Simple Radiometric Determination of Strontium-90 in Seawater Using Measurement of Yttrium-90 Decay Time Following Iron- Barium Co-Precipitation

ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34 1277 2018 © The Japan Society for Analytical Chemistry

Simple Radiometric Determination of Strontium-90 in Seawater Using Measurement of Yttrium-90 Decay Time Following Iron- Barium Co-precipitation

Mitsuyuki KONNO*,** and Yoshitaka TAKAGAI*,***†

*Faculty of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960–1296, Japan **Environmental Radiation Monitoring Centre, Fukushima Prefecture, 45-169 Sukakeba, Kaibama, Haramachi, Minamisoma, Fukushima 975–0036, Japan ***Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima 960–1296, Japan

A radiometric quantitative methodology of 90Sr in seawater was developed using a measurement of the 90Y decay time following iron-barium co-precipitation. With calculations of its decay time, the radioactivity of 90Sr can be indirectly determined under conditional environmental samples. In addition, to avoid the interference of other radionuclide, the prepared samples were measured using a germanium semi-conductivity detector; then, the deposited radioactivity was subtracted from the actual measurement values of beta-ray counting. In this paper, the seawater samples were collected within 2 km around Fukushima Daiichi Nuclear Power Plants during the term from October 2011 to March 2012. This method showed good linearity between the 90Sr concentration and the total beta counting following the proposed method, with a correlation coefficient of 0.99 in seawater sample analysis. No interference that was caused by other radionuclides, such as radioactive cesium, was not observed in the quantification of 90Sr. The whole process requires 12 h to quantify 90Sr; this time is 1/40 shorter than traditional milking-low background gas-flow counting method. The lower limit of detection (average value n = 60) of the 90Sr radioactivity was shown to be 0.03 Bq/L (uncertainty 4.2%).

Keywords Radioactive strontium, iron-barium co-precipitation, yttrium-90, simple radiometric determination, seawater

(Received April 2, 2018; Accepted July 7, 2018; Advance Publication Released Online by J-STAGE July 20, 2018)

2 weeks to 1 month to obtain the results of measurements. For Introduction this reason, the monitoring of pure β-ray emitting radionuclides is time-consuming compared with the determination of γ-ray Radioactive strontium (89Sr (half-life (HLT): 50.53 d1) or 90Sr emitting radionuclides, and a shortening of the measurement is (HLT: 28.79 y1)) is one of β-ray emitting radionuclides, and is thus desirable. widely known to be a representative fission product of U and As methods for analyzing radioactive Sr in a short time, Pu. Sr is an element belonging to the same family as Ca, known quantitative analysis by inductively coupled plasma mass to form deposits in bone when incorporated into the body,2,3 and spectrometry (ICP-MS)6–9 and solid-phase material, obtained by cause long-term internal exposure, for which it is one of the selective extraction of Sr,10 are subjects of studies. However, in most important monitoring radionuclides in nuclear disasters. these analytical methods, the lower limit of quantification is Amongst environmental samples, seawater monitoring is very about 0.3 to 1.0 Bq/L, and interference is caused in the case of important not only for environmental protection, but also for a sample containing elements of high concentration in a matrix.11,12 international influence.4 Alternatively, the measurement of total β radioactivity is also In the accident at TEPCO’s Fukushima Daiichi Nuclear Power well known as an analytical means. In this method, β-rays Plants (NPP), which occurred on March 2011, various artificial emitted from a sample are counted without energy division, and radionuclides were released into the environment, and today the radioactivity is determined by comparing with a standard many agencies are monitoring the released radionuclides.2,3 sample. Since the total β radioactivity does not measure α-rays Detailed nuclide analysis can be carried out in a short time by and γ-rays, the difference in quantitative values is small as monitoring the γ-ray emitting radionuclides with a Ge compared with the measurement method of milking-LBC, and it semiconductor detection device. However, in the case of is excellent as a simple monitoring method of pure β-ray analyzing pure β-ray emitting radionuclides, such as 89Sr or 90Sr, emitting radionuclides. Many samples, such as airborne dust, especially in the case of the conventional method (Milking-Low drinking water, rainwater, seawater etc., are being analyzed due Background Gas-Flow Counting Method (LBC)),5 it takes about to this convenience.13–18 Amongst them, regarding in particular the method for analyzing the total β radioactivity of seawater,14 † To whom correspondence should be addressed. it is known that total β radioactivity can be measured with a E-mail: [email protected] comparatively simple operation via co-precipitating metallic 1278 ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34 elements. By using the cobalt sulfide co-precipitation method, LBC-4202 (Hitachi ALOKA Medical Co., Ltd., Tokyo, Japan), 59Fe, 60Co, 65Zn, 106Ru, and the like, can be co-precipitated was used. The detector was a GM counter, which was equipped efficiently. Furthermore, the iron-barium co-precipitation with an anti-coincidence calculation function with a center method19–21 is an efficient method for the co-precipitation of 90Y counter and a guard counter. This detector was shielded with (HLT: 64.0 h1) and 137mBa (HLT: 2.55 min1) by applying a basic 100 mm lead, and a thin polyethylene terephthalate film with a

