In March 2011, the Fukushima Daiichi Nuclear Power Station (FDNPS) suffered reactor core overheating and fuel melting following the Great East Japan Earthquake and tsunami, producing complex microparticles from vaporized and rapidly solidified nuclear and structural materials. The chemical states and local structures of key elements in these particles, particularly uranium and plutonium, remain poorly constrained. Here, we present the synchrotron-based micro-focused X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) study of microparticles recovered from inside Unit 2 of FDNPS. The particles contain uranium, zirconium, and trace plutonium uniformly incorporated into chemically homogeneous oxide matrices. Two types were identified: uranium-rich particles with cubic UO2 and mixed U-Zr oxides with tetragonal ZrO2, the latter persisting at room temperature, indicating rapid cooling from a high-temperature metastable phase above 1650 °C. Both uranium and plutonium are mainly in the +4 state, with localized valence increases in zirconium-rich regions, suggesting redox-driven charge compensation during crystallization. These results provide direct evidence of melt evolution, actinide mixing, and oxidation-state preservation during severe reactor accidents, informing models of core degradation and strategies for safe decommissioning at FDNPS. Following the Great East Japan earthquake and tsunami in 2011, reactor core overheating and fuel melting in the Fukushima Daiichi Nuclear Power Station produced microparticles from vaporized and rapidly solidified nuclear material for which chemical structures remain underexplored. Here, the authors present synchrotron-based X-ray absorption fine structure and X-ray diffraction studies of microparticles recovered from inside Unit 2 of the station, identifying uranium-rich particles with cubic UO2 and mixed U-Zr oxides with tetragonal ZrO2, the latter indicating rapid cooling from a high-temperature metastable phase above 1650 °C.

Precise separation and purification of f-block elements are important and challenging especially for the reduction of nuclear waste and the recycling of rare metals but are practically difficult mainly because of their chemical similarity. A promising way to overcome this difficulty is controlling their oxidation state by nonchemical processes. Here, we show resonance-enhanced multiphoton charge transfer in actinide complexes, which leads to element-specific control of their oxidation states owing to the distinct electronic spectra arising from resonant transitions between f orbitals. We observed oxidation of trivalent americium in nitric acid. In addition, we found that the coordination of nitrates is essential for promoting the oxidation reaction, which is the first finding ever relevant to the primary process of photoexcitation via resonant transitions of f-block elements. The resonance-enhanced photochemical process could be used in the nuclear waste management, as it would facilitate the mutual separation of actinides, such as americium and curium.

Batch sorption experiments were performed to investigate the sorption mechanism of Se on montmorillonite under reducing conditions in deep geological environments. Based on Eh–pH diagrams and ultraviolet–visible spectra, Se was dissolved as selenide (Se(–II)) anions under the experimental conditions. The distribution coefficients (Kd; m3 kg−1) of Se(–II) indicated ionic strength independence and slight pH dependence. The Kd values of Se(–II) were higher than those of Se(IV), which also exists as an anionic species. X-ray absorption near edge spectroscopy showed that the oxidation state of Se-sorbed on montmorillonite was zero even though selenide remained in the solution. These results suggest that Se(–II) was oxidized and precipitated on the montmorillonite surface. Therefore, it is implied that a redox reaction on the montmorillonite surface contributed to high Kd values for Se(–II).

