ANALYTICAL DEVELOPMENTS FOR PVC RADIOOXIDATION STUDY, IN SIMULATED DISPOSAL CONDITIONS

Radioactive waste is generated by the use of radioactive materials in industry, research and medicine, as well as in nuclear power plant. The management and disposal of such waste is, therefore, a very important issue. In France, the estimated volume for the low intermediate-level long-lived radioactive waste is around 43,000 m3 according to the inventory at end 2018 (Andra, 2020). This category includes, among others, organic materials such as chlorinated and fluorinated polymers, polyolefins, polyurethanes and polyacrylates. Chlorinated polymers, like polyvinyl chloride (PVC), are one of the most used. Under disposal conditions, PVC can be exposed to different processes of degradation and in particular to radiooxidation process that implies the concomitant action of gamma radiation and oxygen (Colombani et al., 2007; Decker, 1976; Miller, 1959). PVC radiooxidation leads to structural changes in the polymer such as chain scission or cross-linking (Miller, 1959) and the main identified reaction byproducts are HCl and H2 gases and some carbonyl compounds and carboxylic acids (Colombani et al., 2007; Decker, 1976). Most of the authors describe the radiooxidation mechanisms of the PVC matrix and less attention has been paid to the identification of organic degradation products. Indeed, the radiooxidation of PVC induces the formation of water-soluble molecules likely to migrate in groundwater and thus interact with disposal surrounding materials as well as with the radionuclides. These anionic compounds can form complexes with the radionuclides present in the waste packages (Barthelemy and Choppin, 1989; Keith-Roach, 2008; Reiller et al., 2017) and thus modify their properties by increasing their mobility and ultimately the risk of dissemination in the environment. This work proposes an extensive study of the organic degradation products resulting from the radiooxidation of formulated PVC using powerful analytical techniques: thermal desorption coupled with GC-MS (TD-GC-MS) and liquid extraction with solvents or solubilization-precipitation approach coupled with GC-MS. This was combined with a diffusion study of these byproducts in interstitial cementitious water. The objectives are (i) to identify the byproducts and set their kinetics, (ii) to discriminate their source (PVC polymer or additives) and (iii) to identify leaked compounds in the solution. Thus, pure resin (powder) and three PVC formulations (films) containing a plasticizer, diisononyl phthalate (formulation 1), the same plasticizer and a heat stabilizer, metals organic derivatives (Ca/Zn) (formulation 2) and an industrial PVC, containing the same type of additives and others used as lubricant (stearic acid, calcium stearate, talc). They were exposed at different doses of γ radiation from a 60Co source, to study the formation/degradation kinetics of byproducts. Then, these PVC samples were put in alkaline water (pH=13.3) to simulate the storage conditions of nuclear waste. The exhaustive analysis of PVC radiooxidation by-products was carried out in two steps. A first study on the polymer to obtain a general insight into degradation products formed inside the matrix; then, a study on the leaching solutions. This second study aims to determine the different families of molecules contributing to the Total Organic Carbon (TOC) contained in the water. The results allowed, thanks to TD-GC-MS, to highlight the increase of the concentration of total Volatile Organic Compounds (VOC) according to the irradiation dose and PVC formulation. In addition, many compounds such as ketones, alcohols and carboxylic acids were also identified by GC-MS. The latter, well-known as radionuclides complexing agents, were identified and quantified in PVC leachates, by Ion Chromatography coupled to Mass Spectrometry (IC-MS). The impact of the PVC formulation on the type of by-products formed and identified in the leachates was established. Pure PVC leads to byproducts resulting from the polymer chain radiooxidation such as carboxylic acids (carbon atoms > 4) while phthalic acid was mostly produced for industrial and formulated PVC. This formation was attributed to the alkaline hydrolysis and radiooxidation of DINP plasticizer.