IMPACT OF SULPHATE IONS RELEASED FROM ION EXCHANGE RESINS UNDER RADIOLYSIS PROCESS ON THE PROPERTIES OF HYDRATED CEMENT ENCAPSULATING MATRIX

摘要

Ion exchange resins are commonly used during industrial treatment of spent nuclear fuel. Some of these resins are intermediate level radioactive wastes which may be damaged by radiolysis process, releasing sulphate ions from the backbone directly into the cement based encapsulation material. This work consists in an experimental study of the impact of released sulphates into hydrated cement matrix. The tests were performed on the one hand on cement coated cationic resins, alone or mixed with anionic resins, and on the other hand on simplified system consisting of pure hydrated CEM III/C cement. As progressive releasing of sulphate ions directly into the matrix was very difficult to simulate during laboratory experiments, tests were carried out under following conditions: thin samples were placed into an aqueous solution containing a concentration of sulphate ions corresponding to the amount that could be released under resins radiolysis. Variations in the chemical composition of the curing solution, dimensional stability, mineralogy and microstructure of the sample were studied during several months. The sulphate attack of cement paste and cement coated cationic resins led to delayed ettringite formation near the sample surface exposed to curing solution. Ettringite precipitated as massive crystals perfectly mixed with the calcium-silicate-hydrate, which are the main cement hydration products. No disruptive expansion was detected at a macroscopic scale. Concerning the mix of cationic and anionic resins, delayed ettringite formation was detected as well. Here however ettringite is precipitating in needle form: on the surface exposed to solution, into the cement matrix, near the surface, localised in the interfacial transition zone between cement matrix and resins, into the cement matrix, near the surface, progressively replacing the portlandite that fulfilled the cracks of anionic resins. No disruptive expansion was detected at a macroscopic scale. Therefore, the CEM III/C and the cement-coated resins could be considered as having a good external sulphate attack resistance during the study period. Further experiments were conducted on cement paste by including sulphate ions directly into the casting solution. The samples were placed into ultra pure water. During the experiment period, the expansion detected was moderate, and didn't lead to cracking. Therefore, the material involved was considered as having a good resistance to internal sulphate attack.