Study of a pyrolysis process for epoxy resin elimination of embedded nuclear fuels

This study aims to develop a thermal treatment for the elimination of epoxy resin from spent nuclear fuel by pyrolysis. In order to suppress gas production from polymer's degradation by radiolysis in confined storage conditions, these fuels, which have been previously embedded in epoxy resin for the purpose of metallographic preparation, must be separated from the resin. To ensure the safety and universality of the process for removing the organic material, a pyrolysis process in an inert atmosphere of argon is selected. The main objective of the treatment is to minimize the final hydrogen content in the residual char while maintaining the integrity of the fuel in order to avoid the release of fission products and the expansion of the material, which occurs in oxidizing atmosphere. In this work, a general methodology is proposed in order to understand the physico-chemical phenomena involved during the pyrolysis of the resin and to acquire relevant data for the development of the process. Experimental investigations were mainly carried out in a pilot-scale furnace which was coupled with a series of filtration devices to recover the pyrolysis products for characterization. Most of pyrolysis gases have been measured on line with micro gas chromatography columns. For some pilot experiments, a FTIR analyzer allowed to characterize heavier and nitrated compounds in the gas. The resin, the tars and the char were characterized by elemental analysis (CHON). All the measurements of the pyrolysis products' elemental compositions and weights have been adjusted by the method of data reconciliation. The results make it possible to establish a precise mass balance with the definition of model molecules (CxHyOzNw) for tars and char and the stoichiometric coefficients for the overall pyrolysis reaction. At 450 C, the mass distribution of pyrolysis products is: 78.74% tars, 5.82% water, 12.84% char and 2.60% permanent gases. The parametric studies carried out with regard to the char ratio and its hydrogen content have shown that the predominant parameters are, in order of importance: the temperature, the plateau time and the heating rate. The influence of argon flow rate and resin mass is negligible. For a further investigation of the char, NMR spectroscopy was applied to identify its molecular structure changes with pyrolysis temperature. Furthermore, the characterization of tars by TD-GC/MS reveals four main compounds: dodecanol, tetradecanol, phenol and 4-isopropylphenol. Both these quantitative and qualitative information help to define the process conditions for the removal of epoxy resin by pyrolysis with respect to the final objective, i.e. minimizing the hydrogen content in the residual char while respecting the safety constraints. (author)