Vertical migration rates of fallout ^239+240Pu and ¹³⁷Cs in soils of forests in Rokkasho, Japan

Abstract

The active test using actual spent nuclear fuels was carried out in the Japan's first commercial nuclear fuel reprocessing plant in Rokkasho, Japan from March 2006. Since vertical migration rates of ^239+240Pu or ¹³⁷Cs in soil are important factors for assessing behavior of those nuclides in the environment around the plant, we investigated their vertical profiles in soil in Rokkasho, and estimated the migration rates. The vertical distribution of ^239+240Pu and ¹³⁷Cs were obtained for the forest soil in several sampling sites. Their inventories were larger than the values obtained in un-cultivated fields in Rokkasho by our previously work. Mean ²⁴⁰Pu/²³⁹Pu atom ratio of 0.178±0.014 for all samples indicated that the Pu in the samples was derived from global fallout. The activity ratio of ^239+240Pu/¹³⁷Cs was 0.030, which is also typical for weapon testing fallout. Vertical migration velocities of the nuclides in soil up to 15 cm deep were calculated from their historical fallout patterns and current vertical distributions. Their migration rates ranged from 0.1 to 0.6 cm y^-1 and within the range of reported values in European forest soils. There are negative correlation between the migration rate of Pu or ¹³⁷Cs and ignition losses of the soils, and positive one between the rates and mean diameter of soil particles. This work was supported by a grant from Aomori prefecture, Japan.