Les nouveaux procédés de décontamination des effluents liquides et des solides du nucléaire
New Decontamination Processes for Liquid Effluents and Solid Materials
CEA - Marcoule / Advanced Decontamination Processes Laboratory (LPAD) (France)
To face the future challenges in the nuclear industry as dismantling and decommissioning, the Advanced Decontamination Processes Laboratory (Laboratoire des Procédés Avancés de Décontamination - LPAD) is developing new decontamination processes for the nuclear industry to protect workers and to reduce strongly the quantity of secondary wastes produced.
First, two decontamination processes for liquid nuclear wastes are under studies: the “coprecipitation process” and the “column process”.
The processes using solid precipitates are the most versatile and thus the most commonly used to perform decontamination of low- and medium-active aqueous effluent. For decades, the coprecipitation process has been used and thus the best chemical conditions were defined thanks to systematic experiment plan. Nevertheless, the chemical mechanisms involved remain unclear.
To bring out improvement ways, it is necessary to control the two coupled mechanisms involved in the process: the formation of adsorbent particles and the uptake of radionuclides.
The approach is based on the study of the impact of process parameters (reactor design, mixing, mean residence time…) on the two steps. To support the experimental investigations, a modeling study at the reactor scale is initiated to distinguish the influence of each process parameter. The aim is to determine the decisive parameters which allow the increase of the decontamination efficiency in a defined reactor design.
Moreover, many studies have been devoted for the cesium removal and recovery from nuclear liquid waste. A restricted number of inorganic exchangers allowed the cesium removal, but often they could not be eluted or needed prohibitive volumes of saline eluent, leading to important amounts of secondary waste. There is a need to developed new materials that can be used in a column process to adsorb cesium in a reversible way. Other advantage, the extraction of cesium being reversible, the stripping of cesium is carried out in deionised water; this property leads to very high concentration factors. Characterization studies, such as adsorption isotherms on new materials, breakthrough curves for fixed bed operation show promising results for cesium separation.
Secondly, the advanced decontamination processes laboratory is developing three new decontamination processes for solid materials to reduce strongly the quantity of secondary wastes produced with regard to the classic techniques (soaking solutions, spraying of abrasives or high pressure water): gels, foams and surfactants aqueous solutions.
Process Using Drying Gels
In 1990, the first mineral gel formulations were defined: however, in spite of an efficiency of the process turned out, a phase of rinsing by means of aqueous solution was necessary. To reduce more the volume of secondary wastes, fundamental studies are going on to improve usual formulations. The aim is not to reduce but to avoid completely secondary liquid contaminated waste. At the conclusion of 5 years of studies the first drying gel process was world patented in 2001. The advantages of the process are then considerable because the gel formulation is mineral, can be pulverized like paint, dry onto the surface by forming cracks to lead to solid tiles easily recovered by vacuum cleaning or by brushing. Then the studies continued to develop different formulations to treat several materials as lead, aluminum, iron, and stainless steel and to optimize the process by introducing few quantities of surfactants in order to reduce the mass fraction of the mineral viscosifier.
Recently, real decontamination operations of hot cells (ISAI in Marcoule, Gascogne and Cyrano in Fontenayaux-Roses) by drying gel process was achieved by the LPAD and revealed a decontamination factor included between 16 and 25 on stainless steel.
Decontamination Process Using Aqueous Foams
In the frame of the future dismantling of nuclear facilities, the foam decontamination process has been assessed as an alternative technique to liquid decontamination. Because decontamination foam is a non stable two-phase fluid within the aqueous phase representing not more than 10 % of the total volume, it strongly decreases the amount of chemicals used during the decontamination processes and the secondary nuclear waste volume. Moreover using foams allows the decommissioning of complex shape facilities.
A decontamination foam comprises at least one surfactant to generate the foam and one or more chemical reactants to achieve the dissolution of the contaminants at the solid surface. Recently, new decontamination processes, called “static foam process”, were developed using new foams stabilized by biodegradable non-ionic surfactants: alkyl polyglucosides (APGs) and viscosifiers or particles. This allows to increase the foam lifetime and thus the contact time of chemical reactants with the reactor walls.
Surfactants in Solutions Decontamination Process
The LPAD started to developed new acidic surfactant liquid solutions to remove organic matter located at the surfaces of equipment used in reprocessing facilities in 2002. The aim of developing acidic formulations is to avoid sodium hydroxide in the final liquid wastes to ease glass conditioning. These equipments are often covered with radioactive Tributylphosphate (TBP). In 2006, we developed specific acidic surfactants formulations containing Pluronics and characterized their degreasing effectiveness. The idea is to combine emulsification and wetting power. A specific experiment was performed to follow the kinetic of TBP drop detachment from stainless surfaces. We have confirmed the two ways of TBP detachment depending of the kind of surfactant: the roll-up mechanism and the emulsification.
Finally, the degreasing process needs a high TBP solubilization. In this way, results of turbidity measurements are presented. These experiments evidenced synergy phenomena with two specific surfactants. Moreover, the emulsions existing when the concentration of TBP is superior to what micelles can contain are proved to be stable upon several weeks without any phase separation. So, for all these scientific reasons, this association of surfactants is now in 2007 considered as the best potential degreasing formulation for future rinsing operations of nuclear facilities.
© SFEN 2008