Isotopic ratio of 135Cs/137Cs as a new tracer of radiocesium released into the environment from the Fukushima nuclear accidentIsotopic ratio of 135Cs/137Cs as a new tracer of radiocesium released into the environment from the Fukushima nuclear accident
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident led to severe releases of radionuclides into the environment. Identifying the release source, the nuclear reactors or the spent fuel pools (SFPs), is an important issue for the accurate estimation of the released amount of radionuclides. Currently the isotopic ratio of 134Cs/137Cs has been widely used to trace the FDNPP released radiocesium. However, the short half-life (2.06 y) of 134Cs will make this tracer unavailable in several years. Here, by applying a newly developed ICP-MS/MS technique , we successfully measured 135Cs/137Cs isotopic ratios in heavily contaminated environmental samples (litter, lichen and soil) (137Cs activity ranging from 0.15-4.65 MBq/kg) collected from April 2011 to May 2013 in the 20-50 km zone around the FDNPP, and found that the release, if any, of 137Cs from the Unit 4 SFP due to the explosion and subsequent fire was negligible. Combined with the information given by the 240Pu/239Pu isotopic ratio fingerprint we could conclude, for the first time, that the Unit 2 reactor was the main release source of FPs in the FDNPP accident. The obtained 135Cs/137Cs isotopic ratio is proposed as a new tracer for applications in long-term estimation of environmental behavior of released radionuclides.