The interaction of Fe-II and Si is at the heart of many critical geochemical processes in diverse natural and engineered environments. The resulting Fe-II-silicate phases play important roles in regulating the concentrations and bioavailability of Fe-II and Si, as well as serve as sinks for trace and hazardous elements. Therefore, a detailed understanding of their structural characteristics and the underlying formation mechanisms may provide insights useful to predicting their reactivity and stability under different conditions. In this work, co-precipitates with different Si/Fe-II ratios (0.5, 1.0 and 2.0) were synthesized under anoxic and reducing conditions at different solution pH (7, 9 and 11). Thermodynamic calculations from solution chemistry data were carried out and co-precipitates were studied using X-ray diffraction (XRD), infrared (IR) spectroscopy and Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). Thermodynamic calculations predict the formation of phyllosilicate phases such as greenalite in all the samples. Solid characterization data, however, reveal significant structural variabilities and phase heterogeneity. Incipient phyllosilicate structures tend to be more pronounced in samples from pH 9 and 11, while at neutral pH conditions, polymeric silicate phases like amorphous SiO2 become predominant. These variabilities are possibly linked to heterogeneous formation processes arising from relative solubility differences between SiO2 and Fe (OH)(2). High pH (>8) conditions favor the polymerization of Fe(OH)(2) layers which likely serve as templates for layered silicate formation, while neutral pH conditions favor the precipitation of polymeric silicate phases like amorphous SiO2 to which aqueous Fe species may adsorb. In samples from Si/Fe-II 0.5, relatively well developed Fe(OH)(2) layers were identified from the EXAFS data. Increasing Si/Fe-II ratios lead to amorphous SiO2 precipitation and inhibition of Fe(OH)(2) polymerization, resulting in smaller phyllosilicate domains embedded in a polymeric SiO2 matrix. The results of this work may be useful in interpreting structural variabilities of Fe-II-Si phases observed both in nature and in engineered environments. Such variabilities may influence subsequent phase recrystallization processes as well as reactivity towards environmentally relevant elements such as radionuclides. (C) 2019 Elsevier Ltd. All rights reserved.

To determine the equilibrium constant for ferroselite (FeSe2(cr)) dissolution reaction, FeSe2(cr) solubility experiments were performed at 298 +/- 1K from both the over- and under-saturation directions with Fe-Se precipitates that were aged at 348K. X-ray diffraction (XRD) analysis detected only FeSe2(cr) as the Se solid phase in the equilibrated precipitates. The E-h values of the equilibrated suspensions ranged from -188.6 to -4.9mV vs. standard hydrogen electrode (SHE) and the pH values ranged from 6.00 to 8.76. Based on the available thermodynamic data, Se-4(2-) and Fe2+ are thermodynamically stable within this E-h-pH range. Agreement between the solubility data obtained from the over- and under-saturation directions lends credence to the attainment of equilibrium at 298 +/- 1K. The thermodynamic interpretations using the specific ion interaction theory (SIT) model showed that E-h values and the concentrations of Se and Fe are well represented by the 2FeSe(2)(cr) solubility reaction (2FeSe(2)(cr) 2Fe(2+) + Se-4(2-) + 2e(-)) with log(10)K(o) = -17.09 +/- 0.28. The obtained log(10)K(o) value falls within the uncertainty limits of the log(10)K(o) value calculated from the available thermodynamic data.

An electrochemical investigation of selenium species, using cyclic voltammetry, has been carried out for the purpose of determining the molar entropy of the Se(VI)/(IV) couple. To obtain the ion interaction coefficient between HSeO4− and Na+, ϵT(HSeO4−, Na+), the following reaction involving HSeO4− as an oxidant and the uncharged species (H2SeO3) as the reductant was used: The Se(VI)/(IV) half-wave potentials were measured in acidic sodium nitrate solutions as a function of the molality of Na+ ranging from 0.500 to 2.00 mol kg−1. The temperatures for the measurement were 288, 298, 308 and 323 K. Specific ion interaction theory was used to calculate the HSeO4−/H2SeO3 standard redox potential, , and ϵT(HSeO4−, Na+) at each temperature. The following molar entropy was derived from the temperature dependence of : The value of ϵT(HSeO4−, Na+) at 298 K was determined to be 0.29 ± 0.03 kg mol−1. The following ∂ϵ/∂T was derived from the temperature dependence of ϵT(HSeO4−, Na+):

