Radiolysis Products of Nitrate-Acetate Aqueous Solutions and Their Influence on pH of the Solutions

Abstract

The influence of the components of nitrate!acetate aqueous solution and products of their radiolysis on pH of the solutions was experimentally studied in a wide dose range. The results can be used to estimate the speciation and migration conditions of radionuclides in a reservoir bed of underground disposed sites of liquid radioactive waste.Here were continue our previous study [1] on radiochemical behavior of nitrate3acetate solutions used in deep disposal of nuclear waste [2]. In this study we examined experimentally the behavior of the radiolysis products of nitrate3acetate solutions and their influence of pH of aqueous solutions with the aim to estimate the speciation and migration conditions of radionuclides in a reservoir bed.The composition of the radiolysis products of aqueous acetic acid solutions was studied insufficiently [2,3]. The data on radiolysis of aqueous nitrate solutions were reviewed in the monograph [3]. However, the behavior of radiolysis products of acetates in aqueous nitrate solutions exposed to prolonged irradiation has not been studied quantitatively. EXPERIMENTALWe studied aqueous solutions simulating the macrocomposition of radioactive waste and containing 0.125 M nitric acid, 0.24 M acetic acid, and 1.17 M sodium nitrate. The pH of nonirradiated model CH 3 COOH3NaNO 3 , NaNO 2 3CH 3 COOH3NaNO 3 , and NaNO 2 3NaNO 3 solutions was measured. The composition of the model solutions was similar to that of irradiated solutions.The kinetic curves of decomposition of nitric and acetic acids on exposure to g-ray radiation of 60 Co and accumulation of their radiolysis products were studied in a temperature-controlled glass cell at 23oC and a dose rate of 1.2 Gy s !1 .The content of nitric and acetic acids and of liquid acidic radiolysis products of acetic acid in the samples was determined by potentiometric titration, with the determination error not exceeding 10%. The concentration of sodium nitrite was determined by the calorimetric procedure described in [4]. The pH was measured with an Ekotest-2000 pH meter. RESULTS AND DISCUSSIONThe potentiometric titration procedure used allowed determination of the concentration of nitric acid and the total concentration of acetic acid (HAc) and its acidic radiolysis products. The dependences of the concentrations of HNO 3 , acidic organic compounds (HAc + products), and NaNO 2 in the solutions containing 0.125 M HNO 3 , 0.245 M HAc, and 1.2 M NaNO 3 and of pH of these solutions on the radiation dose are presented in Table 1 and Fig. 1. Table 1. Concentration of components (M) and pH of nitrate!acetate solutions at different radiation doses ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ Dose, ³ HAc + ³ HNO 3 ³ NaNO 2 ³ pH MGy ³ products ³ ³ ³ ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ 0 ³ 0.245 ³0.125 ³0