Electrodialysis reprocessing of salt regenerates from the special water purification system of a nuclear power plant

Abstract

During the operation of a single power-generating unit of a nuclear power plant with a VVt~R reactor, approximately 300 m3/yr of liquid radioactive wastes with a salt content of 200 g/liter are produced in the course of the technological purification of contaminated waters. In accordance with the concept adopted, the wastes must be solidified with production of bitumen or cement compounds. Their volume, however, decreases by only a factor of 2. In this connection it appears promising to reprocess high-salt-content solutions directly at the nuclear power plant and to extract valuable components from them. To this end, we assessed the use of electrodialysis process for separating acids and alkalides from the regenerates of the water purification system (WPS). This method has the advantage that the apparatus is relatively cheap to build and automate and acids and alkalides can be obtained for reuse, and the volume of the wastes is decreased in the process. The investigations were performed on two types of salt systems: boron-containing alkali regenerates of the WPS-2 setup and nitric-acid regenerates of the WPS-5 setup. The objective of the reprocessing of regenerates of the first type was to extract boric acid and potassium hydroxide, and the objective for the second type of regenerates was to obtain nitric acid and sodium hydroxide. Figure 1 shows the sorption-electrodialysis scheme of the apparatus for reprocessing boron-containing alkali regenerates. This scheme is assembled at the Novovoronezh nuclear power plant. The initial solutions to be reprocessed consisted of desorbate and washing waters from the ion-exchange filters of the WPS-2 setup. A I~DU-1-400 commercial electrodialysis apparatus with a total area of -130 m 2, equipped with bipolar and catonite membranes, was used for reprocessing the regenerates on the apparatus. The characteristics of the heterogeneous ionite membranes, employed in the electrodialysis unit, and also the MB-2 membranes for comparison are presented in Table 1. It follows from the data in Table 1 that the electrical resistance of the MB-3I bipolar membranes is lower than that of the MB-2 membranes. This is explained by the presence of ionogenic phosphoric-acid groups, which are strong catalysts, which increase the rate of constant of the transfer of H + and OH- in the bipolar membrane by a factor of 107, in the MB-3I material [1]. In this connection, the MB-3I membranes could be expected to yield boric acid and potassium hydroxide with less consumption of electricity than in [2]. The potassium ions are transferred from the desorbate through the cationite membranes into the washing waters until a pH of 5.2-5.8 is obtained in the solution being desalinized (desorbate). This pH corresponds to a K + content of 3070 mg/liter in the boric acid solution. The process of transferring potassium ions and the production of boric acid was conducted in a regime with repeated circulation of the solutions through the corresponding chambers of the apparatus. The results of the operation of the EDU-1-400 setup on real boron-containing alkali regenerates are presented in Tables 2 and 3. Table 2 displays the average results for several series of tests of different duration, depending on the initial composition of the regenerates. The volume of the reprocessed solutions in each experiment was equal to 0.6 m 3 and the circulation rate through the loop of the apparatus was equal to 4-5 m3/h. In the course of the experiments the strength of the current, the voltage on the apparatus, the pH of the desorbate, and the concentrations of H3BO 3 and KOH in the solutions were monitored.