High-Temperature Gaseous Reaction of Cesium with Siliceous Thermal Insulation: The Potential Implication to the Provenance of Enigmatic Fukushima Cesium-Bearing Material

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.

Simulant molten core-concrete interaction experiments in view of understanding Fukushima Daiichi Nuclear Power Station Cs-bearing particles generation mechanism

The Fukushima Daiichi accident resulted in the release of a novel form of radioactive Cs contamination into the environment, called Cs-bearing microparticles (CsMP). CsMPs constitute a substantial portion of the radioactive pollution near the nuclear power station and traveled beyond several hundred kilometers. Extensive characterization of the CsMPs revealed an amorphous silica matrix, along with Cs and other minor or trace elements such as Fe and Zn. This study explores the unclear generation mechanism of CsMPs by conducting experimental molten core concrete interactions (MCCI) as a source of Si and analyzing the resultant aerosols. The findings demonstrate that MCCI is in capacity to produce spherical submicronic and micronic particles, primarily composed of amorphous silica and incorporating elements akin to CsMPs. A humid atmosphere is found to favour an even closer chemical composition. Examination of the internal structure of the synthesized particles unveils pores and numerous crystalline nanoinclusions possibly serving as nucleation sites for CsMP formation through the condensation of Si-rich vapors.

Radiocesium-bearing microparticles found in dry deposition fallout samples immediately after the Fukushima nuclear accident in the Kanto region, Japan

Radiocesium released by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident still exists in the environment in two forms: adsorbed species on mineral particles in the soil and microparticles containing radiocesium mainly composed of silicate glass (CsMPs). CsMPs are dispersed not only around the FDNPP but also over a wide area of the Kanto region. The behavior and characteristics of CsMPs must be investigated to evaluate the impact of the FDNPP accident. Deposited particles including radiocesium were wiped from metal handrails on balconies and car hoods using tissue papers at six locations in the Kanto region (Tokai village, Ushiku City, Abiko City, Chiba City, Kawaguchi City, and Arakawa Ward) between March 15 and 21, 2011. CsMPs were isolated from the samples, and their characteristics were investigated. In total, 106 CsMPs derived from Unit 2 were successfully separated from 13 tissue paper samples. The radiation images of the two types of CsMPs discovered in Ushiku City demonstrate that CsMPs can easily become susceptible to fragmentation over time, even in the absence of weathering effects.