Behavior of uranium series nuclides in sediments from 'H-rotou' Kanamaru district, Yamagata Prefecture

In order to investigate the geochemical behavior of uranium in the sediment, the U-series nuclides in the sediment samples taken from 'H-rotou' at Kanamaru district, Yamagata Prefecture, were determined. There were two distinct U anomalous layers in the Kamagui Formation (Tertiary arkosic sandstones and conglomerates); one is at 3.15 m below the top, and the other is at 6.3 m. The upper one is rich in Th-230 over U-234. The lower layer contains the highest U though Ra-226 is depleted. The contents of U have not apparent correlation with those of other elements, which suggests that U was concentrated not as primary U-containing minerals but as secondarily formed materials. Typical U minerals were not observed. As almost half of U is in easily soluble form, uranium is assumed to exist as very fine minerals or easily soluble material on adsorbent. The fact that Th-230/U-234 activity ratios of the samples in upper layer are over 2 suggests that U has suffered from strong leaching within 300 thousand years. On the other hand, the U-234/U-238 and Th-230/U-234 activity ratios in the lower layer are near unity. Ra-226 in both upper and lower layers is not in radioactive equilibrium with Th-230. These facts suggest that the U in the lower layer has retained within 1 million years under the environment in which Ra-226 has moved. (author)