Study of the PVC degradation by radiolysis and/or basic hydrolysis in radioactive waste disposal facilities

Ce travail s’inscrit dans le cadre des études menées sur le stockage géologique profond des déchets radioactifs de haute activité (HA) et de moyenne activité à vie longue (MA-VL), et plus particulièrement de l’amélioration de la représentation du comportement des déchets organiques MA-VL en situation de stockage. A ce titre, il s’est intéressé à la dégradation du PVC suivant deux processus: la radiolyse et l’hydrolyse basique. Ainsi, différents PVC ont été irradiés et lixiviés dans une solution alcaline. L’identification et la quantification des sous-produits dans le PVC par TD-GC-MS et GC-MS a mis en évidence la formation de cétones, alcools, esters et acides carboxyliques. Leur origine et leurs voies de formation ont été proposées, avec un effet de la formulation sur leur nature et quantité. Les acides mono et dicarboxyliques à chaîne courte proviennent de la dégradation du PVC et ceux à chaîne longue des additifs. De plus, la caractérisation du PVC lixivié et l’analyse des lixiviats ont montré que la majorité des produits formés sous irradiation migrent en solution. La quantification des acides carboxyliques (représentant jusqu’à 79% du COT) par CI-MS a permis de discriminer les processus d’hydrolyse basique et de diffusion contrôlant leur relâchement en solution. Ces derniers dépendent majoritairement de la dose d’irradiation et du type de PVC. Le régime diffusif est majoritaire dans les premiers jours de lixiviation et l’hydrolyse basique par la suite. Enfin, il a été démontré par des techniques analytiques complémentaires (FIA-QToF et RMN 1H) que des oligomères de PVC radiooxydé peuvent être relâchés en solution à la suite de réactions de substitution nucléophile.This work is part of the studies carried out on the deep geological repository of high-level (HLW) and long-lived intermediate-level (ILW-LL) radioactive waste, and more particularly on the improvement of the representation of the ILW-LL organic waste behaviour in a disposal situation. In this context, it was focused on the degradation of PVC by two processes: radiolysis and basic hydrolysis. Thus, several types of PVC were irradiated and leached in an alkaline solution. The identification and quantification of irradiated PVC by-products by TD-GC-MS and GC-MS showed the formation of ketones, alcohols, esters and carboxylic acids. Their source and formation pathways were proposed, with an effect of the formulation on their nature and quantity. The short-chain mono- and dicarboxylic acids come from the degradation of PVC and the long-chain ones from additives. Moreover, the characterization of the leached PVC and the analysis of the leachates showed that most of the products generated under irradiation migrate in solution. The quantification of carboxylic acids (representing up to 79% of the TOC) by IC-MS allowed to discriminate the basic hydrolysis and diffusion processes controlling their release in solution. The latter depend mainly on the irradiation dose and type of PVC. The diffusive regime is predominant in the first days of leaching and basic hydrolysis thereafter. Finally, it has been proved by complementary analytical techniques (FIA-QToF and 1H NMR) that oligomers of radiooxidized PVC can be released in solution following nucleophilic substitution reactions

Additives as a major source of radiolytic organic byproducts of polyvinyl chloride (PVC)

International audiencePolyvinyl chloride (PVC), one of the most polymeric materials present in the radioactive waste, is exposed to radiooxidation process during disposal and generates many byproducts. Commercial PVC materials usually contain many additives to provide desired properties and to improve their stability during shaping and lifetime. The additives generally delay PVC radiolysis limiting the formation of some PVC transformation compounds, but they can indubitably generate byproducts as well. Hence, the present work investigates the role of the additives in formulated PVC on the nature and kinetics of the released radiolytic compounds. Four PVC samples, one resin, two PVC formulated on purpose and one commercial PVC containing different additives (at least di-isononyl phthalate (DINP) as plasticizer) were exposed to irradiation. Analyses by thermal desorption-GC-MS and solvent extraction coupled with GC-MS were undertaken to identify radio oxidative products. Higher amounts of volatile and non-volatile organic compounds were detected in formulated PVC compared to PVC resin. Some are products coming from the degradation of the polymer chain while the others result mainly from the radiolytic degradation of DINP plasticizer. Commercial PVC containing the widest range of additives (among other lubricants) produced additional byproducts coming from their degradation. Based on the identification and evolution of the radio oxidized products, mechanistic pathways involved in the radiolytic degradation of formulated PVC are proposed