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 ¹²⁹I (T_1/2 = 1.57 × 10⁷ year) and ³⁶Cl (T_1/2 = 3.01 × 10⁵ year). ¹²⁹I and ¹³¹I are both produced by ²³⁵U fission in nuclear reactors. Being isotopes of iodine, these nuclides are expected to behave similarly in the environment. This makes ¹²⁹I useful for retrospective reconstruction of ¹³¹I distribution during the initial stages of the accident. On the other hand, ³⁶Cl is generated during reactor operation via neutron capture reaction of ³⁵Cl, an impurity in the coolant or reactor component. Resulting ³⁶Cl/Cl ratio within the reactor is thus much higher compared to that in environment. Similar to ¹²⁹I, ³⁶Cl is expected to have leaked out during the accident and it is important to evaluate its effects. In this study, ¹²⁹I concentrations were determined in several surface soil samples collected around F1NPP. Average ¹²⁹I/¹³¹I ratio was estimated to be 26.1 ± 5.8 as of March 11, 2011, consistent with calculations using ORIGEN2 code and other published data. ³⁶Cl/Cl ratios in some of the soil samples were likewise measured and ranged from 1.1 × 10⁻¹² to 2.6 × 10⁻¹¹. These are higher compared to ratios measured around F1NPP before the accident. A positive correlation between ³⁶Cl and ¹²⁹I concentration was observed.