Measurement of long-lived radionuclides in surface soil around F1NPP accident site by Accelerator Mass Spectrometry

In March 2011, vast amounts of radionuclides were released into the environment due to the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident. However, very little work has been done concerning accident-derived long-lived nuclides such as I-129 (T-1/2 = 1.57 x 10(7) year) and Cl-36 (T-1/2 = 3.01 x 10(5) year). I-129 and I-131 are both produced by U-235 fission in nuclear reactors. Being isotopes of iodine, these nuclides are expected to behave similarly in the environment. This makes I-129 useful for retrospective reconstruction of I-131 distribution during the initial stages of the accident. On the other hand, Cl-36 is generated during reactor operation via neutron capture reaction of Cl-35, an impurity in the coolant or reactor component. Resulting Cl-36/Cl ratio within the reactor is thus much higher compared to that in environment. Similar to I-129, Cl-36 is expected to have leaked out during the accident and it is important to evaluate its effects. In this study, I-129 concentrations were determined in several surface soil samples collected around F1NPP. Average I-129/I-131 ratio was estimated to be 26.1 +/- 5.8 as of March 11, 2011, consistent with calculations using ORIGEN2 code and other published data. Cl-36/Cl ratios in some of the soil samples were likewise measured and ranged from 1.1 x 10(-12) to 2.6 x 10(-11). These are higher compared to ratios measured around F1NPP before the accident. A positive correlation between Cl-36 and I-129 concentration was observed. (C) 2015 Elsevier B.V. All rights reserved.