NH4Cl–NH3 mixture to a mixed solution of Fe(III)-Ba(II) ions. gold vapor deposit, which was attached to the β-ray detection The authors consider that it is possible to efficiently perform part of the detector. For quenching, a mixed gas of helium measurements of the total β radioactivity with the proper use of 99%–isobutane 1% was used. the chemical co-precipitation method, as described above. For γ-ray measurements, a GC4020 type germanium Since, however, in total β radioactivity analysis the calculated semiconductor detector (manufactured by Canberra Japan, values are obtained without any energy division of β rays Tokyo, Japan) was used. The resolution was 1.87 keV, with a emitted from a sample, the quantitative determination of a relative efficiency of 44%. A self-absorption correction function specific β-ray emitting nuclide proved to be difficult. Due to and a sum-peak correction function were employed. this disadvantage, although total β-radioactivity analysis is a ELAN DRC-II inductively coupled plasma mass spectrometer simple means for measuring radioactivity, even if 90Y as a (PerkinElmer, Inc., Shelton, CT) was employed with ultrapure 90 daughter nuclide of Sr can be iron-barium co-precipitated, the CH4 (>99.9999%) a reaction gas in a collision-reaction cell (the method can be hardly used for the quantitative determination of dynamic reaction cell (DRC) to measure the concentration of 90Sr due to co-precipitation with other β-emitting radionuclides. multielement. On the other hand, Bau19 reported that by adjusting the pH from 3.6 to 6.2, in the presence of iron, an Fe(III) hydroxide Reagents and preparation precipitate is formed, and thereby rare-earth elements can be In this research, unless specified otherwise, special-grade scavenged. This report relates to verification with stable reagents of Wako Pure Chemical Industries, Ltd. were used. For isotopes, and it does not assume fission products occurring in an the iron carrier, 2.1 g of Fe(III)Cl3·6H2O was dissolved in 20 to actual nuclear power plant. Table S1 in Supporting Information 30 mL of 1 M HCl. This solution was transferred to a volumetric shows a part of radionuclides assumed to be fission products flask and filled up to 250 mL with distilled water. For the Ba among rare earths and Y (REY). In the total β radioactivity carrier, 17.8 g of BaCl2·2H2O was dissolved in ion-exchanged analysis, the radioactivity of pure β-ray emitting radionuclides water and made up to 1000 mL to prepare a concentration of can be calculated by subtracting the contribution of γ-ray 10 mg Ba2+/mL. A certified 90Sr solution was used 10.47 Bq/g emitting radionuclides as measured by a Ge semiconductor (relative expansion uncertainty: 1.2%) and manufactured by detector. Otherwise, if radionuclides having short half-life time, Japan Isotope Association. such as 214Pb and 214Bi and radionuclides reaching transient The standard solution of radioactive Cs was prepared as equilibrium, such as 140Ba and 140La, are detected in the γ-ray follows. The radioactive Cs was extracted from the soil sample spectrum, the interferences of those radionuclides need to be (134Cs: 92 and 137Cs: 250 Bq/g; 12:00 June 28, 2014) collected at subtracted from the measurement value of LBC. In addition, a location about 3 km southwest of the Fukushima Daiichi NPP another β emitter radionuclide, 32P (1.71 MeV), was considered by the following procedure: 200 g of soil was extracted to 2 L to be an inhibitor of measurements using LBC; however, the P of conc. HCl with heating. This was filtered with glass fiber was able to be separated by Fe-Ba co-precipitation (the stable P filter paper (manufactured by ADVANTEC, GA-200). Next, isotopes were distributed into water-phase). NaOH (granitic solid) was gradually added to this solution to

The authors concluded that most of the co-precipitated adjust the pH to 10 or more; then, 100 g of Na2CO3 was added radionuclides are radionuclides with a half-life of 1 month or and stirred. After the precipitate was sufficiently settled, the longer, and focused on the facts that (1), only 90Y has short a supernatant was separated by decantation and the volume was HLT (64.1 h), (2), it has a large β-ray energy (2.28 MeV) and made up to a constant volume. After quantitative analysis with (3), it is a pure β-ray emitting nuclide. It is conceivable that this the Ge semiconductor detector, this was used as a standard could be applied to low-concentration environmental radioactive Cs solution and diluted with ultrapure water as the radioactivity analysis, especially seawater analysis, derived case required for use. from nuclear disasters satisfying fixed conditions. Here, the fixed conditions include: (a) two weeks or more have passed Fe-Ba co-precipitation and LBC analysis since a nuclear disaster, and (b) the total radioactivity First, 10 mg portion, each Fe3+ (carrier) and Ba2+ (carrier) concentration is sufficiently low to the environmental level. In were added to 1 L of a seawater sample and stirred thoroughly; addition, to avoid the interference of other radionuclide, we then, 2 g of NH4Cl was added and dissolved completely while considered that the prepared samples would be measured using heating with a gas burner. Heating was continued and when the a germanium semi-conductivity detector, and then the deposited sample solution reached approximately 80°C, aqueous NH3 was radioactivity was subtracted from the actual measurement values added slowly. After aqueous NH3 had been added, a white- of beta-ray counting. turbid solution arose, and then the solution became colored

