexene and bycyclohexyl is molecular. These molecular yields have to be subtracted from the total yields of those... 

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