Innovative processes for the treatment of radioactive liquid wastes

The nuclear industry produces a wide range of liquid radioactive wastes. Many of these wastes need treatment to reduce the quantities of radioactive contaminants to levels allowed for disposal. To treat these contaminated streams, various processes can be used with varying degree of efficiency according to the characteristics (ionic strength, compositions) of waste streams. The processes using solid precipitates are the most versatile and can be divided into two groups. When particles are directly introduced in the effluent, it is an adsorption process, and when particles are formed in situ, it is a coprecipitation process. In the reprocessing plant of La Hague and in nuclear research centres of CEA (Commissariat a l'Energie Atomique), the coprecipitation process has been chosen for many years to perform decontamination of liquid wastes which can not be concentrated by evaporation. The significance of coprecipitation hinges on the fact that the solubility of a minor constituent coprecipitated with a carrier is smaller than the solubility of its pure solid. The coprecipitation process has significant consequences on subsequent steps. Not only the settling and the filtration are made easier if the crystals are large, but also the stability of radionuclides in the binding agent (concrete or bitumen) is higher if they are captured in the crystalline structure of the precipitate, so the control of the coprecipitation step is essential to carry out a practical and efficient decontamination. As a consequence we chose to study the impact of process parameters (reactor design, mixing, mean residence time) on the formation of precipitates and in fine on their ability to uptake radionuclide. (authors)