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 Pu-239 and Pu-240 by ICP-MS. The average Pu-240/Pu-239 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 Pu-240/Pu-239 ratios between the individual samples analyzed, although the Pu239+240 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 Pu239+240 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 Pu-240/Pu-239 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 Pu-240/Pu-239 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 Cs-137, Co-60, and Sb-125 were also discussed.