Co-Precipitation Method of BaS and Na2S for Treatment of Radioactive Waste Liquid

The treatment of radioactive waste liquid from nuclear power plants has always been a challenge, mainly due to the large production of radioactive waste liquid, uneven levels of radioactive pollution, and complex composition. At present, the treatment of radioactive waste liquid in domestic nuclear power plants is mainly based on the degree and composition of radioactive pollution in the wastewater, using methods such as direct drying in barrels, ion exchange, or imported mobile adsorption equipment. Although the chemical co-precipitation was studied relatively earlier than most of the modern radioactive waste water treatment methods, it has not been applied in engineering. The purpose of this study is to explore the feasibility of using chemical co-precipitation method for the actual treatment of radioactive waste liquid from nuclear power plants and trying to answer the question why the chemical co-precipitation method has not been applied to practice radioactive waste water treatment. According to the theory of soft-hard acid-base theory(HSAB), sulfides are selected as precipitants for removal of radioactive heavy metal ions in this work. Theoretically, they can achieve very ideal removal effects on the main heavy metal nuclides in radioactive waste liquid, such as 54Mn, 58Co, 60Co, 110Agm, and 125Sb. This work compares the treatment effects of two sulfur ion donors, sodium sulfide and barium sulfide, on actual radioactive waste liquid from a domestic nuclear power plants, under identical precipitation condition. The carrier chemicals used for 137Cs, 60/58Co, 125Sb, 110Agm are CsCl, CoCl2•6H2O, SbCl3, AgNO3 respectively, and the concentrations of carrier nuclides for target radionuclides are 0.5 g/L each. Because most of the carrier chemicals are chlorides, to avoid any interference with the co-precipitation of the 110Agm and 137Cs, the carrier chemical AgNO3 was first added to the radioactive waste water. After AgNO3 was well dissolved, the carrier CsCl was then added followed by the addition of sodium tetraphenyl boron to precipitate 137Cs. Then the other carrier chemicals were added followed by the addition of sulfide and pH adjustment. It is found that barium sulfide has a better treatment effect on actual radioactive waste liquid than sodium sulfide. One treatment by barium sulfide can remove more than 99% radioactive nuclides except 125Sb, and reduce the radioactivity in the sample radioactive waste liquid to the standard discharge level. The removal of 125Sb is in the range of 90%-95% depending on the initial concentration of 125Sb. Barium sulfide can be used as an efficient precititant for treatment of radioactive waste water from nuclear power plant. The unsatisfactory removal of 125Sb needs further study. This work provides a preliminary discussion on the reasons why barium sulfide has a better treatment effect than sodium sulfide.