Pyrolysis of Sodium Dibutyl Phosphate–Laden Radioactive Liquid Waste

Abstract

Alkaline hydrolysis of organic radioactive liquid waste, specifically, spent PUREX solvent, is an established process that yields three distinct phases: n-dodecane (top), water-soluble organic phosphate (dibutyl phosphate) along with butanol (middle, a product of alkaline hydrolysis), and unreacted alkali (bottom). Managing the middle layer (ML) poses significant challenges due to its high phosphate content (300–350 g/L), substantial radioactivity (gross α 515–1,500 Bq/mL, gross β-γ 1,453–2,500 Bq/mL), and complex composition. Various methodologies have been tested to destroy organic components or separate radionuclides from the ML, including dilution and dispersion, microfiltration, direct cementation, chemical precipitation, and pyrolysis. Among these methods, pyrolysis has successfully demonstrated complete mineralization, converting sodium dibutyl phosphate to Na3PO4 and effectively separating and isolating the radioactive content from the ML using an indigenously designed pyrolyzer. In this work, comprehensive studies on the thermal, spectroscopic, and radiometric properties of the ML, as well as the immobilization of residues, were conducted by employing a thermogravimetry analyzer (TG-DSC) coupled with an evolved gas analyzer, Fourier-transform infrared spectroscopy, high-purity germanium (HP-Ge) gamma (γ) spectrometry, and the International Atomic Energy Agency (IAEA) 28-day chemical durability test. The resulting cement waste form contained 20 wt% undissolved residue from ML pyrolysis and separated radionuclides achieved a leachability index (L) greater than 6. This indicates that the cemented waste form meets the acceptance criteria, ensuring safe and effective long-term disposal. The combination of alkaline hydrolysis (AH) and pyrolysis presents an effective method for managing organic radioactive liquid waste (OLW) from spent PUREX solvent in reprocessing plants. AH separates the waste into three layers, with the middle layer (ML) containing high levels of dibutyl phosphate (DBP) and radioactivity. Pyrolysis of the ML in an inert atmosphere successfully mineralizes Na-DBP to Na3PO4 and isolates radioactive content. Thermogravimetric, spectroscopic, and radiometric analyses confirmed the complete degradation of DBP and the formation of nontoxic by-products. The residue from pyrolysis, containing radionuclides, was immobilized in a cement waste form with a leachability index greater than 6, meeting disposal criteria. This process, demonstrated in lab scale, ensures efficient treatment of OLW. The organic layer from pyrolysis can be incinerated, while the aqueous layer is mixed with low-level waste for disposal. The conditioned cement waste form ensures safe, long-term disposal, offering a comprehensive solution for OLW management with regulatory compliance and minimal environmental impact.