Evaluation of ^239+240Pu, ¹³⁷Cs and natural ²¹⁰Pb fallout in agricultural upland fields in Rokkasho, Japan

Abstract

The background distributions of ^239+240Pu and ¹³⁷Cs fallout in agricultural soil were investigated in Rokkasho where Japan's first commercial nuclear fuel reprocessing plant is now being constructed. The mean inventories of ^239+240Pu and ¹³⁷Cs in three fields with non-yam-cultivation history were 116 Bq·m⁻² and 3.4 kBq·m⁻², respectively. The mean atomic ratio of ²⁴⁰Pu/²³⁹Pu for all studied fields was 0.18±0.04, and was similar to that of global fallout. The ^239+240Pu concentrations correlated very well with ¹³⁷Cs (r = 0.97) in spite of heavy disturbance of the soil, and the activity ratio of ^239+240Pu/¹³⁷Cs was 0.037±0.007, which is a typical value for global fallout. These results showed that the nuclides had similar behavior in agricultural upland fields in Rokkasho. Since ²¹⁰Pb is steadily deposited from the atmosphere to the land, this nuclide could be an index for the degree of disturbance of a field and of soil lost from the field. The ratio of excess ²¹⁰Pb inventory in the soil to the equivalent inventory of atmospheric ²¹⁰Pb deposition was 96%, and indicated that soil was not lost from the cultivated fields.