55. Determination of Uranium Isotopes an Soil and Plant Samples Collected around the Uranium Conversion Building on the JCO Grounds Following the Criticality Accident

Satoshi Yoshida, Yasuyukt Muramatsu, Keiko Tagami, Shigeo Uchida, Tadaaki Ban-mat, Hidenori Yonehara and Sarat Kumar Sahoo

Keywords: JCO criticality accident, uranium, ²³⁵U ²³⁵U, isotope ratio, soil, plant, ICP-MS

Appreciable amounts of gaseous fission products and activation products were observed in some soil and vegetation samples collected near the JCO campus immediately after the criticality accident on September 30, 1999. However, there were almost no data on the levels of U in the environment around the uranium conversion building. Since enriched U (18.8% of ²³⁵U by mass) was used in this facility the ²³⁵U/²³⁸U ratio might be useful in assessing the possible contamination of U. In the framework of studies on environmental effects of the criticality accident, we collected soil and plant samples on the JCO grounds. This study determines the U isotopes (²³⁵U and ²³⁸U) in these soil and plant samples.

Surface (O-5 cm) soil samples were collected around the uranium conversion building on October 7, 1999. Sampling points were decided considering the distance and direction from the center of the building. After removing stones, the soils were ovendried at 80C until constant weight, and ground into powder. Plant samples belonging to four spectes were collected around the building on October 26 1999. In the laboratory, leaves were oven-dried at 80 C until constant weight and pulverized with a blender. Samples (0.1 g for soils and 0.2 g for plants) were digested with HNO₃ , HF and HCIO₄. A microwave digester (CEM, MDS-2000) was used for heating the samples. After digestion, the samples were evaporated to dryness on a hot plate. Then, the residues were dissolved in l-2% HNO₃ to yield the sample solutions. Decomposition and analysis were duplicated for each sample. ICP-MS (Yokogawa PMS-2000) was used for the analysis of ²³⁵U and ²³⁸U. ²³²Th was also measured for soil samples. Standard reference materials such as GSJ-JB-la (basalt) and NIST-SRIVI-1573a (tomato leaves) were used to validate the analytical procedure. Concentrations of U in soils were comparable to the U values for common Japanese soils. However the U/Th ratios for these soil samples were markedly higher than the control value, suggesting the possible contamination of U. The ²³⁵U/²³⁸U atom ratios in soil and plant samples were notably hngher than the natural ratio, 0.00725. The highest values, 0.0162 for soil and 0.0193 for plant were found in samples collected near the conversion building as shown in Fig. 20. However, a relatively high ratio was observed at the site more than 50 m from the building, indicating that enriched U may have been released not only from the conversion building, but also from other unknown sources. The ²³⁵U/²³⁸U atom ratios were in general higher in plants than those in soils collected at the same potnts.

Fig.20. 235U/238U atom ratio in soils and plants with function of distance from the JCO uranium conversion building.

Publication:
Yoshida, S., Muramatsu, Y., Tagami, K., Uchida, S. Ban-nai, T., Yonehara, H. and Sahoo, S.: J Environ. Radioactivity, 50, 161-172, 2000.