Fundamental Study on the Solidification of Radionuclides with Hydrous Ti IV Oxide Modified with Either Si or Zr
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FUNDAMENTAL STUDY ON THE SOLIDIFICATION OF RADIONUCLIDES WITH HYDROUS Ti"v OXIDE MODIFIED WITH EITHER Si OR Zr Hiromichi Yamazaki and Yasushi Inoue Department of Nuclear Engineering, Tohoku University, Sendai 980-77, Japan ABSTRACT The potentiality of hydrous Ti'v-Si oxide (TS) and hydrous Tilv-Zr oxide (TZ) for the selective sorption and immobilization of radioactive Co 2,, Eu3÷,ThI and U0 22' ions has been studied experimentally: The distribution coefficients and sorption capacity for these ions at various conditions in solutions, the structure and the composition of the exchangers in relevant ionic forms at high temperatures, and the leaching rates from the calcined exchangers into water and 0.01M HNO 3 solution at room temperature. These results were discussed together with the results previously obtained for Cs' and Sr2÷, in order to evaluate the performance of TS and TZ as solidification media for radioactive liquid waste. TS takes up these cations more than TZ does in acidic solutions due to the higher acidity of the exchange site. In TZ, transition metal ions show high affinity despite the small distribution coefficients for representative metal ions. The dissolution of Co2÷,Eu 3÷,and Th' is very slow from both calcined exchangers due to the crystallization of titanates of these cations. The leaching rates of Cs', Sr2÷ and UO 22÷from calcined TS are much slower than those from TZ, indicating the formation of titanium silicate containing these cations. Based on these results, TS is considered to be superior to TZ in the sorption capacity and fixation capability of various kinds of radionuclides and is considered as a promising material for the solidification of radwaste. INTRODUCTION A method for separating radionuclides from a huge volume of liquid waste and immobilizing them to a stable solid form is called for the safety management of radioactive liquid waste produced in the nuclear fuel cycle. Inorganic ion exchangers are a group of promising materials for this purpose, since they have unique selectivity for radionuclides and can be converted into stable waste forms suitable for disposal by simple thermal treatment.', Along this line, the solidification of radionuclides with zeolites and calcium titanate has been thoroughly studied as an alternative process of radioactive waste management. 3'4 However, it is necessary to survey the performance of various inorganic ion exchangers in order to select the most promising solidification medium for radioactive liquid wastes of different compositions. Hydrous TiIv oxides, modified with either Si or Zr, are promising for use as solidification materials of radionuclides in a huge volume of liquid wastes, since they have larger ion-exchange capacity than simple hydrous metal oxides.5' 6 As solidification media of Cs* and Sr2+, hydrous Ti"v-Si oxide (TS) was found to be superior to hydrous Ti'V-Zr oxide (TZ), as was previously reported;" the ion-exchange capacity for Cs* and Sr2. was much larger on TS than on TZ, and the leaching rates from the calcined materials we
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