Reconstruction of uranium and plutonium isotopic signatures in sediment accumulated in the Mano Dam reservoir, Japan, before and after the Fukushima nuclear accident

Hugo Jaegler,Fabien Pointurier,Silvia Diez-Fernández,Alkiviadis Gourgiotis,Hélène Isnard,Seiji Hayashi,Hideki Tsuji,Yuichi Onda,Amélie Hubert,J Patrick Laceby,Olivier Evrard

doi:10.1016/j.chemosphere.2019.03.064

Abstract

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with 137Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios (240Pu/239Pu, 241Pu/239Pu and 242Pu/239Pu) and for 137Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, 236U/238U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U.

Highlights

The 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident released significant volumes of radionuclides into the environment
As early as in April 2011 U ratios measured in aerosols collected at the same location returned to the global fallout level. These results suggested that U was only released in trace amounts during the FDNPP accident
In the sediment core collected in the Hayama Lake, the 240Pu/239Pu, 241Pu/239Pu and the 242Pu/239Pu isotope ratios measured in the bottom layer (31 cm, MD7) were within the range of values for the global fallout reported in the literature

Summary

Introduction

The 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident released significant volumes of radionuclides into the environment. Non-volatile Pu isotopes including 239Pu (Ti/2=24,110 y, alpha-decay), 240Pu (Ti/2=6,563 y, alpha-decay), 241Pu (Ti/2=14.35 y, beta-decay) and 242Pu (Ti/2=376,000 y, alpha-decay)(Evrard et al, 2014; Xu et al, 2016; Yamamoto et al, 2014; Zheng et al, 2012b) and U isotopes including 238U (Ti/2=4.469x109 y, alpha-decay), 235U (T1/2=7.038x108 y, alpha-decay), 234U (T1/2=245,500 y, alpha-decay) and 236U (T1/2=23.42x106 y, alphadecay)(Sakaguchi et al, 2014; Schneider et al, 2017; Shinonaga et al, 2014; Yang et al, 2016), were detected in environmental samples collected in the Fukushima Prefecture These radionuclides may originate from two main sources: the global fallout associated with the nuclear atmospheric weapon tests(Yamamoto et al, 2014) and the FDNPP accident. Implications regarding the fate and the geochemical behaviour of actinides in the catchments draining the main radioactive plume in the Fukushima Prefecture are discussed

Materials and methods

Findings

A Mano dam