The Natureal analogue study in uranium deposit, 1; A Study on radioactive disequilibrium of uranium-series nuclides

Nohara, Tsuyoshi   ; Seo, Toshihiro ; Ochiai, Yoji

The Tsukiyosi uranium deposit, which is called Tertiary sandstone-type, can be regarded as a potentially useful analogue for geological isolation of radioactive wastes in Japan. In order to understand the geochemical behaviour of U-series nuclides (U-238, U-234, Th-230, Ra-226) under reducing condition, a study on radioactive disequilibrium of rocks was carried out. A total of 185 rock specimens were collected from 6 boreholes and gallery in the uranium mineralized zone, which is situated in paleo-river system, in the direction of east-west. In the system of groundwater flow, the mineralized zone was divided into four: the upper, the middle, the lower, and the lowermost reaches from the west to the east. The initial mineralization were occurred at the period of 10 Ma, and mostly focused on the middle reaches. The following results are obtained by the chemical analysis and alpha and gamma spectorometry. Activity ratios of U-234/U-238 and Th-230/U-234 (for 185 specimens) are mostly within the range of 1.2 and 0.8. In the middle reaches to lower reaches of groudwater flow, activity ratios of Th-230/U-234 are occasionally less than unity. In the middle reaches, the ratios of Th-230/U-234 are less than unity at particularly highly mineralized parts, while the ratios are nearly equal to unity at poorly mineralized parts. Activity ratios of Ra-226/Th-230 (for 185 specimens) were a wide range between 0.6 and 5.1. Combination of the result above-mentioned with chemical properties and decay constants of the nuclides leads to the following conclusions. (1)Uranium and thorium have not migrated significantly during the last one million years. The magnitude of the uranium migration is probably within the range of several ten centimeter. (2)Uranium at the middle to lower reaches has been concentrated during the past several hundred thousand years. It is noteworthy that uranium of severely mineralized parts at the middle reaches have highly redistributed. (3)Radium has been probabl