Incorporation and Accumulation of Strontium-90 in the Hard Tissue of Animals and Their Relationship with Strontium-90 Pollution in the Environment

Substantial amounts of radionuclides including strontium-90 ( 90 Sr) were released by the Fukushima Daiichi Nuclear Power Plant (FNPP) accident. In the present study, we describe and discuss the presence of 90 Sr in the ex-evacuation zone of the FNPP accident and its relationship with 90 Sr activity concentration in the hard tissue of animals. We found that the activity concentration of 90 Sr in the hard tissue exhibited a positive correlation with 90 Sr pollution in their corresponding terrestrial and marine environments. Hard tissues, such as the teeth, bones, and otoliths, of animals and fishes could serve as useful tools in assessing 90 Sr pollution in the environment during the period of the formation of those tissues.


Introduction
Strontium-90 ( 90 Sr) has been released to the environment from various nuclear disasters in the past, such as nuclear weapon tests [1], the Chernobyl Nuclear Power Plant accident [2,3], release of contaminated radionuclides from nuclear fuel processing plants at Sellafield and Mayak facilities [4,5], and sea disposal operations by the former Soviet Union [6]. Strontium-90 was released also by the Fukushima Daiichi Nuclear Power Plant (FNPP) accident in 2011 and remained in the terrestrial and marine environments due to its long physical half-life of 28.8 years. Its long biological half-life and bone-seeking property may have adverse effects on the bone marrow, and special attention should be paid to the behavior of 90 Sr. Strontium-90 easily transfers to terrestrial and marine biota because it is rather soluble [7,8]. However, the relationship between 90 Sr in the environment and in animal body is still unclear. In this chapter, we describe and discuss the presence of 90 Sr in the environment and its migration into the teeth and bones of cattle abandoned in the ex-evacuation zone of the FNPP accident. We also discuss 90 Sr activity concentration in the otolith of marine fishes around FNPP. Through the discussion, we suggest that 90 Sr activity concentration in the hard tissue of animal body might reflect the extent of 90 Sr pollution in the environment.

90 Sr in the Environment
The Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), reported that 0.1-6 kBq/m 2 of 90 Sr and 0.3-17 kBq/m 2 of 89 Sr were detected in soil within a 20-km radius from FNPP (the ex-evacuation zone) [9]. Strontium-90 is known to have been present in the Japanese environment before the FNPP accident, which probably stemmed from the atmospheric nuclear weapon tests conducted during the 1950s-1980s. However, the existence of 89 Sr in soil of the ex-evacuation zone indicates that the 90 Sr pollution was caused by the FNPP accident, because 89 Sr has a relatively short half-life of 50.5 days and more than 50 years have passed since the atmospheric nuclear weapon tests were discontinued.
It was reported that the amount of 90 Sr released by the FNPP accident into the atmosphere is smaller than those of volatile radionuclides (noble gases, iodine, tellurium, and cesium). The amount of released 90 Sr is estimated to be three orders of magnitude smaller than that of cesium-137 ( 137 Cs) [3]. The nuclear fuel of FNPP did not exceed the temperature of 2,700 K that is necessary for the volatilization of refractory elements, such as 90 Sr and actinides [10]. Schwantes et al. estimated that most of radioactive Sr retained inside the reactors [11]. These reports suggest that the area polluted by 90 Sr might be smaller than that by 137 Cs attributed to the FNPP accident.
It was reported that 90 Sr activity concentration in soil and vegetation samples is one to four orders of magnitude lower than that of 137 Cs [12]. Sahoo et al. showed that 90 Sr activity concentration in soil from the ex-evacuation zone of the FNPP accident ranged from 3 to 23 Bq/kg, whereas 137 Cs activity concentration ranged from 0.7 to 110 kBq/kg [13], which is consistent with the previous reports [3,10]. The ratio of 90 Sr/ 137 Cs in soil widely varies depending on the area examined.
The FNPP accident caused 90 Sr pollution not only in the terrestrial environment but also in the marine environment. Since the most of 90 Sr was presumed to remain in the reactor of FNPP, atmospheric release of 90 Sr was speculated to be minor. The total amount of 90 Sr released into the sea was estimated in the range from 0.09-0.9 PBq [14] to 1-6.5 PBq [15], and 90 Sr activity concentration in surface seawater near the harbor of FNPP was 0.2-400 kBq/m 3 [15]. The activity concentration of 90 Sr was an order lower than that of 137 Cs from April 2011 to February 2012, except for December 2011 when 90 Sr and 137 Cs activity concentrations were equivalent due to the discharge of 90 Sr-contaminated wastewater [16]. In the Pacific Ocean 15 km east from FNPP, 90 Sr activity concentration was two orders lower than that in seawater near FNPP. Owing to diffusion in offshore regions of Fukushima Prefecture, the pollution is believed to be limited to the sea around FNPP. These suggest that the 90 Sr pollution of the terrestrial and marine environments are limited to the vicinity of FNPP.

