Application of Mass Spectrometry for Analysis of Cesium and Strontium in Environmental Samples Obtained in Fukushima Prefecture

Abstract

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

1 Introduction

On the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP), fission products (FP) such as radioactive Cs and Sr were widely released. The amounts of FP generated in each reactor were calculated by using ORIGEN code [1]. Many studies of radioactive Cs and Sr were performed to estimate external and internal exposures and to analyze the source of radioactive nuclides. These studies were typically performed by γ-ray spectrometry of ¹³⁴Cs (T_1/2 = 2.06 y) and ¹³⁷Cs (T_1/2 = 30.2 y) for the analysis of radioactive Cs and by β-spectrometry of ⁹⁰Sr (T_1/2 = 28.9 y) for that of radioactive Sr.

In addition to ¹³⁴Cs and ¹³⁷Cs, radioactive ¹³⁵Cs (T_1/2 = 2.3 × 10⁶) is also generated during the operation of reactors. Because of the difference in the generation process and the half-life of radioactive Cs, the isotopic ratios of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs have been used for analyzing the operations of nuclear facilities [2–6]. Naturally occurring Sr has four stable isotopes (⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr), on the other hand, and the isotopic composition of Sr generated in reactors [1] are totally different from the natural abundance [7]. From the analysis data of the isotopic compositions, thus, the information on the origin of radioactive nuclide release would be obtained. The mass spectrometry provides the isotopic compositions of elements. Although mass spectrometry has been used for the analysis of radioactive nuclides and actinides, few studies have reported the analysis of radioactive Cs and Sr.

The purpose of the present study is to analyze Cs and Sr isotopes in environmental samples in Fukushima prefecture for source analysis and safety assessment. Although the amounts of radioactive Cs and Sr released in this accident were very huge, the contaminated environmental samples show the small radioactivity per unit weight of the contaminated environmental samples, since the contaminated area is very wide. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive Cs and Sr were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

2 Experimental

2.1 Irradiation of UO₂ for Study of Radioactive Cs and Sr

10 mg of UO₂ of natural uranium was irradiated for 3 h at the Kyoto University Research Reactor with the neutron flux 5.5 × 10¹² n/s cm². From the calculation with ORIGEN-II code [8], the amounts of the major radionuclide of Cs and Sr were estimated as 7.4 × 10⁻¹¹ g (¹³⁷Cs) and 4.5 × 10⁻¹¹ g (⁹⁰Sr), respectively. After standing for ca. 2 days, radioactive Cs and Sr were recovered and analyzed.

2.2 Recovery of Cs and Sr

2.2.1 Isolation of TRU Elements

Cs and Sr were recovered with UTEVA^TM-resin (100–150 μm, Eichrom Technologies), Sr-resin (100–150 μm, Eichrom Technologies), ammonium phosphomolybdate (AMP), the cation exchange resin DOWEX^TM 50WX8 (100–200 mesh), and the anion exchange resin DOWEX^TM 1 X 8 (100–200 mesh).

The irradiated UO₂ was dissolved in 8 M HNO₃ (TAMAPURE-AA-100) and was evaporated to dryness at 403 K. 8 M HNO₃ was added and the insoluble residues removed by centrifugation. After centrifugation, H₂O₂ (TAMAPURE-AA 100) was added for the preparation of 8 M HNO₃/0.3 % H₂O₂ sample solution to isolate TRU elements such as U and Pu by the extraction chromatography with UTEVA-resin [9].

Three milliliter of the UTEVA-resin conditioned with diluted nitric acid was filled into a column of 54 mm in length and 6.5 mm in diameter and pretreated with 10 mL of 8 M HNO₃/0.3 % H₂O₂ before loading the solution. After loading the solution, the UTEVA-resin was rinsed with 8 M HNO₃ to elute alkaline earth metal elements [10]. The effluent was evaporated to dryness and dissolved in 10 mL of 3 M HNO₃ solution for the extraction chromatography with Sr-resin.

