53. Determination of Uranium Isotopes by ICP-MS in Soil Core Samples Collected around the Reconversion Facilities

Satoshi Yoshida, Yasuyuki Muramatsu and Keiko Tagami

Keywords: JCO criticality accident, uranium, ²³⁵U, ²³⁸U, isotope ratio, soil, ICP-MS

Naturally occurring ²³⁵U and ²³⁸U are long-lived isotopes, and the ²³⁵U/²³⁸U abundance ratio is the same everywhere, 0.00725 in atom ratio, regardless of sampling location, rock type, and degree of weathering. Hence any deviation from the natural ratio attests to a soil affected by human activity, with the well-known exception of the Oklo reactor. After the 1999 criticality accident in Tokai-mura, we collected surface soil and plant samples around the uranium conversion building in JCO and observed the higher ²³⁵U/²³⁸U atom ratios than the natural ratio. However, we could not conclude whether the main source of the enriched U was the criticality accident or not. In the present study, U isotopes were determined with ICP-MS for seven soil core samples collected on the JCO grounds, in order to evaluate the isotope composition of excess U in soils.

In the framework of studies on environmental effects of the criticality accident, three soil core samples were collected at open areas beside the uranium conversion building (S9, S12A, S12B) and the other four soil cores were collected at deciduous forests (S6, JF1, JF2, JF3). Samples were cut every 1 to 5 cm depth using a spatula on site. After removing stones, the soil samples were oven-dried at 80 ^oC until of constant weight, and then ground into powder. The samples from organic layers of the forests were oven-dried and pulverized with a blender. Samples (usually 0.1 g) were digested with HNO₃, HF and HClO₄. Quadrupole-ICP-MS (Yokogawa PMS- 2000) or high resolution-ICP-MS (Finnigan MAT ELEMENT) was used for the analysis of ²³⁵U and ²³⁸U. Standard reference materials such as GSJ-JB-1a (basalt) were used to validate the analytical procedure.

The ²³⁵U/²³⁸U ratios were higher than the natural ratio in most samples. The highest ratio observed was 0.0262. Although vertical profiles of the ²³⁵U/²³⁸U ratio differed among the soil cores, the ratios tended to be high at the surface and decreased with depth. The U concentration also changed with depth. The percentages of ²³⁵U in the excess U, estimated from the positive correlation between U concentration and the ²³⁵U/²³⁸U ratio in soil samples, were less than 4 % by mass (mostly 1 - 3 %), and were much lower than the enrichment of the U used in the uranium conversion building at the time of the criticality accident (18.8 %) (see Fig. 18). These findings indicate that enriched U had been released before the criticality accident during the U processing at JCO in connection with the reconversion of light water reactor fuel. Since the range of the U concentrations found was comparable to the range of uncontaminated Japanese surface soils, the amount of U added to the soil was judged negligible from a radiation protection viewpoint.

Publications:
1) Yoshida, S., Muramatsu, Y., Tagami, K., Uchida, S., Ban-nai, T., Yonehara, H. and Sahoo, S.: J. Environ. Radioactivity, 50, 161-172, 2000.
2) Yoshida, S., Muramatsu, Y. and Tagami, K.: Environ. Sci. Technol. (in press).

Fig.18.

Relationships between U concentration and 235U/238U ratio in soil core samples. Curves: calculated mixing curves of the enriched U with certain enrichments (percentage of 235U by mass) and natural soil with 1.1 g/g (dry wt) of U.

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