90 Sr Activity Concentration in the Hard Tissue of Abandoned Cattle After the FNPP Accident
We previously reported activity concentration and specific activity of 90 Sr in the teeth of cattle stayed in the ex-evacuation zone ( Fig. 5.1) of the FNPP accident [17]. Because of chemical similarity between Sr and calcium (Ca), the teeth incorporate Sr during their formation period (calcification period) and retain it until they fall out or are worn down. As the amount of 90 Sr accumulated in the teeth is assumed to reflect the amount of 90 Sr incorporated into the body during the formation of the teeth, we hypothesized that the assessment of 90 Sr activity concentration in the teeth might provide useful information about the degree of 90 Sr contamination in the environment. Figure 5.2 shows the relationship between 90 Sr activity concentration in hard tissues (teeth and mandibular bones) of cattle and in soil from the ex-evacuation zone (Okuma Town and Kawauchi Village) after the FNPP accident. Activity concentrations of 90 Sr in the teeth were 150-830 mBq/g Ca (ratio of 90 Sr radioactivity to Ca weight in the tooth) (average, 470 mBq/g Ca) for Okuma Town and 100-310 mBq/g Ca (average, 170 mBq/g Ca) for Kawauchi Village [17]. The deposited amount of 90 Sr in soil was 94-1500 Bq/m 2 (average, 740 Bq/m 2 ) for Okuma Town and 39-380 Bq/m 2 (average, 200 Bq/m 2 ) for Kawauchi Village [9]. We selected Iwate Prefecture as the control area because it is approximately 250 km north from FNPP and is considered free from FNPP-related 90 Sr pollution. As shown in Fig. 5.2a, 90 Sr activity concentration in cattle teeth was significantly correlated with that in soil; the higher the 90 Sr activity concentration in soil, the higher the activity concentration in cattle teeth. A similar correlation was also found between 90 Sr activity concentration in soil and that in the mandibular bone (Fig. 5.2b). Metabolic turnover rates (modeling and remodeling) of the mandibular bone in the cattle are not well- Relationship between teeth and soils. (b) Relationship between bones and soils. Data of 90 Sr activity concentration in the teeth were obtained from our previous report [17]. Data of 90 Sr activity concentration in soil were obtained from Japanese government reports [9,18]. Each symbol indicates the average value with error bars showing the maximum and minimum values known. However, a major part of the original bone that formed before the FNNP accident was believed to be replaced by new bone formed after the accident, because, at the time the cattle were killed, 16-25 months had already passed since the accident. Thus, 90 Sr activity concentrations in hard tissues (both in the teeth and bones) reflect environmental 90 Sr levels during the formation of the tissues.

90 Sr Pollution in Fish
Miki et al. reported 90 Sr activity concentrations in marine fish before and after the FNPP accident (Fig. 5.1) [19]. Activity concentrations higher than the background level before the FNPP accident were detected in 4 of 26 specimens collected just outside of the ex-evacuation zone, whereas 90 Sr activity concentrations in all samples collected offshore the neighborhood of Fukushima Prefecture were under the detection limit. The report pointed out that 90 Sr was detected only in marine fish living in the ocean near FNPP. Higher 137 Cs activity concentration was found in the whole body of teleost fish; the activity ratio of 137 Cs to 90 Sr was 5-190 times higher than that before the accident.
Regarding 90 Sr activity concentration in marine biota, the otolith of fish is a hard tissue composed of CaCO 3 and might be a unique index of 90 Sr pollution. In the process of otolith calcification, Sr is incorporated with Ca and is not metabolized once it is formed. It is reported that the β-ray count rate from otoliths of Japanese rockfish correlates with the activity concentration of radioactive Cs and 90 Sr in the whole body of fishes in the main harbor of FNPP. On the other hand, no β-rays were detected from fish collected outside of the main harbor of FNPP [20].
The otolith could reflect 90 Sr pollution in the living area of the fish. However, the use of otoliths as a biomonitor may not be feasible for detecting low levels of pollution, such as that outside FNPP harbor, because the amount of 90 Sr would be too small in the otolith. If more sensitive determination methods, such as massspectrometric quantification [21], were developed, 90 Sr in otoliths could be determined even with low radioactivity of the nuclide.

