Surface analyses of cesium hydroxide chemisorbed onto type 304 stainless steel

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.

Isotopic signature and nano-texture of cesium-rich micro-particles: Release of uranium and fission products from the Fukushima Daiichi Nuclear Power Plant

Highly radioactive cesium-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) provide nano-scale chemical fingerprints of the 2011 tragedy. U, Cs, Ba, Rb, K, and Ca isotopic ratios were determined on three CsMPs (3.79–780 Bq) collected within ~10 km from the FDNPP to determine the CsMPs’ origin and mechanism of formation. Apart from crystalline Fe-pollucite, CsFeSi₂O₆ · nH₂O, CsMPs are comprised mainly of Zn–Fe-oxide nanoparticles in a SiO₂ glass matrix (up to ~30 wt% of Cs and ~1 wt% of U mainly associated with Zn–Fe-oxide). The ²³⁵U/²³⁸U values in two CsMPs: 0.030 (±0.005) and 0.029 (±0.003), are consistent with that of enriched nuclear fuel. The values are higher than the average burnup estimated by the ORIGEN code and lower than non-irradiated fuel, suggesting non-uniform volatilization of U from melted fuels with different levels of burnup, followed by sorption onto Zn–Fe-oxides. The nano-scale texture and isotopic analyses provide a partial record of the chemical reactions that occurred in the fuel during meltdown. Also, the CsMPs were an important medium of transport for the released radionuclides in a respirable form.

Caesium-rich micro-particles: A window into the meltdown events at the Fukushima Daiichi Nuclear Power Plant

The nuclear disaster at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 caused partial meltdowns of three reactors. During the meltdowns, a type of condensed particle, a caesium-rich micro-particle (CsMP), formed inside the reactors via unknown processes. Here we report the chemical and physical processes of CsMP formation inside the reactors during the meltdowns based on atomic-resolution electron microscopy of CsMPs discovered near the FDNPP. All of the CsMPs (with sizes of 2.0–3.4 μm) comprise SiO₂ glass matrices and ~10-nm-sized Zn–Fe-oxide nanoparticles associated with a wide range of Cs concentrations (1.1–19 wt% Cs as Cs₂O). Trace amounts of U are also associated with the Zn–Fe oxides. The nano-texture in the CsMPs records multiple reaction-process steps during meltdown in the severe FDNPP accident: Melted fuel (molten core)-concrete interactions (MCCIs), incorporating various airborne fission product nanoparticles, including CsOH and CsCl, proceeded via SiO₂ condensation over aggregates of Zn-Fe oxide nanoparticles originating from the failure of the reactor pressure vessels. Still, CsMPs provide a mechanism by which volatile and low-volatility radionuclides such as U can reach the environment and should be considered in the migration model of Cs and radionuclides in the current environment surrounding the FDNPP.