The aqueous solubility of BaSeO4 (cr) was studied at 23 +/- 2 degrees C as a function of Na2SeO4 concentrations (0.0001 to 4.1 mol kg(-1)) and equilibration periods (3 to 596 d). The equilibrium, approached from both the under-and over-saturation directions, in this system was reached rather rapidly (<= 3 d). The SIT and Pitzer's ion-interaction models were used to interpret these data and the predictions based on both of these models agreed closely with the experimental data. Thermodynamic analyses of the data show that BaSeO4 (cr) is the solubility-controlling phase for Na2SeO4 concentrations <0.59 mol kg(-1). The log(10) K-0 value for the BaSeO4 (cr) solubility product (BaSeO4 (cr) Ba2+ + SeO42- calculated by the SIT and Pitzer models were very similar (-7.32 +/- 0.07 with Pitzer and -7.25 +/- 0.11 with SIT). Although the BaSeO4 (cr) solubility product and Ba concentrations as a function of Na2SeO4 concentrations predicted by both the SIT and Pitzer models are similar, the models required different sets of fitting parameters. For examples, 1) interpretations using the SIT model required the inclusion of Ba(SeO4)(2)(2-) species with log(10) K-0 = 3.44 +/- 0.12 for the reaction (Ba2+ + 2SeO(4)(2-) Ba(SeO4)(2)(2-)), whereas these species are not needed for Pitzer model, and 2) at Na2SeO4 concentrations >0.59 mol kg(-1) it was also possible to calculate the value for log(10) K-0 for the solubility product of a proposed double salt (Na2Ba(SeO4)(2)(s) 2Na(+) + Ba2+ + 2SeO(4)(2-)) which for the SIT model is -(8.70 +/- 0.29) whereas for the Pitzer model it is -(9.19 +/- 0.19). The ion-interaction/ion-association parameters hitherto unavailable for both the SIT and Pitzer models required to fit these extensive data extending to as high ionic strengths as 12.3 mol kg(-1) were determined. The model developed in this study is consistent with all of the reliable literature data, which was also used to extend the model to barium concentrations as high as 0.22 mol kg(-1) and pH ranging from 1.4 to 13.8, in addition to selenium concentrations as high as 4.1 mol kg(-1).

The latest available thermodynamic data for palladium and tin were critically reviewed and the selected values were included into the JAEA-TDB for performance assessment calculations for geological disposal of radioactive high-level and TRU wastes. We made sure that the selected data are internally consistent with other data included in the compilation. This critical review specifically addressed thermodynamic data for 1) the palladium-hydroxide-chloride system, and 2) the solid oxides and hydroxido complexes of Sn(IV). We also selected thermodynamic data for other tin reactions from critical review of tin by the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development (OECD/NEA). Furthermore, we refined some thermodynamic data for Protactinium to estimate more reliable solubility values. We prepared text files of the updated thermodynamic database (JAEA-TDB) for geochemical calculation programs of PHREEQC, EQ3/6 and Geochemist's Workbench. Use of the Bronsted-Guggenheim-Scatchard Model (SIT) for ionic strength corrections was applied to the PHREEQC database. A CD-ROM is attached as an appendix. (J.P.N.)

The solubility of Ba(SeO4, SO4) precipitates was determined as a function of the BaSeO4 mole fractions, ranging from 0.0015 to 0.3830, and time with an equilibration period extending to as long as 302 days. Equilibrium/steady state conditions in this system are reached in <= 65 days. Pitzer's ion interaction model was used to calculate solid and aqueous phase activity coefficients. Thermodynamic analyses showed that the data do not satisfy Gibbs-Duhem equation, thereby demonstrating that a single-solid solution phase does not control both the selenate and sulfate concentrations. Our extensive data with log(10) [Ba] ranging from -3.6 to -5.9 mol kg(-1), log(10) [SeO4] ranging from -3.6 to -5.2 mol kg(-1), and log(10) [SO4] ranging from -4.0 to -5.3 mol kg(-1) can be explained with the formation of an ideal BaSeO4 solid solution phase that controls the selenium concentrations and a slightly disordered/less-crystalline BaSO4(s) (log(10) K degrees(sp) = -9.5instead of -10.05 for barite) that controls the sulfate concentrations. In these experiments the BaSO4 component of the solid solution phase never reaches thermodynamic equilibrium with the aqueous phase. Thermodynamic interpretations of the data show that both the ideal BaSeO4 solid solution phase and less-crystalline BaSO4(s) phase are in equilibrium with each other in the entire range of BaSeO4 mole fractions investigated in this study.

