Degradation of the polyacrylonitrile-based UP2W material under cementitious conditions

Abstract

UP2W is a polyacrylonitrile-based filter aid material used in nuclear power plants, which is disposed in repositories for low and intermediate level radioactive waste. The degradation of UP2W was investigated in a series of batch experiments in the presence and absence of portlandite, Fe(0) and NaOH, which simulate the hyperalkaline, reducing conditions expected in repositories with cementitious engineered barriers. Degradation experiments were performed under Ar atmosphere at T = 22 or 80 (±2) °C. Aliquots of the supernatant solutions and retrieved solid phases were systematically characterized for ca. 5 years using a multi-method approach.The evolution of dissolved organic carbon shows a strong contrast between NaOH- and Ca(OH)2-buffered systems. In the absence of Ca, an early, sharp increase in the dissolved organic carbon is observed, and its magnitude correlates with the initial NaOH concentration.In Ca(OH)2-buffered systems, dissolved organic carbon remained very low (<10 ppm) up to 600 days, but steadily increased afterwards reaching ∼ 50 ppm at t ≈ 1800 days. Size-exclusion/liquid chromatography coupled with organic carbon, ultra-violet and organic nitrogen detection (LC-OCD-UVD-OND) confirmed the presence of significantly smaller fragments in solution compared to NaOH-systems, which however were found to increase with time.Overall observations underline that hydrolysis of the nitrile functional groups, chain scission of the polymer backbone and cross-linking of polymer fragments in the degradation leachates play a key role in the progress of the degradation reaction. Chain scission is hindered in the presence of Ca, possibly due to the complexation of Ca with amide and carboxylate intermediates and the consequent decrease in electron density on the β-carbon atoms. This study improves the mechanistic understanding and quantitative description of UP2W degradation in cementitious environments relevant for L/ILW disposal.