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

In 2020 the ETF project STAB-SLAK was launched. The main objective 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 characteristics of the slags (i.e. the chemical components and the cooling rate after production). A range of waste stream compositions was first determined by ONDRAF/NIRAS, and then thermodynamic calculations considering a 3:1 flux with soda-lime glass were done by InsPyro N.V. to optimise 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, which were used in leaching and expansion tests to study their stability. This technical report provides the results of the dissolution (leaching) tests carried out at SCK CEN including static, semi-dynamic and dynamic tests. The tests were performed under anoxic conditions at 40 °C in a synthetic cementitious solution with a pH of 12.5 (IPM water) using a low slag surface area to solution volume (10 m-1). The leaching data of the static dissolution tests performed with the slags of Cycle 1 and Cycle 2 were statistically processed by ONDRAF/NIRAS. The elements constituting the slags can be categorised into three groups: fast-leaching elements for B and Pb, intermediate-leaching elements for Al, Ba, Cr, Cu, Si and Zn, and slow-leaching elements for Fe and Ni. The statistical treatment of the results also enabled to build a quantitative dissolution model that enables to predict the average dissolution rate of an element on the basis of the ratios Fe2O3/SiO2 and MgO/SiO2 and the group the element belongs to. Additional leaching tests consisting of semi-dynamic and dynamic tests were started with two slags from the centre points of the domain investigated, which have a similar composition but a different cooling rate. These two types of tests aim at confirming the hypotheses related to the interpretation and translation of the lab-scale tests to the preliminary safety assessments. The latter use the obtained leaching data as an input and provide information on the disposability of the waste. To ensure confidence in the built model, these leaching tests will be continued as part of the STAB-SLAK 2.0 project, which is funded by ONDRAF/NIRAS, to collect additional data. In addition, the literature data and insights on Nb, which together with Pu is responsible for most of the long-term radiological impact, did not provide sufficient information for a classification of Nb in one of the three groups (slow-, intermediate- and fast-leaching elements). Therefore, slags containing Nb were produced, and leaching tests were started. The results available so far suggest that Nb is not a fast-leaching element.