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Scope. The radiation chemistry of aqueous systems containing chlorinated hydrocarbons is investigated using a multi-pronged approach employing 60Co gamma ray and alpha particle irradiation experiments in conjunction with diffusion-kinetic modeling incorporating track structure simulations. The goal is to determine mechanisms, kinetics, and yields for the formation of potentially explosive gases and corrosive agents, such as H2 and HCl, respectively, in the radiolysis of water-organic mixtures. The information obtained is of a fundamental nature, but the radiation chemical systems studied are found throughout the DOE portfolio and are important in radioactive waste remediation and management. Program Highlights. Radiation-induced production of H2 and HCl from chlorinated hydrocarbons. 60Co gamma-radiolysis experiments and stochastic kinetic modeling have been used to investigated the radiation-induced yield of H2 and Cl- from aqueous solutions of 1,2-dichloroethane (1,2-DCE) and 1,1-dichloroethane (1,1-DCE) over the concentration range 1-80 mM. In deoxygenated solution, the yield of H2 from both 1,2-DCE and 1,1-DCE solutions decreases as the concentration of DCE is increased. The decrease in the H2 yield shows that the reaction of H atom with DCE does not lead to the production of H2. This observation is unexpected and reflects the reverse of the effect seen in the gas phase, where the reaction of H atom with 1,2-DCE and 1,1-DCE leads to the production of H2. The yield of Cl- from 1,2-DCE and 1,1-DCE solutions increases slightly from 2.8 ions/100eV to 3.6 over the concentration range 10-50 mM, demonstrating the increased competition of the DCE with intra-track processes. Comparison of the measured yields of Cl- with the predictions of stochastic kinetic modeling shows that the reactions of eaq- with 1,2-DCE and with 1,1-DCE are quantitative, and that the reaction of H atom with both DCEs leads to the production of Cl- (and Haq+). In aerated solution, the yield of Cl- from 1,2-DCE and from 1,1-DCE solutions is very significantly higher ({approx} x 3-4) than from deoxygenated solution. Furthermore, the observed yield is both dose and dose rate dependent. The mechanisms for Cl- production in aerated aqueous solutions of 1,2-DCE and of 1,1-DCE are currently under investigation. Rate coefficients for the reaction of eaq- and -OH with chlorinated hydrocarbons. There is considerable disagreement over the rate coefficients for the reaction of the primary radiation-produced reducing and oxidizing radicals from water, eaq- and -OH respectively, with 1,2-DCE and with 1,1-DCE. Electron pulse-radiolysis experiments monitoring the decay of eaq- have been used to measure the rate coefficients: 1,2 DCE eaq- + CH2Cl-CH2Cl ' CH2Cl-CH2- + Cl- k1 = 2.3 x 109 dm3 mole-1 s-1 1,1 DCE eaq- + CH3Cl-CHCl2 ' CH3-CHCl- + Cl- k2 = 3.5 x 109 dm3 mole-1 s-1 while competition kinetic experiments were employed to determine the rate coefficients: 1,2 DCE -OH + CH2Cl-CH2Cl ' CH2Cl-CHCl- + H2O k3 = 1.8 x 108 dm3 mole-1 s-1 1,1 DCE -OH + CH3Cl-CHCl2 ' CH3-CCl2- + H2O k4 = 1.1 x 108 dm3 mole-1 s-1 The values obtained are similar to those measured by Asmus and co-workers, but there is a significant discrepancy from the estimate of Getoff and co-workers for k1. Rate coefficient for the reaction of OH with thiocyanide ion. The rate coefficient for the reaction of the -OH radical with a chlorinated hydrocarbon is obtained by a competition experiment, in which the change in the radiation-induced yield of (SCN)2-- from an aqueous SCN- solution is monitored on the addition of the hydrocarbon. The mechanism for the radiation-induced formation of (SCN)2-- from a SCN- is complex and involves a number of equilibria. Careful electron pulse radiolysis experiments have been performed and analyzed, employing the full, complex reaction mechanism, to re-evaluated the rate coefficient for the fundamental reaction -OH + SCN- ' (HOSCN)-- k5 = 1.4 x 1010 dm3 mole-1 s-1 This reaction is central to the experimental determination of the rate coefficient of a solute with OH using the competition method. The value obtained for k5 is about 30% higher than that generally employed, and this has a major effect on the measured values for k3 and k4

Hazardous and Corrosive Gas Production in the Radiolysis of Water/Organic Mixtures in Model TRU Waste