2.2.2 Recovery of Strontium

The solution was loaded to the Sr-resin conditioned with diluted nitric acid and filled into a column of 54 mm in length and 6.5 mm in diameter up to 3 mL. This effluent was evaporated at 403 K and the residue dissolved in 0.05 M HNO₃ for the recovery of Cs. After washing of the Sr-resin with 3 M HNO₃, Sr was recovered with 20 mL of 0.05 M HNO₃, evaporated to dryness, and dissolved in 10 μL of 1 M HNO₃.

2.2.3 Recovery of Cesium

After adding of 0.1 g of AMP to the Cs solution and stirring for several hours, the supernatant was removed from the mixed solution by centrifugation. A 20 mL 3 M ammonium hydroxide (TAMAPURE-AA 100) solution was used to dissolve the residue for subsequent anion-exchange ion chromatography.

After the final conditioning [11], a 3 mL portion of the anion-exchange resin was added to a column of 54 mm in length and 6.5 mm in diameter. The sample solution was added to the column, and the resulting eluate was collected and heated to dryness. The residue was dissolved in 20 mL of 0.1 M HNO₃ for the final purification with the cation-exchange ion chromatography.

The cation-exchange resin conditioned with hydrofluoric acid (TAMAPURE-AA-100), etc. [12] was filled into a column of 42 mm in length and 5.0 mm in diameter up to 1.5 mL. After loading the sample solution, the resin was washed with diluted nitric acid followed by 20 mL of 1.5 M HCl (TAMAPURE-AA 100) to recover Cs. The effluent was heated to dryness, and the residue was dissolved in 20 μL of 1 M HNO₃ for the analysis of the isotopic composition of Cs.

2.3 Analysis of Isotopic Composition of Cesium and Strontium

Isotopic compositions of Cs and Sr were measured with a TIMS (Triton-T1, Thermo Fisher Scientific). A 1 μL aliquot of each solution was loaded onto a rhenium filament with a TaO activator [13]. The standard material of SRM987 [14] was used as a reference material of mass spectrometry of Sr. The mass spectra of radioactive Cs and Sr were obtained with a secondary electron multiplier detector (SEM) because of the low total amounts of radionuclide loaded onto the filament.

2.4 Analysis of Environmental Samples

The plant samples were obtained from the south area of Iitate village, the northeast area of Okuma town, the southeast area of Futaba town, and southwest area of Futaba town in Fukushima prefecture from November 2012 and May 2013 (Table 4.1). The samples were washed three times with pure water and dried at 373 K. About 2.5 g of the dried samples was incinerated with a ring furnace at 873 K and dissolved in concentrated HNO₃ at 403 K and evaporated to dryness. 20 mL of 8 HNO₃ was added and the insoluble residues removed by centrifugation for the preparation of recovery of Cs and Sr. Recovery of Cs and Sr from environmental samples was also carried out by the same manner described above.

Table 4.1 List of samples and results of ⁸⁷Sr/⁸⁶Sr isotopic ratio measurement

The concentration of ⁸⁸Sr was measured with an inductively coupled quadrupole mass spectrometer (ICP-QMS, HP-4500, Yokoagawa) and radioactivity of ⁹⁰Sr by Cherenkov counting [15]. The total concentration of radioactive Cs was measured by γ-spectrometry. The sample solutions were prepared as 50 ppm of ⁸⁸Sr in 1 M HNO₃ for the analysis of Sr and 5000 Bq/mL for ¹³⁷Cs in 1 M HNO₃ for the analysis of Cs. The mass spectra of radioactive Cs and Sr were obtained with a SEM, while those of stable Cs and Sr were obtained with Faraday cup detector, since the amounts of stable nuclide were much larger than those of radionuclide.

3 Results and Discussion

3.1 Isotopic Analysis of Radioactive Cs and Sr from Irradiated UO₂

Figure 4.1a shows the mass spectra of Cs recovered from the irradiated UO₂. In this measurement, ¹³⁵Cs, ¹³⁶Cs, and ¹³⁷Cs were detected: ¹³⁴Cs was not detected, because of the difference in the generation scheme. The observed isotopic ratios of ¹³⁵Cs/¹³⁷Cs and ¹³⁶Cs/¹³⁷Cs were obtained as 0.9103 ± 0.0008 and 0.00022 ± 0.00001. From our calculation with ORIGEN-II code [8], the loading amounts of ¹³⁵Cs, ¹³⁶Cs, and ¹³⁷Cs in this time were about 3.5, 0.03, and 3.7 pg respectively. This means that the femtogram level of Cs is detectable by TIMS.