An electrochemical investigation of selenium, using cyclic voltammetry, has been carried out to determine the standard redox potential of the following reaction: The Se(VI)/(IV) half-wave potentials were measured in alkaline sodium perchlorate solutions as a function of molality of a sodium ion. Extrapolation of the experimental data to the standard state using specific ion interaction theory yields the following standard redox potential and ion interaction coefficients:

We additionally selected thermodynamic data for solid and gaseous phases of nickel, selenium, zirconium, technetium, thorium, uranium, neptunium, plutonium and americium to our thermodynamic database JAEA-TDB for geological disposal of radioactive waste of high-level and TRU wastes. We thermodynamically obtained equilibrium constant from addition and subtraction of Gibbs free energy of formation on nickel, selenium, zirconium, technetium, thorium, uranium, neptunium plutonium and americium, which were selected in the Thermochemical Database Project by the Nuclear Energy Agency in the Organisation for Economic Co-operation and Development. Furthermore, we collected and updated thermodynamic data on iodine, changed master species of technetium(IV), and added thermodynamic data on selenium due to improving reliability of the thermodynamic database. We prepared text files of the updated thermodynamic database (JAEA-TDB) for geochemical calculation programs of PHREEQC, EQ3/6 and Geochemist's Workbench. These text files are contained in the attached CD-ROM and will be available on our Website (http://migrationdb.jaea.go.jp/). (author)

研究論文「Se(IV)/Se(VI)の標準電極電位に関するサイクリックボルタンメトリーによる実験的研究. 原子力バックエンド研究 Vol. 16 (2009).」において，対象の反応の式量電位を与える式が誤っていた．本報告は，その訂正記事である．

Japan Atomic Energy Agency (JAEA) established the thermodynamic database (JAEA-TDB) for performance assessment of geological disposal of high-level radioactive waste (HLW) and TRU waste. Twenty-five elements which were important for the performance assessment of geological disposal were selected for the database. JAEA-TDB enhanced reliability of evaluation and estimation of their solubility through selecting the latest and the most reliable thermodynamic data at present. We evaluated and estimated solubility of the 25 elements in the simulated porewaters established in the “Second Progress Report for Safety Assessment of Geological Disposal of HLW in Japan” using the JAEA-TDB and compared with those using the previous thermodynamic database (JNC-TDB). It was found that most of the evaluated and estimated solubility values were not changed drastically, but the solubility and speciation of dominant aqueous species for some elements using the JAEA-TDB were different from those using the JNC-TDB. We discussed about how to provide reliable solubility values for the performance assessment.

To evaluate the long-term performance of a geological disposal system for high-level radioactive wastes (HLW), selenium-79 (Se) solubility and its solubility-limiting solid phase(s) were investigated in the presence of iron (Fe) under reducing conditions. Se is one of the key radionuclides in the safety assessment of a HLW repository because Se forms anions and has a low coefficient of sorption onto geological materials. The formation of Fe-Se solids is expected due tothe release of Fe(II) by Fe overpack corrosion. Experiments in both bentonite-equilibrated and pure waters were performed in the presence of Fe under reducing conditions. With bentonite (Kunigel V1®), the experimental conditions were within the bounds of the FeSe2(cr) stability field when plotted on an Eh-pH diagram; however, no Se solid phases could be identified in the experiments by X-ray diffraction (XRD). With pure water, the experimental conditions were in the same FeSe2(cr) stability field. After 1 month, the experimental system was under a transitional condition which moved toward true equilibrium from oversaturation and Se concentration decreased as a likely result of the precipitation of FeSe2(cr), although it remains possible that FeSe(cr) controlled Se concentration if the water had an initial pH = 7. Se(cr) could not control Se concentration because Se concentration decreased to below Se solubility, even though XRD identified Se(cr) as the dominant solid phase. After 2 and 3.5 months, the transformation from Se(cr) to the Fe-Se solid phase (FeSe2(cr), FeSe(cr)) could be identified by XRD.

Sorption and diffusion of Cs in the sedimentary rock from the Horonobe generic URL were studied from the viewpoints of reliability of experimental evaluation and model prediction, focusing on its applicability to intact systems. The distribution coefficient, K(d), for Cs was measured under the same chemical conditions by both batch sorption and diffusion experiments. To obtain reliable parameters of sorbing species for intact rock, through-diffusion experiments coupled with multiple curve analysis including tracer depletion, breakthrough and depth-concentration curves were examined, and resulted in good agreement with those predicted by conventional transport models using only one set of retardation parameters. The K(d) values obtained by the diffusion tests using intact rock were consistent with those obtained by the batch tests with crushed rock. The sorption behavior was modeled by considering additive ion-exchange reactions for illite and smectite, which were assumed to be dominant sorption minerals based on microscopic observation and their known sorption mechanism. The model predicted the K(d) values obtained by the series of experiments reasonably well.