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Thermochemical analysis for cesium and iodine species and their impact on aqueous pH under severe accident conditions

シビアアクシデント条件下におけるセシウム及びヨウ素化学種の化学熱力学解析とそれら化学種が液相pHに与える影響評価

塩津 弘之 ; 石川 淳 ; 丸山 結 

Shiotsu, Hiroyuki; Ishikawa, Jun; Maruyama, Yu

The effect of chemical species on solution pH in S/C pool was investigated on the basis of parametric analysis with the integral severe accident code, THALES2, after performing the thermochemical analysis for Cs-I-Mo-B-C-O-H systems under severe accident conditions of BWR. In the results of thermochemical calculation without B$$_{4}$$C oxidation, Cs mainly took a chemical species of Cs$$_{2}$$MoO$$_{4}$$(c, g) at oxidation condition and partially form CsOH(g) and Cs(g) above $$sim$$1500 K at the reducing condition owing to Mo reduction. In addition, B$$_{4}$$C oxidation resulted in formation of major Cs species of CsBO$$_{2}$$(g) above 1800 K at a wide range of H$$_{2}$$/H$$_{2}$$O ratio. However, Cs$$_{2}$$MoO$$_{4}$$(c) could be preferentially formed under a H$$_{2}$$O rich oxidizing condition with low concentration of B$$_{4}$$C below 1800 K. In case of I, although CsI was dominate species in various conditions, HI fraction was increased with increase of B$$_{4}$$C amount above 1500 K. Based on THALES2 analysis, 80$$sim$$90% of Cs was retain in S/C pool by large scrubbing effect, resulting in the change of apparent pH at each Cs and I species in range of 4.8$$sim$$8.0. Especially, the case with higher fraction of Cs$$_{2}$$MoO$$_{4}$$ and HI shows the lower pH, indicating importance of their fraction because of larger formation of volatile I species such as I$$_{2}$$ and CH$$_{3}$$I by the chemical reaction at lower pH.

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