Fig. 4.1

Mass spectra of Cs. (a) Recovered from UO₂ irradiated in KUR. (b) Recovered from environmental sample from Fukushima prefecture (Reproduced from Ref [11])

Figure 4.2 shows the mass spectra of Sr both of stable (a) and radioactive (b) isotopes. At the measurement of 2.6 days later, ⁸⁹Sr, ⁹⁰Sr, and ⁹¹Sr were detected. From our calculation with ORIGEN-II code [8], the loading amounts of ⁸⁹Sr, ⁹⁰Sr, and ⁹¹Sr in this time were about 3, 4, and 0.04 pg respectively. This means that the femtogram level of Sr is also detectable by TIMS.

Fig. 4.2

Mass spectra of Sr. Stable isotopes (a) were obtained by measurement of Sr of SRM987 with a Faraday cup detector, and radioactive isotopes (b) were obtained by measurement of Sr recovered from UO₂ irradiated in KUR with a secondary electron multiplier detector

The measured isotopic ratios were 0.80 for ⁸⁹Sr/⁹⁰Sr and 0.01 for ⁹¹Sr/⁹⁰Sr showing the agreement with the calculated value (0.79 for ⁸⁹Sr/⁹⁰Sr and 0.01 for ⁹¹Sr/⁹⁰Sr). Because the half-life of ⁹¹Sr is 9.5 h, the mass spectrum of ⁹¹Sr disappeared at the measurement of 31 days later. The measured isotopic ratio of ⁸⁹Sr/⁹⁰Sr is 0.53 showing the agreement with the calculated value of 0.54. At the measurement of 574 days later, only the mass spectrum of ⁹⁰Sr was observed because the half-life of ⁸⁹Sr is 50.5 days. This means that ⁸⁹Sr/⁹⁰Sr could not be analyzed by using a typical mass spectrometer after Sep. 2012, if we obtain the sample containing femtogram level of ⁹⁰Sr. The isotopic ratio of ⁹⁰Sr/^stableSr would be therefore needed for our purpose.

3.2 Analysis of Isotopic Compositions of Cs and Sr from Environmental Samples

3.2.1 Analysis of Cs

Figure 4.1b shows three peaks, representing ¹³⁴Cs, ¹³⁵Cs, and ¹³⁷Cs, were observed on the typical mass spectra of Cs recovered from environmental samples obtained in Fukushima prefecture [11], while the peak representing ¹³⁶Cs was not observed because of the half-life (T_1/2 = 13.2 d). From the calculation with ORIGEN-II code [1], the isotopic ratio of ¹³⁶Cs/¹³⁷Cs in the fuel was estimated as ca. 0.00032. This value shows the same order compared with that of the irradiated UO₂, suggesting that we could obtain the three isotopic ratios of ¹³⁴Cs/¹³⁷Cs, ¹³⁵Cs/¹³⁷Cs, and ¹³⁶Cs/¹³⁷Cs until July 2011. Since there are three reactors in FDNPP, three isotopic ratios would bring the important information for the source analysis of radioactive Cs in the contaminated area in Fukushima prefecture.

Although we could not obtain the isotopic ratio of ¹³⁶Cs/¹³⁷Cs after July 2011, we can obtain the two-dimensional map with the isotopic ratios of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs as shown in Fig. 4.3. All of the isotopic ratios of ¹³⁵Cs/¹³⁷Cs showed less than 0.4. This value was also much smaller than reported isotopic ratios of global fallout (∼0.5 for Chernobyl accident and ∼2.7 for nuclear weapon testing, corrected to March 11, 2011 [11]) and the long half-life of ¹³⁵Cs (T_1/2 = 2.3 × 10⁶ y), meaning that only the isotopic ratio of ¹³⁵Cs/¹³⁷Cs would also provide the information for the origin of radioactive Cs among Chernobyl accident, nuclear weapon testing, and FDNPP accident for the long term.

