Distributions of artificial radionuclides, ^239+240Pu and ¹³⁷Cs in un-cultivated, agricultural upland and paddy fields in Rokkasho, Japan

Abstract

Japan's first commercial nuclear fuel reprocessing plant is now being constructed in Rokkasho, Japan. Since small amount of radionuclides will be released from the plant to the environment in normal operation, it is important to investigate the background levels of the radionuclides before full operation in order to evaluate them released in future. Distributions of ^239+240Pu and ¹³⁷Cs in soil samples collected from un-cultivated, agricultural upland and paddy fields in Rokkasho were examined. Inventories of ^239+240Pu in those fields were 111±52 Bq m^-2, 81±36 Bq m^-2 and 81±22 Bq m^-2, respectively, and those of ¹³⁷Cs were 3.2±1.5 kBq m^-2, 2.4±0.9 kBq m^-2 and 2.3±0.8 kBq m^-2, respectively. Variation ranges of their inventories in the agricultural fields were larger than the range in the un-cultivated field, because surface soil of the agricultural fields was removed or added with soil from other places in many cases. Concentrations of both nuclides in 25cm below were under detection limits in the un-cultivated fields. On the other hands, the nuclides were detectable under 1m depth for some cases in the agricultural upland and paddy fields. Atomic ratios of ²⁴⁰Pu/²³⁹Pu were 0.179±0.038, 0.164±0.023 and 0.175±0.014 in the upland, the paddy field and the un-cultivated field, respectively. These values agreed with the ratio of global fallout Pu within the range of one standard deviation.