| Research Abstract |
Geological disposal of high level radioactive wastes (HLRW) has been discussed in the world as well as in Japan. Solidification of radioactive wastes is necessarry for the geological dosposal because the wastes is put in the underground area. One of the purpose of present study is to evaluate the solidified glass of the wastes from the geological disposal poit of view. Firstly, borosilicate glass of 15 kinds of chemical composition. The borosilicate glasses are composed pribcipally of SiO_2, Al_2O_3 and Na_2O.The chemical compositions (wt %) of the major constituent elements of borosilicate glasses are as follows. The variation is that SiO_2=45-50, Al_2O_3=5-10, B_2O_3=15-17, and Na_2O=10-11. In addition to the principal elements, CaO (5-10wt%), SrO (7.5-15)and Cs2O (7.5-15) are included in some borosilicate glasses. Devitrification of the glasses depends on the chemical composition. However, all glasses are altered to devitrification in hydrothermal condition. Mineral species obtained
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in the present heating experiments are grouped in three categories that (1) zeolite phases, (2) B-bearing minerals and (3) silicate minerals. Zeolite group minerals are mostly predominant. Zeolite phases include analcite-pollucite, mordenite-Sr mordenite, heulandite-Sr heulandite, gumelinite-Sr gumelinite, and wairakite-Sr wairkite. It is characterized that all zeolite mmeral species establish solid solution between Na or Ca and Sror Cs. 15 kinds of starting borosilicate glasses are categorized into 6 types, that is, NSrCs, CNSrCs, CNSr, CNCs, NSr, and NCs. N and C indicats Na_2O and CaO,respectively. It is recognized considerably that zeolite mineral species dipends on strongly the chemical composition of starting borosilicate glasses. At the heating temperature of 200ﾟC,NSrCs and NSr glasses produced anaicite-pollucite predominantly, and heulandite-Sr heulandite. CNSrCs, CNSr and CNCs glasses are crystallized to analcite-pollucite, and mordenite-Sr mordenite. NCs glass produced only analcite-pollucite. On the other hand, in the heating experimet at 250ﾟC,CNSrCs glass produced analcite-pollucite, and wairakite-Sr wairakite.
The chemical compositions of analcite-pollucite series mineral species in paragenetic relation with heulandite-Sr heulandite arecaracterized as follows. (1) Cs concentration in analcite series mineralspecies increases with the increace of heating time. (2) Cs contents in analcite series mineral species vary widely with the increace of he ating time. (3) The Na/Cs atomic ratios in analcite series mineralspecies vary from 0.9 to 1.5 with heating time. It results from the chemical caracteristics that Sr substitutes Na or Cs in analcite-pollucite mineral species at longer heating time. On the other hand, analcite series mineral species in coexistence with mordenite show relatively linear relationship between Na and Cs contents. It is considered that Sr prefers crystallographically mordenite series mineral species. Less
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