Treatment Of Cationic Waste Resins By Molten Salt Oxidation Technology

Nuclear energy is one of the recognized clean energy sources in the world.It brings many benefits to human society and has irreplaceable attraction.Low-level radioactive spent resins comprise the main part of solid radioactive wastes in nuclear industry（in terms of volume）.Traditional radioactive waste resins treatment methods such as cementation have some disadvantages.The final waste form is much larger than original resins and some radioactive nuclides may be leached into contacting water etc.Incineration is easy to cause radionuclides to escape.Strictly speaking,an ideal treatment method has not been found yet.In this paper,molten salt oxidation method is used to explore the suitable molten salt system,the destruction process of resins and the influencing factors.The main contents of the paper are as follows:Firstly,the suitable molten salt system for treating cationic resins was screened,and the oxidation behaviors of cationic resins in chloride,carbonate and other molten salt systems were studied.Chloride molten salt does not have the effect of intercepting the polluting gas sulfur dioxide.Carbonate and nitrate can absorb sulfur dioxide to form sulfate.Eutectic salt can achieve higher weight loss and volume reduction than pure salt.The melting point of molten salt is a key factor in the selection of molten salt systems.The ternary eutectic salt Li₂CO₃-Na₂CO₃-K₂CO₃ is more suitable for MSO of waste resins than other molten salt systems.According to the thermogravimetric analysis of cationic resins under air and argon conditions,the decomposition of resins from room temperature to 800℃can be divided into three stages:volatilization of water,the destruction of functional groups and polymer matrix.The first-order kinetic model G（α）=-ln（1-α）has a high degree of fitting to the destruction process of the resins at 520-680℃.The addition of oxygen can increase the activation energy（E）required for the destruction of the polymer matrix.The formation of sulfate in the waste salt proves that the molten carbonate captures a part of sulfur after the sulfonic acid groups are destroyed.The formation and partial decomposition of sulfate occurr at 400～500℃and 500～700℃,respectively.The sulfur bridge and sulfoxide make the residues more stable which are difficult to continue oxidation.According to the thermogravimetric analysis of the mixture of cationic resins and carbonate Li₂CO₃-Na₂CO₃-K₂CO₃ under air and argon conditions,carbonate can catalyze the destruction process of resins and advance the termination temperature of functional groups destruction.Finally,the study of simulating radionuclide ⁹⁰Sr with ⁸⁸Sr was carried out,and metal cations also play a catalytic role in the destruction of resin functional groups,which can reduce the activation energy（E）.Under the same failure condition,the simulated waste resins are more completely destroyed than the original resins,and many pores are formed on the surface.The result indicates that most of strontium included in the simulated spent resins remained in the form of Sr₂CO₃ and stably exists in molten salt.The rejection rate of Sr element is 93.4%,and MSO can achieve effective interception of radionuclides.With the increase of oxidation temperature and time,the graphitization degree of residue increases.Li OH added in molten salt can be used as catalyst to promote the destruction of resins.