核種移行評価モデルにおけるRaの共沈現象の検討

Development on coprecipitation model of Ra for radionuclide migration analysis

吉田 泰*; 吉川 英樹

Yoshida, Yasushi*; Yoshikawa, Hideki

Raが他の元素に伴って共沈反応する際に、従来は例えばCa塩との共沈に際しては、モル比を保ったまま液相から固相に変化するとの仮定でモデル化や評価がなされていた。本報では、共沈実験により得られた元素分配比と固液のCa/Ra存在比(モル比)が異なる点に着目して、共沈反応によるRa分配比を分配係数に換算して表現する分配モデルを提案するとともに、例として降水系還元性高pH(FRHP)地下水反応間隙水条件における分配係数の換算値を示した。分配係数の換算値を考慮して降水系還元性高pH(FRHP)地下水反応間隙水の条件において4n+2系列核種の人工バリアからの核種移行率を計算した結果、Raの人工バリアからの移行率が4n+2系列で最も高くなることが示されたものの、人工バリアからの核種放出率が最も高いCsの約25分の1であった。

A concentration of Ra limited by coprecipitation with calcite used to be estimated by a model of distribution assuming that constant mol ratio of Ra/Ca is preserved in solid and liquid phase. According to experiments, it has been revealed that mol ratio of Ra/Ca in solid differs from that in liquid. This difference was evaluated by a partition coefficient(D). In this study, a partition model was established to evaluate an effect of coprecipitation of Ra in calcite with D. In the model, D was converted to a distribution coefficient(Kcp) and the effect of incorporation of Ra was examined. As an example, this model was adopted under a condition of FRHP porewater and a converted Kcp was conducted. Release rates of 4n+2 series nuclides from engineered barrier with this model were also calculated. As a result of calculation, release rate of Ra was largest in their series on an outer surface of engineered barrier. This release rate was more than one order magnitude lower than that of Cs.