Abstract
Chemical reactions between metal and nitrate ion have been studied to determine their roles in chemical conditioning of a radioactive waste disposal repository and its surrounding environment. Immersion tests and rest potential measurements under hyper-alkaline and high NaNO3 concentration conditions were conducted to elucidate and improve model predictions of chemical interactions between carbon steel and NO3− in highly concentrated solutions of nitrate salts. Potentiostatic electrolysis experiments in the concentrated solution with a carbon steel electrode, as the working electrode, were also conducted to determine the electrochemical rate equation for NO3− reduction to NO2−.
Experimental results of potentiostatic electrolysis of the concentrated solutions showed that a linear electrochemical rate equation for NO3− reduction to NO2−, as used in the previous model, gave too high a rate under the conditions of high NaNO3 concentration (≧1 mol dm−3). To ameliorate this effct, a non-linear equation was derived assuming a Langmuir type adsorption process of NO3− as a precursor process of discharge, and the parameter for the equation was determined by curve fitting using the data acquired under the conditions of potential −0.85 V vs. SHE and pH 12.5. The equation was incorporated in the model, and was used to analyze the results of ampul tests and rest potential measurements. The modified model can estimate the tendency of time dependent variation of chemical species and of rest potentials in the highly concentrated solution of NaNO3.