Migration of 90 Sr from the Environment to Hard Tissues of Animals
Strontium in soil exists in two forms; one is an insoluble solid form, and the other is the soluble form (exchangeable fraction) that can be dissolved in water or other aqueous solutions such as ammonium acetate [22,23]. Considering the migration of 90 Sr from soil to hard tissues of animals, 90 Sr in the latter form is believed to be important because Sr isotopes in the exchangeable fraction would be transferable in terrestrial biota. According to our analysis of soil from the ex-evacuation zone [23], only 1-9% of Sr in soil was in the exchangeable fraction, whereas 37-84% of 90 Sr in soil was in the exchangeable fraction. Consequently, the specific activity of 90 Sr ( 90 Sr/total Sr ≈ stable Sr) in the solid form was low (23-33 Bq/g Sr in Kawauchi Village and 11-36 Bq/g Sr in Okuma Town), and specific activity in the exchangeable fraction was high (200-330 Bq/g Sr in Kawauchi Village and 130-1200 Bq/g Sr in Okuma Town). As both 90 Sr and stable Sr behave similarly in the biota, the specific activity of 90 Sr would basically be identical in the migration route of 90 Sr from soil to plants and plants to animals (Fig. 5.3). In fact, specific activities of 90 Sr in cattle teeth (260-640 Bq/g Sr in Kawauchi Village and 380-1400 Bq/g Sr in Okuma Town) were in a similar level as those in the exchangeable fraction in soil. After the FNPP accident, cattle in the ex-evacuation zone were released to the polluted field and allowed to graze on contaminated grasses. Once it enters the body, Sr is known to be incorporated in hard tissues during their formation period and accumulate there. The main chemical form of Sr in the teeth and bones is hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , in which any fraction of Ca may be replaced by Sr with little change in the apatite structure. 90 Sr incorporated in the hard tissue may exist as stable Sr X Ca (10-X) (PO 4 ) 6 (OH) 2 in the apatite crystal. Thus, 90 Sr activity  90 Sr would be accumulated in the hard tissues of fish, such as the bones, teeth, and otoliths. Therefore, the hard tissue of fishes may also provide useful information about 90 Sr pollution in the marine biota, as previously reported [20].

Summary and Perspectives for the Future Study
We described several aspects of 90 Sr pollution in the environment after the FNPP accident and 90 Sr activity concentration in the hard tissue of animal body. This article reviewed 90 Sr pollution in soil and seawater, hard tissues of abandoned cattle, and contaminated marine fishes around FNPP after the accident. Although the degree of environmental pollution has gradually been declining for the last several years, continuous research is necessary for clarifying 90 Sr mobility in the environment affected by the FNPP accident. Hard tissues such as the teeth, bones, and otoliths have the potential to reflect 90 Sr pollution of the environment during their formation period. It should be emphasized that 90 Sr specific activity in the teeth during their formation period and that in the exchangeable fraction in soil is at the same level [23]. This means that 90 Sr specific activity in the teeth may serve as a useful tool for understanding the migration route of 90 Sr in terrestrial biota. Furthermore, as the amount of 90 Sr in the teeth may be proportional to the amount of 90 Sr incorporated into the body during teeth formation, 90 Sr in the teeth provides a useful index for the individual assessment of internal exposure to radiation. Estimation of individual exposure dose is essential for understanding the biological effect of radiation, especially animals affected by the FNPP accident. In this sense, the examination of radioactive nuclides including 90 Sr in hard tissues provides useful information about the exposure dose to radiation. Nevertheless, further studies are required to elucidate the relationship between 90 Sr in hard tissues and internal exposure dose to the nuclide.
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