The NOx system in nuclear waste. 1997 annual progress report

'The authors highlight their results from the title project. The project is a coordinated effort of the three Co-PIs to assist the Safety Programs at the Hanford and other DOE Environmental Management Sites. The authors present in the report their observations and interactively discuss their implications for safety concerns. They focus on three issues: (1) Reducing radicals in the NOx system The authors show that the only reducing radical that lasts longer than a few ns in typical waste solutions, and is capable of generating hydrogen, is NO{sub 3}{sup 2-}. The authors measured the lifetime of this species across the whole pH range (3 {le} pH {le} 14) and found it to be shorter than -15 \265s, before it dissociates to give the strongly oxidizing NO, radicals. They found that it reacts with many proton donors (H{sup +}, phosphate, borate, NH{prime}, amines) in a reaction that is not merely an acid-base equilibrium reaction but is probably a dissociative proton transfer. They estimate the redox potential from theoretical considerations and obtain an experimental verification. They conclude that it is highly unlikely, although thermodynamically possible, that this radi-cal will generate hydrogen in waste solutions. (2) Aging of organic chelators and their degradation products by NO, Methodologies to study the degradation of organic substrates (including the important waste components, formate and oxalate) to CO;, or carbonate, by NO, were developed. This radical dimerizes and disproportionates to nitrate and nitrite. Therefore, mineraliza-tion of the organic substrates competes with the disproportionation of NO,. Among the organic substrates, formate and oxalate are also mineralized but because they are of low fuel value their mineralization is not very helpful, yet it consumes NO,. (3) Interfacial processes in aqueous suspensions Yields of charge transfer from solid silica particles to water and other liquids were meas-ured. If the particles are small enough, essentially all of the charge that is originally depos-ited in the solid escapes into the liquid. This implies that the solid/liquid interface does not provide a significant barrier to the transfer of charges into the solution when the particles are very small (I 20 nm). Electrons may reach the liquid and generate hydrogen, for example. On the other hand, the same mechanism may also provide a pathway for oxidative aging of organics by holes even when the organic is dissolved in the liquid or adsorbed on the solid surface. The authors have started to study reactions of NO,. Methodology and instrumentation to measure reactions of relevant organic radicals with NO, and with its parent NO, were developed. Because of low extinction coefficients, conductivity will be the method of choice.