Fig. 4.3

¹³⁵Cs/¹³⁷Cs (atomic ratio) vs ¹³⁴Cs/¹³⁷Cs (activity ratio). Error here means ±2SE. Data of OKM01, FTB01, FTB35R and ITT07 were reproduced from Ref [11]. Both of isotopic ratio was corrected to March 11, 2011. Single asterisk (*) represents calculation results from estimation of radioactive nuclides with ORIGEN-II code [1]. Double asterisk (**) represents values reported for ¹³⁴Cs/¹³⁷Cs activity ratio in polluted water [22]

3.2.2 Analysis of Sr

The FP of Sr in each reactor has mainly five isotopes [1]: two stable isotopes of ⁸⁶Sr and ⁸⁸Sr and three radioactive isotopes of ⁸⁹Sr, ⁹⁰Sr, and ⁹¹Sr. The relationship between the isotopic ratio of radioactive Cs and that of Sr estimated by ORIGEN Code calculation [1] is plotted in Fig. 4.4. In addition to the radioactive isotopes, the stable isotopes of Sr generated in each reactor show the characteristic profile. This suggests that the stable isotopes of Sr could be also used for the analysis of the FP of Sr.

Fig. 4.4

Estimated relationship between isotopic ratios of ¹³⁴Cs/¹³⁷Cs, ⁸⁹Sr/⁹⁰Sr and ⁸⁷Sr/⁸⁶Sr and that of ¹³⁵Cs/¹³⁷Cs. Isotopic ratios were estimated by calculation results with ORIGEN Code [1]. Open circle means isotopic ratio of ⁸⁹Sr/⁹⁰Sr. Closed circle represents isotopic ratio of ⁸⁷Sr/⁸⁶Sr

Among the isotopic ratios of stable isotopes, the isotopic ratio of ⁸⁷Sr/⁸⁶Sr is important in the field of the geological chronology [16], because ⁸⁷Sr is generated by the β-decay of ⁸⁷Rb having the half-life of 4.9 × 10¹⁰ y. Thus, the isotopic ratio of stable isotopes, in this study, will be focused on the isotopic ratio of ⁸⁷Sr/⁸⁶Sr.

The certified value for SRM987 of the isotopic ratio of ⁸⁷Sr/⁸⁶Sr showing the 95 % confidence intervals is 0.71036 ± 0.00026 [14]. The averaged measurement value was obtained as 0.71025 ± 0.00002 (n = 26) showing the agreement with the certified value.

In this study, the variations in the isotopic ratio of ⁸⁷Sr/⁸⁶Sr were normalized with that of SRM987; this would be expressed as delta-value (δ_87/86) in per mill notation as the following equation:

$$ {\delta}_{87/86}=\left(\frac{\left({}^{87}\mathrm{S}\mathrm{r}{/}^{86}\mathrm{S}\mathrm{r}\right)\mathrm{sample}}{\left({}^{87}\mathrm{S}\mathrm{r}{/}^{86}\mathrm{S}\mathrm{r}\right)\mathrm{S}\mathrm{R}\mathrm{M}987}\right)\times 1000. $$

The samples of ITT01 to ITT07 were prepared by the division of one sample. The δ_87/86–values of samples ITT01 to ITT07 in Table 4.1 agreed within the error showing the reproducibility of the isotopic ratio measurement including chemical treatment. From the δ_87/86–values of samples ITT01 to ITT07, the averaged δ_87/86–value of them was obtained to be δ_87/86 = −3.14 ± 0.06 ‰.

The results of the isotopic ratio measurements for all samples are summarized in Table 4.1 and shown in Fig. 4.5a. The result of the measurement for the reference material of IAEA-156: Radionuclides in clover [17] is also included. This reference material contains ca. 0.0075 Bq/g in June 2015. The δ_87/86-values of the samples of Okuma range from −1.4 to −4.4, while those of Futaba range from −2.5 to −4.2. It is found that these values have significant difference, by comparison with the δ_87/86-value of Iitate samples.

