Activities of boron (B) and silicon (Si) in the Fe-Si-B liquid phase were evaluated by thermodynamic approach to clarify the effect of Si on the stable cesium (Cs) species formation at the temperature range from 1500 K to 2000 K. Although Si exists only as a minor alloying element of the stainless steel (SS) at the initial condition, Si may be controlling element when considering the chemical state of Cs in accidental conditions. The calculated activities were applied to the evaluation of chemical reactions of Cs-Si-O and Cs-B-O systems assumed in the Fukushima Daiichi Nuclear Power Station (FDNPS) unit 2. Consequently, production of 1.27 kg of the Cs-bearing particle at 2000 K was possibly found and it is considered to be specific phenomenon to the FDNPS unit 2 accident progression. Moreover, the following two findings were obtained that the increasing of Si resulted in an increase in activity of B in the melt, and Cs could not be released as CsBO2; instead, could be trapped on the surface of the SS-B4C condensed phase as the Cs-B-O species.
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