Radionuclide pollution inside the Fukushima Daiichi exclusion zone, part 2: Forensic search for the "Forgotten" contaminants Uranium-236 and plutonium

In the course of the Fukushima Daiichi nuclear accident (March 2011), large quantities of radionuclides, mainly of the volatile elements such as cesium and iodine were released to the environment. In small amounts non-volatile elements such as plutonium and uranium have also been released. For this study vegetation, litter and soil drill core samples were taken in 2013 and 2015 in the contaminated area in Fukushima prefecture. In 2015 the samples were taken as close to the sampling locations of the 2013 campaign as possible. To investigate the origin of plutonium and uranium the isotopic ratios ²⁴⁰Pu/²³⁹Pu and ²³⁶U/²³⁸U were measured using accelerator mass spectrometry. While the determined ²⁴⁰Pu/²³⁹Pu ratios indicate global fallout as the plutonium source for most samples, reactor plutonium of the reactors of Fukushima Daiichi is identified unambiguously in some vegetation and litter samples. This assumption is corroborated by the detection of ²³⁶U. None of the samples contained higher plutonium activity concentrations than in the rest of Japan, caused by global fallout. This holds even for the samples where the reactors were identified as the source from the isotope ratios. This and the strong localization, even for neighboring sampling sites, indicate that the overall small quantities of plutonium were most likely released in the form of particulate matter. - Highlights: • Investigations of actinides in environmental samples from Fukushima Prefecture. • Plutonium was released most likely in a particulate form. • U-236 signal was found in environmental samples found.