Thermal Decomposition of Nitrated Tri-n-Butyl Phosphate in a Flow Reactor

Tri-n-butyl phosphate (TBP) is a universal nuclear extractant, commercially used in the PUREX process for the last 60 years. However, it is prone to nitration and thermal degradation, and as a consequence a red-oil event may be initiated under several operating conditions resulting in severe pressurization of vessel/cell if venting is inadequate. In this work, an attempt was made to understand the reaction pathway of thermal decomposition of nitrated TBP in a flow reactor at atmospheric pressure. Many reaction products were identified and quantified by instrumental methods like HPLC-RI and GC-TCD. The experimental data was analysed with a power law model and the apparent rate constants were estimated. The activation energy for thermal decomposition of nitrated TBP, assuming an Arrhenius type of temperature dependency, was estimated to be ？kJ·mol？1. The effect of both varying temperature and concentration of nitric acid on conversion of TBP into degradation products and products distribution was experimentally studied. Based on the experimental observations, a reaction mechanism framework for thermal decomposition of nitrated TBP is proposed. 1. Introduction In nuclear fuel reprocessing industry, solvent extraction is a cost-effective process for separation of unused uranium and bred plutonium from the complex fuel matrix [1]. There are many solvents which can effectively extract uranium, plutonium, or thorium from nitric acid solutions but tri-n-butyl phosphate (TBP) is one of the important organic solvents utilized during the acid extraction step in separation process at reprocessing facilities [2, 3]. This is because of its overall superiority in operation, safety, physical properties, radiation resistance, and economics. One of the most desirable attributes of TBP is its high flash point, 146°C, compared with other solvents. The reported boiling point and the density of TBP at 25°C is 289°C and 0.973？g/mL, respectively [4–6]. Most solvent extraction operations are conducted at ambient conditions without heating TBP and have been performed safely for decades. After the extraction process, raffinate containing fission products and dissolved TBP remains as highly active rad-waste which needs to be concentrated in the evaporator for storage and further management. During the evaporation process, water is evaporated continuously which results in separation of dissolved TBP from aqueous layer. At 130°C, “red oil” (complexes of TBP and HNO3) is formed which further undergoes exothermic reactions. With continual concentration of aqueous phase, the rate of