Background distributions of ~(239+240)Pu and ~(137)Cs of upland soil in Rokkasho, Japan

摘要

The background distributions of Pu and ~(137)Cs in soil were investigated in Rokkasho, Aomori Prefecture, where the first commercial nuclear fuel reprocessing plant in Japan is now being constructed. Soil core samples to 1m depth were collected at 13 upland fields in Rokkasho and control sites in Hachinohe and Hirosaki. Since fields under yam (Dioscorea babatus) cultivation, which is a common crop in Rokkasho, were dug to approximately 1 m depth at harvesting, depth profiles of fallout radionuclides are heavily disturbed for most fields in Rokkasho. The mean inventories of ~(239+240)Pu and ~(137)Cs in three fields with no yam cultivation history were 116 Bq m~(-2) and 3.4 kBq m~(-2), respectively and similar to values in Hachinohe. However, the inventories were approximately a half of those in Hirosaki. The mean ratio of ~(240)Pu/~(239)Pu for all studied fields was 0.18 +- 0.04, and similar to that of global fallout. The Pu concentrations correlate very well with ~(137)Cs (r = 0.97) in spite of heavy disturbance of soil, and the ratio of ~(239+240)Pu/~(137)Cs was 0.037 +- 0.007, which is a typical value for global fallout.