Concentrations of ²³⁹Pu and ²⁴⁰Pu and Their Isotopic Ratios Determined by ICP-MS in Soils Collected from the Chernobyl 30-km Zone

Soil samples collected from three forest sites within the 30-km zone around the Chernobyl reactor were analyzed for 239Pu and 240Pu by ICP-MS. The average 240Pu/239Pu atom ratio in contaminated surface soil samples, values of which are scarce in the literature, was 0.408. There were almost no differences in the 240Pu/239Pu ratios between the individual samples analyzed, although the 239+240Pu levels varied very widely (i.e. from 6.3 to 1430 Bq kg-1 dry weight) depending on the distance from the reactor and on the soil layers investigated. This result corresponded to area-related activities for 239+240Pu between 1.1 kBq m-2 and 13.3 kBq m-2. It was estimated that about half of the Pu migrated from the organic layers to the underlying mineral layers. The 240Pu/239Pu ratio observed in the Chernobyl area was much higher than that attributed to weapons fallout (ca. 0.18). The high ratio was related to the high burn-up grade of the reactor fuel. The 240Pu/239Pu ratio observed might be used as a "fingerprint" in identifying the distribution of Chernobyl-derived Pu in the environment and in distinguishing it from other sources, e.g. global fallout. Relationships between the concentrations of Pu and those of 137Cs, 60Co, and 125Sb were also discussed.