The radioactive fission product 90Sr has a long biological half-life (˜18 y) in the human body. Due to its chemical similarity to calcium it accumulates in bones and irradiates the bone marrow, causing its high radio-toxicity. Assessing 90Sr is therefore extremely important in case of a nuclear disaster. In this work 16 soil samples were collected from the exclusion zone (<30 km) of the earthquake-damaged Fukushima Daiichi nuclear power plant, to measure 90Sr activity concentration using liquid scintillation counting. 137Cs activity concentration was also measured with gamma-spectroscopy in order to investigate correlation with 90Sr. The 90Sr activity concentrations ranged from 3.0 ± 0.3 to 23.3 ± 1.5 Bq kg−1 while the 137Cs from 0.7 ± 0.1 to 110.8 ± 0.3 kBq kg−1. The fact that radioactive contamination originated from the Fukushima nuclear accident was obvious due to the presence of 134Cs. However, 90Sr contamination was not confirmed in all samples although detectable amounts of 90Sr can be expected in Japanese soils, as a background, stemming from global fallout due to the atmospheric nuclear weapon tests. Correlation analysis between 90Sr and 137Cs activity concentrations provides a potentially powerful tool to discriminate background 90Sr level from its Fukushima contribution.

In this work, a new 90 Sr analysis method was developed using the Isotopx Ltd., Phoenix X62 thermal ionization mass spectrometer (TIMS). Excellent ion beam sensitivity was demonstrated with the detection of 1 mBq (0.2 fg) 90 Sr on a Daly ion-counting system. The abundance sensitivity for the 90 Sr/ 88 Sr ratio was 2.1 × 10 −10 , and this could ensure measurement of 100 Bq•kg −1 (19 fg•g −1 ) 90 Sr in an environmental sample with 100 μg•g −1 stable strontium concentration. For analytical method validation, 90 Sr was determined in two certified reference materials, for example, wild berry (IRMM-426) and freshwater lake sediment (NIST-4354), for the first time in the history of TIMS. This mass spectrometry method is faster than conventional radiometric techniques; however, interference from 90 Zr and peak tailing on the higher mass side from 88 Sr must be considered for a reliable 90 Sr determination.

No abstract

90 Sr activity concentration was measured in four soil samples (0-10 cm) collected from the exclusion zone around the Fukushima Daiichi Nuclear Power Plant.The open chemical digestion method was used for sample decomposition with a mixture of acids. The strontium separation was achieved with strontium selective resin (Srresin). The activity of 90 Sr was determined with liquid scintillation counter. Owing to the atmospheric nuclear weapon tests, the soil in Japan is contaminated with 90 Sr. Significant Fukushima contamination was not noticed by the results of this study due to background level. The detected 90 Sr activities were 8.9 ± 0.8, 20 ± 1.3, \6.8, and 23.4 ± 1.5 Bq kg -1 , respectively.