Isotopic compositions of (236)U, (239)Pu, and (240)Pu in soil contaminated by the Fukushima Daiichi Nuclear Power Plant accident

Six years after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for (236)U and (236)U/(238)U have remained limited to a few heavily contaminated samples. In the present study, activities of (236)U, (239)Pu, and (240)Pu, along with other U isotopes in 46 soil samples both heavily and lightly contaminated by this accident were measured by inductively coupled plasma–mass spectrometry (ICP-MS) and triple-quadrupole ICP-MS. The (236)U activities and (236)U/(238)U atom ratios in these soil samples were in the range of (0.469–24.4) × 10(−5) Bq kg(−1) and ((0.099–1.35) × 10(−7)), respectively. Higher (240)Pu/(239)Pu atom ratios (0.245–0.312) and (238)Pu/(239+240)Pu activity ratios (0.859–1.62) indicated Pu contamination originated from this accident and global fallout in some samples. For those soil samples along with black substances collected along roads in Fukushima Prefecture, high linear correlations were presented between (236)U activities and (239+240)Pu activities (Pearson’s r = 0.755, p < 0.01), and between (236)U activities and (238)Pu activities (Pearson’s r = 0.844, p < 0.01). The analysis of these soil samples confirmed the release of (236)U, although in trace amounts, during the FDNPP accident.