9. TRU and Nuclear Chemistry

概要

INTRODUCTION: For the reliable prediction of ra- dionuclide migration behavior under relevant radioactive waste disposal conditions, it is necessary to establish a thermodynamic model on the solubility limits of radionu- clides in groundwater. Several actinides and long half-life fission products such as 93Zr are known to exhibit a tet- ravalent oxidation state in repository systems and easily precipitate as an amorphous hydroxide (M(OH)4(am)) due to strong hydrolysis reactions. Although the solubili- ty is primarily controlled by the amorphous hydroxide, M(OH)4(am) is thermodynamically meta-stable, and can be converted to stable crystalline oxide of MO2(cr) by heating or calcinations [1]. Since the decay heat emis- sions are transferred to the groundwater surrounding the waste packages, the repository systems are expected to be under elevated temperature conditions [2]. Therefore, it is important to reveal an impact of elevated temperature on the solid phase transformation of M(OH)4(am) for the prediction of the solubility limits of radionuclides in tet- ravalent oxidation state. On the other hand, a number of literatures have investigated the solubility of MO2(cr) and several studies suggested that the surface of MO2(cr) are possibly hydrolyzed to form hydroxide layer, which is controlling the apparent solubility [3]. Since the solubili- ty product (Ks) of MO2(cr) and M(OH)4(am) differs more than several orders of magnitude, it is of importance to clarify the solid phase transformation between MO2(cr) and M(OH)4(am) for establishing the robust thermody- namic model on the solubility of radionuclides in tetra- valent oxidation state.
In the present study, we focused Zr as one of the fission products in tetravalent oxidation state under the reposito- ry conditions. ZrO2(cr) powder were first measured by a versatile compact neutron diffractometer (VCND) at the B-3 beam port of KUR as references. Then the neutron diffraction patterns of amorphous zirconium deuterium hydroxide (Zr(OD)4(am)) aged at 25 to 90 ºC were col- lected to investigate a temperature effect on the transfor- mation of ZrO2(cr) and Zr(OD)4(am).

EXPERIMENTS: The powder samples of ZrO2(cr) was purchased from WAKO Pure Chem. and put in sam- ple holders without further treatment. The samples of Zr(OD)4(am) was prepared by an oversaturation method. A few grams of ZrCl4 (WAKO Pure Chem.) was dis- solved in D2O to prepare the Zr stock solution ([Zr] = 0.2 M). Portions of deuterium hydroxide (NaOD) were then added in the stock solution to adjust the pHc at pHc = 2.7. The sample suspensions were placed in ovens kept at 25, 40, 60, and 90 ºC for 3 weeks. After the aging periods at each temperature, the pHc of the sample solutions was measured. The solid phases were separated by centrifuga- tion and dried in vacuum desiccator. After the dryness, the powder samples were put in sample holders and measured by a VCND installed at the B-3 beam port of KUR. A Cu monochromator with 40 mm diameter was used to monochromatize the neutron wavelength of 1.0Å. The beam size was approximately 10 mm in width and 30 mm in height. The beam flux was 1.3×105 n/s·cm2 during the operation at the power of 5 MW on KUR.

RESULTS: Figure 1 shows the neutron diffraction (ND) pattern of ZrO2(cr) in comparison with the X-ray diffraction (XRD) patterns [1]. The ND patterns exhibited intense peaks at low d (d = λ/2 sin θ), while the peaks in the XRD patterns showed less intensities. The peak posi- tions of both methods agreed and indicated the mono- clinic structure of ZrO2(cr).
The neutron diffraction pattern of Zr(OD)4(am) after ag- ing at 25 ºC showed a halo pattern typical for amorphous phase. With increasing the aging temperatures, the peaks corresponding to ZrO2(cr) appeared and those after aging at 90 ºC became clear with large intensities, indicating the progress of crystallization by heating. Further inves- tigation is planned by an undersaturation approach from ZrO2(cr), and also for other elements in tetravalent oxida- tion state.

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