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

Here, we report an investigation of the gas-solid reaction between cesium hydroxide (CsOH) and siliceous (calcium silicate) thermal insulation at high temperature, which is postulated as the origin for the formation mechanism of cesium-bearing material emitted from the Fukushima Daiichi nuclear power plant. A developed reaction furnace consisting of two heating compartments was used to study the reaction at temperatures of 873, 973, and 1073 K. Under the influence of hydrogen-steam atmospheric conditions (H2/H2O = 0.2), the reaction between cesium hydroxide vapor and solid thermal insulation was confirmed to occur at temperatures of 973 and 1073 K with the formation of dicalcium silicate (Ca2SiO4) and cesium aluminum silicate (CsAlSiO4). Water-dissolution analyses of the reaction products have demonstrated their stability, in particular, CsAlSiO4. Constituent similarity of the field-observed cesium-bearing materials near the Fukushima Daiichi nuclear power plants with CsAlSiO4 suggests for the first time that gaseous reaction between CsOH with calcium silicate thermal insulation could be one of the original formation mechanisms of the cesium-bearing materials.

Revaporization Behavior of Cesium and Iodine Compounds from Their Deposits in the Steam-Boron Atmosphere.

This paper presents our investigation on cesium and iodine revaporization from cesium iodide (CsI) deposits on stainless steel type 304L, which were initiated by boron and/or steam flow. A dedicated basic experimental facility with a thermal gradient tube (TGT) having a temperature range of 1000-400 K was used for simulating the phenomena. In the absence of boron, it was found that the initially deposited CsI at 850 K could be revaporized as CsI vapor/aerosol or reacted with the carrier gas and stainless steel (Cr2O3 layer) to form Cs2CrO4. The latter mechanism consequently released gaseous iodine that was later accumulated downstream. After introducing boron to the steam flow, a severe revaporization occurred. This, in addition to the revaporized CsI vapor/aerosol, was caused by cesium borate (Cs2B4O7 and CsB5O8) formation, which then largely released gaseous iodine that was capable of reaching the TGT outlet (<400 K). In the case of a nuclear severe accident, our study has demonstrated that an increase of gaseous iodine in the colder region of a reactor could occur after late release of boron or a subsequent steam flow after refloods of the reactor, thus posing its inherent risk once leaked to the environment.