Abstract
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.
Highlights
Radioactive strontium isotopes in the atomic mass number range of 89 to 92 are generated with high cumulative fission yield (5–6%) during thermal neutron fission in a nuclear reactor[1,2]
Compared to volatile radioisotopes of caesium released into the atmosphere (137Cs, ̃15 PBq), non-volatile 90Sr ( ̃0.14 PBq) emission has been estimated two orders lower. 90Sr contamination is expected in environmental samples due to Fukushima accident[9]
̃80% of the 90Sr emission was deposited over the Pacific Ocean, significantly decreasing the terrestrial pollution[10], whereas no 90Sr concentration that would be of radiological concern in soil samples have been reported[11,12] outside the damaged Fukushima Daiichi Nuclear Power Plant (FDNPP) by the authorities
Summary
Radioactive strontium isotopes in the atomic mass number range of 89 to 92 are generated with high cumulative fission yield (5–6%) during thermal neutron fission in a nuclear reactor[1,2]. One year after emission by a nuclear event, 89Sr cannot be determined in environmental samples owing to its short half-life, the identification of the origin of 90Sr could be a challenging task In our case, it consists of discrimination between Fukushima-borne and global 90Sr after decay of 89Sr. Figure 1. It was reported that 89Sr and 90Sr has been detected in 45% and 80% of samples, respectively, with average background level of 90Sr about 3 Bq kg−1 (n = 100)[12,13] Gamma emitter radionuclides such as iodine, tellurium and caesium isotopes[14,15,16,17,18,19,20] have been published and studied much more extensively after the Fukushima accident than the pure beta emitter radiostrontium isotopes[21,22,23,24]. The energy distribution of the emitted electrons in the beta decay is continuous, element specific separation from the interfering beta emitters is necessary for qualitative radioisotopes identification and subsequent measurement, making 90Sr determination complicated, time-consuming procedure with corrosive and aggressive chemicals during sample preparation[1,26]
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