Pilot Test of Precipitation Setup for Dust Supressor and Transuranic Elements Removal From Wastewaters of Chernobyl Nuclear Power Plant

In this paper we report the results of pilot tests of flocculation/precipitation setup for dust-suppressor (DS) and transuranic elements (TUE) removal from wastewaters of Chernobyl Nuclear Power Plant (ChNPP), including those of the Object “Shelter”. Tests were performed on the pilot unit (PU), which included service tank, precipitation tank, and accumulation tank, 300 dm³ each, connected with pipelines with dosing and pumping equipment, and throttle valves providing controlled dosing, mixing, precipitation and mechanical filtration of radioactive wastewaters under different conditions. The reagent compositions used in pilot tests were based on coagulant POLYPACS-30 LF (aluminum polyoxychloride), synthetic cationic flocculants Besfloc K6634, K510CA, K6732 («Kolon Life Science, Inc», South Korea) varying in molecular weight and charge density, and natural cationic flocculant «Chitofloc» (Institute of Chemistry FEBRAS, Russia). The following wastewater parameters were controlled during the pilot tests: pH, dry residue, oxygen consumption, total α- and β-activity, isotope composition, optical density and DS content. The precipitation setup demonstrated lower efficiency DS removal from evaporator concentrates due to high ionic strength suppressing the electrostatic interactions between coagulants/flocculants and oppositely charged colloids of DS and TUE. The residual DS concentration was below 1 mg/L that corresponds to decontamination factor above 300 for the drainage water samples tested. The chitosan-based “Chitofloc” flocculant appeared to be the reagent which was the least sensitive to negative effect of ionic strength; however, the decontamination factor in DS removal was not higher than 5 due to suppressing of electrostatic interactions in high salinity media. Analysis of α-activity of water samples after flocculation/mechanical filtration revealed that TUE were not detected in the drainage water samples with DS content reduced to 2 mg/L that corresponds to TRU decontamination factor above 10000 and confirms immobilization of TUE in DS precipitate.