Isotopic ratios of plutonium in radioactive particles released by the accident of Fukushima Daiichi Nuclear Power Plant

The atom ratios of Pu isotopes have unique values depending on the source of Pu release and are useful for identifying the origin of Pu. To date, various studies have been conducted on Pu analysis originating from the Fukushima Nuclear Power Plant (FDNPP) accident, which was the latest Pu release event. Some study focused on radioactive particles formed in a reactor during the accident to evaluate the Pu contamination originating from the FDNPP accident, assumed to be not influenced by serious Pu contamination from global fallout. However, a small number of samples of analyzed radioactive particles introduced a large uncertainty into the Pu atom ratio estimate. In this study, we analyzed more samples of radioactive particles to investigate the accurate Pu atom ratios from the FDNPP accident. The averaged Pu atom ratios of ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu for 7 particles were precisely determined for 0.369 ± 0.008 and 0.150 ± 0.007, respectively, by the sector field inductively coupled plasma mass spectrometry system (SF-ICP-MS) method. These values are close to the calculated value of the core inventory but are slightly different. In addition, the ratios were similar to reported values for environmental samples around the FDNPP, particularly for black substances and vegetation. This study determined accurate Pu atom ratios from the FDNPP, which provided Pu fingerprint values of FDNPP for future Pu source studies.