Since the Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011, intensive studies of the distribution of released fission products, in particular (134)Cs and (137)Cs, in the environment have been conducted. However, the release sources, that is, the damaged reactors or the spent fuel pools, have not been identified, which resulted in great variation in the estimated amounts of (137)Cs released. Here, we investigated heavily contaminated environmental samples (litter, lichen, and soil) collected from Fukushima forests for the long-lived (135)Cs (half-life of 2 × 10(6) years), which is usually difficult to measure using decay-counting techniques. Using a newly developed triple-quadrupole inductively coupled plasma tandem mass spectrometry method, we analyzed the (135)Cs/(137)Cs isotopic ratio of the FDNPP-released radiocesium in environmental samples. We demonstrated that radiocesium was mainly released from the Unit 2 reactor. Considering the fact that the widely used tracer for the released Fukushima accident-sourced radiocesium in the environment, the (134)Cs/(137)Cs activity ratio, will become unavailable in the near future because of the short half-life of (134)Cs (2.06 years), the (135)Cs/(137)Cs isotopic ratio can be considered as a new tracer for source identification and long-term estimation of the mobility of released radiocesium in the environment.

Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011, the activity ratio of (134)Cs/(137)Cs has been widely used as a tracer for contamination source identification. However, because of the short half-life of (134)Cs (2.06 y), this tracer will become unavailable in the near future. This article presents an analytical method for the determination of the long-lived (135)Cs (t(2/1) = 2 × 10(6) y) and the atomic ratio of (135)Cs/(137)Cs, as a promising geochemical tracer, in environmental samples. The analytical method involves ammonium molybdophosphate (AMP)-selective adsorption of Cs and subsequent two-stage ion-exchange chromatographic separation, followed by detection of isolated radiocesium isotopes via triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS). The AMP-selective adsorption of Cs and the chromatographic separation system showed high decontamination factors (10(4)-10(5)) for interfering elements, such as Ba, Mo, Sb, and Sn. Using ICP-MS/MS, only selected ions enter the collision/reaction cell to react with N2O, reducing the isobaric interferences ((135)Ba(+) and (137)Ba(+)) and polyatomic interferences ((95) Mo(40)Ar(+), (97) Mo(40)Ar(+), (119)Sn(16)O(+), and (121)Sb(16)O(+)) produced by sample matrix ions. The high abundance sensitivity (10(-9) for the (135)Cs/(133)Cs ratio) provided by ICP-MS/MS allowed reliable analysis of (135)Cs and (137)Cs isotopes with the lowest detection limits ever reported by mass counting methods (0.01 pg mL(-1) and 0.006 pg mL(-1), respectively). The developed analytical method was successfully applied to the determination of (135)Cs and (137)Cs isotopes in environmental samples (soil, litter, and lichen) collected after the FDNPP accident for contamination source identification.

An accurate and precise analytical method is highly needed for the determination of Pu isotopes in marine sediments for the long-term marine environment monitoring that is being done since the Fukushima Dai-ichi Nuclear Power Plant accident. The elimination of uranium from the sediment samples needs to be carefully checked. We established an analytical method based on anion-exchange chromatography and SF-ICP-MS in this work. A uranium decontamination factor of 2 × 10(6) was achieved, and the U concentrations in the final sample solutions were typically below 4 pg mL(-1), thus no extra correction of (238)U interferences from the Pu spectra was needed. The method was suitable for the analysis of (241)Pu in marine sediments using large sample amounts (>10 g). We validated the method by measuring marine sediment reference materials and our results agreed well with the certified and the literature values. Surface sediments and one sediment core sample collected after the nuclear accident were analyzed. The characterization of (241)Pu/(239)Pu atom ratios in the surface sediments and the vertical distribution of Pu isotopes showed that there was no detectable Pu contamination from the nuclear accident in the marine sediments collected 30 km off the plant site.

No abstract

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to the release of large amounts of radionuclides into the atmosphere as well as direct discharges into the sea. In contrast to the intensive studies on the distribution of the released high volatility fission products, such as ¹³¹I, ¹³⁴Cs and ¹³⁷Cs, similar studies of the actinides, especially the Pu isotopes, are limited. To obtain the vertical distribution of Pu isotopes in marine sediments and to better assess the possible contamination of Pu from the FDNPP accident in the marine environment, we determined the activities of ^239+240Pu and ²⁴¹Pu as well as the atom ratios of ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu in sediment core samples collected in the western North Pacific off Fukushima from July 2011 to July 2012. We also measured surface sediment samples collected from seven Japanese estuaries before the FNDPP accident to establish the comprehensive background baseline data. The observed results of both the Pu activities and the Pu atom ratios for the sediments in the western North Pacific were comparable to the baseline data, suggesting that the FDNPP accident did not cause detectable Pu contamination to the studied regions prior to the sampling time. The Pu isotopes in the western North Pacific 30 km off the Fukushima coast originated from global fallout and Pacific Proving Ground close-in fallout