Rateconstants measured over the temperature range 2Cb2OO"C are reported for the following reactions: (a) reaction of the hydrated electron with oxygen, the proton, hydrogen peroxide, nitrate, nitrite, nitrobenzene and methyl viologen ; (b) reaction of the hydroxyl radical with another hydroxyl radical and ferrocyanide; (c) reaction of the hydrogen atom with permanganate and oxygen. To evaluate methods of estimating rate constants at high temperatures these rate constants and others in the literature have been fitted to the following equation: kobs = kdiff/(l + kdiff/kreact)where kobs is the measured rate constant for the bimolecular reaction in solution, kdiff is the encounter rate constant of the two reacting species, and k,,,,, is the rate constant that would be measured if diffusion of the species was not rate influencing. With the exception of reactions of the hydrated electron with nitrate and nitrite ions and nitrous oxide, good fits have been obtained to the above equation, and the results demonstrate that few, if any, of the reactions which are pertinent to water radiolysis are truly diffusion controlled at elevated temperatures.

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The extinction coefficients and the decay kinetics of I; and (SCN); have been characterized over the 15-90°C-temperature range. The extinction coefficients of I; at 385 and 725 nm were determined to be 10,000 and 2560M-' cm-', respectively, based on the extinction coefficient of (SCN); at 475 nm being equal to 7600M-' cm-'. At these three wavelengths, all extinction coefficients were constant over the temperature range studied. The rate of decay of both I; and (SCN); was found to be a function of I-and SCN-concentration, respectively, as well as temperature.There is interest in the radiation chemistry of halogen compounds since radioactive iodine, a fission product in nuclear reactors, may be released into the environment under some postulated accident conditions. An understanding of chemical behavior of iodine will help to estimate the extent of the release under a variety of hypothesized conditions and the subsequent transport of the released fraction away from the reactor. The transport of this iodine into the surroundings will depend on its oxidation state and this state will be partially determined by radiation chemical effects [1,21. Before any computer modeling of this system can be completed, many of the discrepancies which exist in the radiation chemistry of iodine compounds in aqueous solutions have to be resolved [ll. In this respect the effect of temperature (15-90°C) on the spectra and the decay kinetics of I;' and (SCN), in aqueous solution are described. The properties of the (SCN), radical ion were characterized so as to extend above room temperature the use of SCN-as a reference solute in competition studies to determine the rate of reaction of OH with other chemicals [31.The radical anions of 1;. and (SCN) , ' were formed by pulse irradiating oxygen or nitrous oxide saturated solutions containing iodide or thio-* Issued as AECL No. 8505.

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The g values for the primary species formed in the y-radiolysis of light and heavy water have been measured as a function of temperature up to 300°C. With the exception ofg(H,) andg(D,), all the g values are consistent with the generally accepted diffusion-kinetic model of spurs, i.e. with an increase in temperature, the g values of the free radicals increased while those of peroxide decreased. The g values for H, and D, increased with temperature which suggests that they are formed by other mechanisms in addition to radical-radical reactions in the spur.Water-cooled nuclear power reactors operate with either light or heavy water as the primary heat-transport medium. Since radiolysis of this water has been associated with corrosion and hydriding of in-core components, computer packages are being developed to model the radiation chemistry of the water as it passes through the reactor core. The computer models require a knowledge of the g values for the primary species formed from both fast neutron and y-radiations [see reaction (l)] as well as the rate constants for all of the reactions involving these species. radiation H'O ___e,, OH, H, H', H'O', HO, (1)The term ' g value' will be used for the yield of the primary species in reaction (1) CQ.s after the ionizing event and ' G value' will be used for the experimentally determined yields from which g values are deduced. This information is required for the operating temperature range (250-3 15 "C) found in nuclear reactors.In this paper, estimates of the g values for the primary species formed in reaction (l), for both light and heavy water, have been made as a function of temperature for low linear energy transfer (LET) radiation. The results presented here supersede those given in our earlier paper for light water.'Experimentally, the estimation of g values at higher temperatures is straightforward. The difficulty is in finding chemical systems which are both thermally stable above 100 "C and whose radiation chemistry is well defined. Pulse radiolysis experiments have a further complication ; the molar absorption coefficient of the species formed has to be known at the temperature being studied. Steady-state radiolysis experiments at high temperatures do not have this problem because all analyses are generally made at room temperature. The approach taken in this report was to estimate the g values of all the primary products over as wide a range of temperature as possible and then ensure that there was a material balance.J.