Cesium retention and release from sulfur polymer concrete matrix under normal and accidental conditions

•Slag and fly ash sulfur polymer concrete as effective hosts for Cs-137 immobilization.•Proposed methodology almost entirely based on waste materials.•Accidental conditions speed up Cs-137 release from sulfur polymer concrete matrix.•Cesium mobilization observed already below matrix melting temperature.•Silicate and aluminosilicate phases proposed as Cs+ ions binding sites. This paper proposes an efficient two-stage process for stabilization and solidification of the Cs-137 isotope in a sulfur polymer concrete (SPC) matrix. Lignite slag (SL) and fly ash (FA) were applied as active fillers for cesium immobilization. To study the release of Cs-137 isotope and determine the tracer activity in the leachates, we applied a slightly modified ANSI/ANS 16.1 protocol and the gamma spectrometry technique. The measured effective diffusion coefficients for the Cs-137 isotope were between 0.84·10−9 and 3.10·10−9 cm2·s−1. Normalized leaching rates were within the range of 1.74·10−5 – 3.85·10−5 g·cm−2·d−1, fulfilling acceptance criteria for radioactive wasteforms. As well as standard leaching under static conditions, we also studied dynamic leaching of SPC samples at increased temperatures and leaching in an aggressive environment. The Cs-137 effective diffusion coefficients were found to increase by 3 – 4 orders of magnitude (10−6 – 10−5 cm2·s−1), while the normalized leaching rate reached values of up to 2.36·10−3 g·cm−2·d−1 after 28 days of leaching. The proposed cesium immobilization mechanism is based on the formation of cesium silicate and aluminosilicate phases, together with effective matrix sealing during the SPC manufacturing process.