November 14, 2018
Plutonium and radiocaesium are hazardous contaminants released by the Fukushima Daiichi nuclear power plant (FDNPP) disaster and their distribution in the environment requires careful characterisation using isotopic information. Comprehensive spatial survey of 134Cs and 137Cs has been conducted on a regular basis since the accident, but the dataset for 135Cs/137Cs atom ratios and trace isotopic analysis of Pu remains limited because of analytical challenges. We have developed a combined chemical procedure to separate Pu and Cs for isotopic analysis of environmental samples from contaminated catchments. Ultra-trace analyses reveal a FDNPP Pu signature in environmental samples, some from further afield than previously reported. For two samples, we attribute the dominant source of Pu to Reactor Unit 3. We review the mechanisms responsible for an emergent spatial pattern in 134,135Cs/137Cs in areas northwest (high 134Cs/137Cs, low 135Cs/137Cs) and southwest (low 134Cs/137Cs, high 135Cs/137Cs) of FDNPP. Several samples exhibit consistent 134,135Cs/137Cs values that are significantly different from those deposited on plant specimens collected in previous works. A complex spatial pattern of Pu and Cs isotopic signature is apparent. To confidently attribute the sources of mixed fallout material, future studies must focus on analysis of individual FDNPP-derived particles.
Isoscapes of 134, 135, 137Cs and 239, 240Pu for part of the Fukushima prefecture surrounding FDNPP. The green marker is used to highlight an anomalous 240Pu/239Pu atom ratio of 0.64. R1, R2 and R3 correspond to ORIGEN estimated isotope ratio values for Reactor Units 1, 2 and 3, respectively27. SW indicates the mean value for the Cs isotope ratios measured to the southwest of FDNPP by Snow et al.19. 240Pu/239Pu atom ratio for Northern Hemisphere integrated global fallout is denoted by NHF28.