Advances in understanding cesium retention on calcium silicate material

Retention or trapping of cesium, one of the radiologically important fission products, in the nuclear reactor becomes a great concern as the occurrence may affect radioactivity in the long term or its environmental fate. Herein the chemical compound of cesium that had been largely trapped on the nuclear reactor structural material of (calcium silicate) thermal insulator in a simulated nuclear accident condition was investigated. A combined pre- and post-water dissolution analysis through infrared (IR) spectroscopy and optical emission spectroscopy (OES) was explored to resolve the characterization difficulty encountered in conventional X-ray diffraction analysis reported in the previous works. This method allowed us to identify for the first time the related large amount of water-soluble cesium in the calcium silicate material after a high-temperature chemical reaction as cesium metasilicate (Cs₂SiO₃). It was evidenced by similar vibrational characteristics of the material to that in the synthesized Cs₂SiO₃ as well as based on the dissolved Cs and Si in the leaching water having a molar ratio of 2.16 ± 0.33. The corresponding 79-98% of the retained cesium in calcium silicate materials in the case study of 700 and 800 °C reactions was of this compound, emphasizing its significance once formed. Thermodynamic considerations further corroborated the higher stability of Cs₂SiO₃ in the cesium-calcium silicate reaction than other cesium silicates such as Cs₂Si₄O₉, Cs₂Si₂O₅, or Cs₆Si₂O₇. This clearly poses a high environmental risk due to the volatility of cesium metasilicate as it may spread out further through the water leak path from a damaged nuclear reactor.