The role of mass spectrometry in the understanding of the polymer degradation in the nuclear context
Résumé
Radioactive waste of Intermediate Level Long Lived Waste (IL-LLW) coming from nuclear laboratories or nuclear industry are conditioned in specific packages for deep underground disposal. These packages may contain a wide variety of polymer polyurethane (PUR), polyvinyl chloride (PVC), polyethylene (PE) and cellulose are the most common polymers and are mainly used as gloves, bottles or glove box bags. Inside the package, the waste is potentially exposed to three phenomena during the temporary storage, the transport and the final disposal (deep underground for French case) radiolysis, thermolysis and hydrolysis. Depending on the intensity of these phenomena, we can observe different effects ranging from slight ageing to almost total degradation of the material.This study attempts to demonstrate the mass spectrometry importance in the understanding of these phenomena. Initially, this technique is used to analyze polymer material before and after deterioration. Thanks to the atmospheric pressure ionization method, such as Atmospheric Solids Analysis Probe (ASAP) source, very high molar mass molecules are ionized and no sample preparation is required. This source is used to perform de-formulation studies which allow us, on the one hand, to identify the additives and monomers, and on the other hand, to reveal molecules due to polymer and additives degradation.In a second phase, the aqueous solutions obtained by hydrolysis of non-degraded and radio-oxidized polymers, are also studied with the objective to understand the degradation process in presence of water. The aim is to identify the water-soluble compounds formed and to explain the impact of the oxidative ageing on the hydrolytic degradation of the polymers. The electrospray (ESI) source coupled with Quadrupole Time Of Flight (Q-TOF) is used to characterize the most polar water-soluble molecules. The structure of these molecules is verified by the exact mass and by MS/MS analyses. Finally, Gas Chromatography Mass Spectrometry (GC-MS) coupling allows us to identify the small organic water-soluble non-polar molecules.The implementation of all these techniques leads to the characterization of molecules formed by the degradation of the radio-oxidized polymer chain. Special attention is paid to the identification of terminal groups which give information about the scission mechanism of macromolecules.