過リン酸石灰および石膏ボードにおけるウランとトリウムの非平衡の検出

Abstract

By chance, large radioactive disequilibrium was experienced between U-234 and U-238 in a commercial phosphate fertilizer. The sample fertilizer was leached in a column successively by using water, 0.1 M HCl, and 8 M HCl, and then the residue was leached and digested with a mixture of hydrofluoric acid, nitric acid and perchloric acid in a pressurized PTFE container. The ratio of U-234/U-238 radioactivities amounted to 8.0 +/- 0.8 in the final fraction of successive leaching/digesting steps. Later, introduced was the method of batch leaching with similar leachates in order to reduce processing time and to get easily additional information upon the radioactivity of Th-230. But little disequilibrium in uranium isotopes was observed till now in the fertilizer in the same package. Possible explanations for this result are: 1) heterogeneity of the sample fertilizer, or 2) different leaching behavior of the sample in between the column and batch method, the latter of which takes shorter processing and aging time than in the former method. Recently, observed was U-234/U-238 disequilibrium in gypsum wallboard samples, whose main component, calcium sulfate, was sometimes supplied commercially as insoluble residues left in the production process of the phosphate fertilizer. The findings indicate that the gypsum wallboard and the fertilizer of interest might include, as an insoluble content, uranium-poor silicate grains, into which extra U-234 atoms might be implanted as Th-234 from outside by alpha-recoil on the decay of the parent U-238, as proposed by P. K. Kuroda long years ago.