Fig. 4.5

Results of ⁸⁷Sr/⁸⁶Sr isotopic ratio measurement for plant samples (a), and isotopic ratio of ⁸⁷Sr/⁸⁶Sr as a function of ¹³⁵Cs/¹³⁷Cs (b). Bark is results of comparison between OKM03 and FTB01. Grass shows results of comparison between FTB35R and FTB35L. Isotopic ratio of ¹³⁵Cs/¹³⁷Cs of OKM01, FTB01, FTB35R and ITT07 were reproduced from Ref [11], and used after time correlation on March 11, 2011. Open square and open circle mean analytical results in this study and results of estimation by calculation results with ORIGEN Code [1]

Though the samples OKM03 and FTB01 are bark samples from the plants of the same family, these showed different magnitudes (Fig. 4.5a and Table 4.1). The isotopic ratio of ⁸⁷Sr/⁸⁶Sr has received attention as the indicator of the production region of plants and reported the δ_87/86–values ranged from −25.0 to 5.5 [18]. As the reason of the difference in the δ_87/86–values among samples OKM03 and FTB01, two origins could be considered: the first is the difference in the δ_87/86–values of soils of sampling point (as the supply source of Sr) and the second is the difference in the degree of the isotope fractionation during the translocation process (considered as the reason of the difference in the isotopic ratio between the parts of the identical organism). Because of the comparison of the δ_87/86–values of the same parts in this case, the difference in the δ_87/86–value among samples OKM03 and FTB01 might be caused by the soils in sampling area.

If the difference of δ_87/86–values between samples OKM03 and FTB01 originated from a difference of contamination level by the FP of Sr, the isotopic ratio may show a correlation as

$$ \begin{aligned} \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{OKM}03}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{OKM}03}\right) &= \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{nat}}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{nat}}\right)\times \left(1\ \hbox{--} X\right)\\ &\quad + \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FP}}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FP}}\right)\times X,\end{aligned} $$

$$ \begin{aligned} \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FTB}01}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FTB}01}\right) &= \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{nat}}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{nat}}\right)\times \left(1\ \hbox{--} Y\right)\\ &\quad + \left({\left[{}^{87}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FP}}/{\left[{}^{86}\mathrm{S}\mathrm{r}\right]}_{\mathrm{FP}}\right)\times Y.\end{aligned} $$

According to the relation and the concentrations of Sr; 72 ppm for OKM03 and 24 ppm for FTB03, the amount of the FP of ⁸⁶Sr contained in the sample OKM03 would be higher than that of FTB01, about 10.3 ng. This is equivalent to ca. 10.5 μg of ⁹⁰Sr (ca. 5.3 × 10⁷ Bq) according to the averaged isotopic ratio of ⁹⁰Sr/⁸⁶Sr of the FP of Sr [1]. ⁹⁰Sr was not found in the plant samples by TIMS and Cherenkov counting having the detection limit of several ten mBq/g [15], however, suggesting that our samples contain ⁹⁰Sr < <10 fg and was less than 1 Bq/g.

Sample FTB35R is roots, while FTB35L is leaves, of the same plant. The δ_87/86-values (Fig. 4.5a and Table 4.1) showed a significant difference. Sample FTB35R shows higher δ_87/86-value compared with sample FTB35L. The isotopic fractionations were observed in some biological processes. For example, the isotopic analysis of Sr [19], Fe [20], and Zn [21] proves that roots are isotopically heavy compared with the aerial parts; the maximum δ_87/86-value was ca. −5.0 for Sr, the maximum δ_56/54-value was ca. −1.4 for Fe, and the maximum δ_66/64-value was ca. −0.26 for Zn, respectively. Since the Cherenkov counting showed the amounts of ⁹⁰Sr in these samples were under the detection limit, the difference in the δ_87/86-value between samples FTB35R and FTB35L might be caused by the isotopic fractionations in the biological processes along with the contamination of sample FTB35R by the soil.

