Study of the Radioactive Liquid Waste Treatment by Coprecipitation: From Modeling to New Process Designs Available to Purchase

The ever increasing pressure to reduce the release of radioactive and other toxic substances into environment requires constant improvement/upgrading of processes and technologies for treatment and conditioning of liquid radioactive wastes. To decontaminate liquid nuclear wastes, the coprecipitation process is the most commonly used in nuclear field because it can be applied to any type of aqueous effluents whatever their composition may be. This process deals with the in situ precipitation of solid particles to selectively remove one or more radioelements. In the nuclear research center of CEA (Commissariat a` l’Energie Atomique), the coprecipitation of ⁹⁰Sr with barium sulphate is the technique used to treat selectively this radionuclide. In a previous study (1), an elaborated model is presented which predicts the radioactive strontium decontamination factor of nuclear waste solutions which can be achieved by using a coprecipitation process with barium sulphate. The originality of this new approach lies in the possibility to simulate the decontamination process in non equilibrium conditions and at the reactor scale. This modelling combined with the resolution of the population balance, enables to identify the influence of process parameters (flow rates, stirring speed…) on crystal size and ultimately on decontamination. Simulations of the strontium coprecipitation with barium sulphate have been performed in continuous and semi batch reactors. Thanks to these simulations, laws of the treatment efficiency variation as a function of several process parameters (mean residence time, stirring speed, BaSO₄ concentration) have been determined and experimentally verified. This study leads to the determination of optimal treatment conditions. Three apparatus (recycling apparatus, fluidised bed and reactor/settling tank) providing these optimal conditions have been successfully tested and offered significant outlooks for the reduction of the residual sludge volume.