Chemical stability of slags stemming from the thermal treatment of low-level radioactive waste (STAB-SLAK): Results of the expansion tests – status 31.05.2025

In 2020 the ETF project STAB-SLAK was launched. One of the objectives of this project was to establish a link between the leaching and expansion behaviour of plasma slags (which are glass composite materials) under alkaline conditions representative of those expected at the surface disposal site, and the chemical composition of the slags. A range of waste stream compositions was first determined by ONDRAF/NIRAS, and then thermodynamic calculations considering a 3:1 flux:waste ratio with soda-lime glass were done by InsPyro N.V. to optimize the production of the slags by thermal treatment. 22 slags were produced in Cycle 1 and 8 slags in Cycle 2, with chemical compositions determined by an experimental matrix designed by ONDRAF/NIRAS. These slags were used in leaching and expansion tests to study their stability. This technical report provides the results of the expansion tests carried out at SCK CEN. The tests were done with cured prisms (40x40x160 mm³) that were produced by mixing the granulated slags with cement and water, and cured afterwards for 28 days. The samples were placed in an incubator with temperature of 60 ± 1.5 °C and relative humidity of 95 ± 5% for 120 days. The evolution of the length was monitored after 7, 14, 28, 60, 90 and 120 days. Compressive and flexural strength were also measured at the end of the tests. The length and weight variation during the tests were very limited, except for the sample produced with the slag R8. The compressive and flexural strength were high for all samples. The results of the expansion tests performed with the slags of Cycle 1 and Cycle 2 were statistically processed by ONDRAF/NIRAS. The analysis of the Cycle 1 results suggested that the SiO2 content plays a major role in the expansion of the samples under these conditions, but a quantitative model based on this insight did not succeed in reproducing well the length evolution of the Cycle 2 test results. Therefore, further analysis was done considering different models based on the data of both Cycle 1 and Cycle 2. This analysis suggested again a significant effect of the SiO2 content, but also of the content of other elements like B2O3 and PbO, of the cooling rate, and of the interactions between these factors. In the follow-up project STAB-SLAK 2.0, funded by ONDRAF/NIRAS, a third set of samples will be produced and submitted to expansion tests.