The isotopic ratios of radioactive Cs in samples FTB01, OKM01, FTB35R, and ITT07 measured by TIMS have been reported in our previous study [11], and that in OKM02 was measured in this study. The relationships between the isotopic ratio of ⁸⁷Sr/⁸⁶Sr as δ_87/86-value of these samples and that of ¹³⁵Cs/¹³⁷Cs are plotted in Fig. 4.5b. The isotopic ratios of ¹³⁵Cs/¹³⁷Cs show the significant difference from the reported values of the global fallout (ca. 0.5 for Chernobyl accident and ca. 2.7 for nuclear weapon testing corrected on March 11, 2011 [11]), while these values agreed with the estimated values with the results of ORIGEN Code calculation [1]. This means that all of the samples are contaminated by radioactive Cs released from FDNPP. The δ_87/86-values of these samples, on the other hand, are far from that of the FP calculated by ORIGEN Code [1]. This suggests that the amount of deposit of 90Sr is very little compared with that of Cs and agrees with our previous report [15].

Although ⁹⁰Sr was not found in the plant samples suggesting that our samples contain ⁹⁰Sr < < 10 fg, typical mass spectrometers have the external analytical precision of ppm level. Assumed that this precision could be applied for the isotopic ratio of ⁹⁰Sr/^stablesSr, the isotopic ratio of ⁹⁰Sr/^stableSr must be higher than 10⁻⁶. For the natural sample, since the Sr concentration ranges from ppb level to several hundred ppm level (Fig. 4.6), the detectable lower limit of the isotopic ratio of ⁹⁰Sr/^stableSr can be evaluated.

Fig. 4.6

Detectable lower limits of ⁹⁰Sr in environmental samples with TIMS. Solid line indicates a limit for ⁹⁰Sr/⁸⁴Sr, and broken line a limit for ⁹⁰Sr/⁸⁸Sr

If ⁸⁸Sr having the natural abundance ca. 82 % was used as reference isotope, the concentration of ⁹⁰Sr should be higher than 1 Bq/g in almost any type of sample. When the isotopic ratio of ⁹⁰Sr/⁸⁴Sr is used, because the abundance of ⁸⁴Sr (ca. 0.56 %) is lower than that of ⁸⁸Sr, the applicable range will become much wider than the case of ⁸⁸Sr (Fig. 4.5). The improvement in the sensitivity of ⁹⁰Sr detection and the obtaining of samples including small amounts of natural Sr will also bring wide applicable range.

4 Conclusions

Cs and Sr recovered from samples were analyzed by TIMS to study the applicability of TIMS for safety assessment and source analysis.

For the study of the recovery/analysis method of Cs and Sr, Cs and Sr were recovered from the natural uranium irradiated at KUR. From the measurement of radionuclide recovered from irradiated UO₂, it was concluded that several tens of femtogram level of radionuclide is detectable.

Cs and Sr were recovered from the environmental samples obtained from Fukushima prefecture and were analyzed by a method based on the results of irradiated UO₂. In the case of the analysis of Cs, it was confirmed that the analysis of the radioactive Cs by TIMS would provide important information for the source analysis. The isotopic ratio of ¹³⁵Cs/¹³⁷Cs was useful for the precise evaluation of the radioactive Cs from FDNPP apart from that of global fallout after the radioactivity of ¹³⁴Cs became below the detection limit of γ-ray measurement.

In the case of the analysis of Sr, on the other hand, the presence of ⁹⁰Sr was not detected in any samples, while the changes in the isotopic ratios of ⁸⁷Sr/⁸⁶Sr were observed. From the discussion for the amount of the FP of Sr, it was conjectured that the changes in the isotopic ratios of ⁸⁷Sr/⁸⁶Sr might be brought by some isotopic fractionation in the biological processes. The evaluation of the detectable lower limit of the isotopic ratio of ⁹⁰Sr/^stableSr suggests that the isotopic ratio of ⁹⁰Sr/⁸⁴Sr is the most suitable index to judge a source of radioactive Sr released during the accident of FDNPP by TIMS.