Formulation of organic-based gels for the decontamination of non-compactable radioactive waste

Radioactive decontamination is a stage of radioactive waste management that aims to remove or reduce residual radioactive contaminant (sometimes in combination with other hazardous materials) from the surface of equipment such as rotors, motors, centrifuges, pumps, motor pumps, columns, etc. or structure, all these materials considered as non compactable radioactive waste, by means of physical (ultrasound or abrasive) or chemical methodologies (solutions of acids, foams or gels, electrochemistry) with the purpose of limiting exposure to radiation and the likelihood of radioactive contamination towards the environment, as well as reduce the costs of storage, maintenance and monitoring of these wastes. In particular, the use of gels as vectors of chemical reagents has been privileged as decontamination techniques in the nuclear industry, since these materials can be easily applied on the surfaces to be decontaminated in the form films on the surfaces, increasing that of Reaction contact time on the contaminated surface in a controlled manner and increasing the decontamination factors, which are obtained regularly by traditional chemical methods, without deteriorating the surface and minimizing the volume of secondary waste generated by the treatment. Therefore, the objective of the present work was to develop a methodology for preparing gels based on two natural polysaccharides: Arabian Gum and Xantana Gum, for the decontamination of metal parts contaminated with radioactive material (non-compactable radioactive waste), with the purpose of reduce the volume of waste and facilitate its management given the biodegradable characteristics of these gels. Gels were formulated at different concentrations of Arabian Gum and Xantana Gum, in the presence and absence of HNO₃ and Co (NO₃)₂, and acetic acid, oxidizing agents that facilitate the decontamination of iron-based metals, varying the concentration of these acids in the gels The gels were applied in stainless steel specimens previously contaminated with Ga-67 or Co-58, forming removable films that when removed from the specimen remove the contaminant present on the surface. The respective Decontamination Factors (Df) were then determined, as well as the decontamination efficiencies, by quantifying the activity of the radioactive material before and after the treatment in the specimen, with the help of a gamma spectrometry system. Additionally, the gels were characterized and the following properties were determined: density and viscosity, morphology by scanning electron microscopy, structure from infrared spectroscopy and the contact angle between the gel and the metal surface of the specimen. The results show that the Df increase with the concentration of the gums and with the concentration of HNO₃. Co content does not cause variations in Df. The best decontamination conditions were obtained in gels prepared based on 5% Xantana Gum and 7% Arabic Gum in 2 M HNO₃, with Df of approximately 200 and 240, and decontamination efficiencies greater than 99%. The viscosity of gels based on Gum Arabic (1-4 Cp) and Xantana (> 24 Cp) increase with increasing concentration; the presence of HNO₃ decreases its density and viscosity, and increasing the temperature decreases its viscosity. Gels are non-Newtonian pseudo-plastic fluids that when prepared with water or with [HNO₃] <0.5 M, can have a hydrophilic or hydrophobic behavior according to their formulation; have microorganisms after 5 days of being prepared; they form smooth, homogeneous and uniform films, which do not damage the surface of stainless steel specimens, and which are easily removable when dried at room temperature. Infrared spectrometry shows that the polymerized galactose present in the gel based on Arabic Gum changes when the concentration of the gum, the concentration of HNO₃, the aging of the gel or the presence of graphene oxide is varied. While in Goma Xantana-based gels, variations occur in OH, carboxylate and enole groups, as the concentration of gum or nitric acid varies. (author)