The solubility of niobium-94 (94Nb) in calcium alkaline solutions is one of the important parameters in safety assessment of intermediate-depth disposal which are assumed to use cementitious materials. Nb solubility and solubility-limiting solid phases of Nb in these systems remain unclear. The oversaturation solubility experiments were performed systematically in the 0.001–0.1 mol dm−3 (M) CaCl2 solutions under alkali conditions, and the characterization of precipitated solid phase controlling Nb solubility was conducted. The negative dependence of Nb solubilities on pH and calcium (Ca) concentration was observed in solubility experiments, and the Ca/Nb molar ratio of precipitated solid phase was 0.66. The pH and Ca dependence of Nb solubilities was reproduced by the reaction with Nb aqueous species Nb(OH)6 − and Ca–Nb oxide with the Ca/Nb ratio of 0.66, e.g., Ca4Nb6O19 (am). With increasing pH, Nb concentrations in the 0.001–0.1 M CaCl2 solutions were significantly lower than those calculated from thermodynamic data without Ca–Nb solid. This work provides systematic evidence that the presence of Ca clearly affects Nb solubility. Since calcium is a major component of groundwater and cement pore water, the Ca–Nb solid phase should be considered in the Nb solubility assessment.

Sorption distribution coefficient (Kd) of niobium-94 on minerals are an important parameter in safety assessment of intermediate-depth disposal of waste from core internals etc. The Kd of Nb on clay minerals in Ca(ClO4)2 solutions were, however, not successfully modeled in a previous study. The high distribution coefficients of Nb on illite in Ca(ClO4)2 solutions were successfully reproduced by taking Ca–Nb–OH surface species into account. Solubility of Nb was studied in Ca(ClO4)2 solutions and the results were reproduced by taking an aqueous Ca–Nb–OH complex species, CaNb(OH)6+, into account in addition to previously reported Nb(OH)6− and Nb(OH)72−. Based on this aqueous speciation model, the Ca–Nb–OH surface species responsible for the sorption of Nb on illite in Ca(ClO4)2 solutions was presumed to be X_OCaNb(OH)6. Although uncertainties exist in the speciation of aqueous Ca–Nb–OH species, the result of this study proposed a possible mechanism for high distribution coefficient of Nb on illite in Ca(ClO4)2 solutions. The mechanism includes Ca–Nb–OH complex formation in aqueous, solid and surface phases.

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Radioactive gas of Unit 1 of the Fukushima Daiichi Nuclear Power Station was released from the exhaust stack shared by Units 1 and 2 through the venting line on March 12th, 2011. In the present study, radiochemical analysis of drain water sampled at the drain pit of the exhaust stack was conducted to study radionuclides released during venting of the Unit 1. Not only volatile 129I, 134Cs and 137Cs but also 60Co, 90Sr, 125Sb and Unit 1-originated stable Mo isotopes were detected. Although Unit 1-originated stable Mo isotopes were clearly detected, their amounts were quite low compared to Cs, suggesting that the formation of Cs2MoO4 was suppressed under the accident condition. Approximately 90% of iodine existed as I− and 10% as IO3− in November 2020. Furthermore, larger amount of 129I than 137Cs was observed, suggesting major chemical form of 131I was molecular iodine rather than CsI at the accident time. The 134Cs/137Cs radioactivity ratio decay-corrected to March 11th, 2011 was 0.86, supported the results that Unit 1 originated radiocesium in environment has smaller 134Cs/137Cs radioactivity ratio than Unit 2 and 3 originated radiocesium.