Therefore, in this study, we applied Fe-Ba co-precipitation to reddish-brown upon the addition of more excess of NH3. The 90 seawater, and the precipitated Y was identified by measuring dropwise addition of aqueous NH3 was continued until a dark- the time-intensity measurement (decay curve) by a beta counter. brown precipitate was formed in the sample solution, and then it With calculations, we indirectly determined the radioactivity of was boiled once. The pH of this solution was adjusted at the 90Sr in seawater. range from 9.0 to 9.6. After boiling, the sample solution was kept at about 80°C for 30 min to let the precipitate grow. Then, the sample solution was allowed to cool to room temperature. Experimental The original whole sample solution (1 L) was filtered using a paper filter (f25 mm, 5B), and the precipitate was collected. Equipment This was washed with 2% NH4Cl and ethanol, heated and dried For β-ray measurements, a low background gas-flow counter, with an infrared lamp, and then β-rays were measured using ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34 1279

LBC. The background count rate was subtracted from the data Table 1 Sampling locations in each measurement. GPS coordinatea Direction or Air location from radiation Counting efficiency in LBC analysis No. b Latitude-N Longitude-E Fukushima 1 dose rate / The counting efficiency in the total β radioactivity analysis is NPP µSv h–1 usually determined using U3O8 or KCl standards. This standard radiation source has a structure in which the above-mentioned A 37.4136500 141.0375333 NPP cooling 0.19 reference material was applied to a sample dish, and its surface water outlet was covered with a plastic film or the like; in most cases the B 37.4314000 141.0374833 NPP cooling 0.07 water outlet shape and the base material were different from the actual C 37.4242833 141.0428333 NPP cooling 0.08 90 measurement sample. When quantifying the result of Sr from water inlet the result of the total β radioactivity as in this study, we D 37.4224667 141.0607333 East 0.02 considered that the difference in the counting efficiency due to E 37.3835500 141.0567833 South-east 0.02 the difference in the sample shape remarkably affected the F 37.4584833 141.0574000 Nouth-east 0.02 result. Although the solvent for dissolving the reference material was desired to be same as the sample solvent, pure water was a. Geographical coordinate values are based on the World Geodetic System (WGS-84) datum. used as a solvent to dissolve the reference material in this study. b. Air radiation dose rates were measured at 1 m above the ground. Therefore, this time, exchanged ion water, containing a fixed The values were calculated as a dose rate of 31 July 2011. Air radiation 90 amount of the Sr standard solution, was prepared and β-rays dose rates were measured using a NaI (Tl) scintillation survey meter. were measured by the above-mentioned method described in the section of “Fe-Ba co-precipitation and LBC analysis” to determine the counting efficiency. The measurement time was 60 min and the counting rate was 0.3 cpm for a gain of 1 Bq. Thereafter, this sample was dissolved in ion-exchanged water

The counting efficiency was calculated according to the and an appropriate amount of a saturated (NH4)2CO3 solution following equation: was added to precipitate Sr(CO3)2. After filtration immediately, through a glass filter (f30 mm, 10 – 16 μm), the precipitate was Ns − N b 1 dissolved in diluted HCl. Fe carrier (Fe3+) and a Y carrier (Y3+) e (%) = × ×100, (1) t Rstd were added, and the mixture was kept in a sealed state for at least 2 weeks, waiting for radiation to occur equilibrium between 90 90 where e (%) represents the counting efficiency, Ns (counts) and Y and Sr. Two weeks later, NH4Cl and aqueous NH3 were 90 Nb (counts) are the count values of the standard sample and the added to the sample solution, and Y was co-precipitated with background, respectively, t (s) is the measurement time, Rstd iron. The time to be precipitate was recorded and the decay (Bq) indicates the amount of the added 90Sr standard substance. correction of the 90Y time was calculated using the time. This Accordingly, the obtained counting efficiency was 30.4%; which precipitate was filtered with a paper filter (f25 mm, 5C), dried, was used to calculate the β-ray radioactivity in this study. and β-rays were measured using LBC. The background count rate was subtracted from data in each measurement. 90Sr analysis of seawater by the official method of Japan (milking process-LBC) Sampling Radioactive Sr in seawater was analyzed according to the Seawater samples collected at 6 locations around the official method5 in Japan. A 40-L portion of a seawater sample Fukushima Daiichi NPP were used (Table 1 and Fig. 1 show was filtered with a paper filter (5A); 40 mL of HCl was then sampling locations). The sample volume was approximately added and the solution was passed through a column (f100 mm, 200 L and the collection was carried out from a ship (Hatsukaze L = 260 mm) packed with an anion exchange resin (Dowex #7, 19-ton) using a pump from a depth of 1 m. The salinities of 50W-X8 manufactured by Muromachi Technos Co., Ltd.). collected seawaters were measured using an IC-2100 series Next, Sr was isolated by flowing 4000 mL of a 1:1 mixture of a ion-chromatograph (TOSOH Corp., Tokyo, Japan) following

2 M aqueous CH3COONH4 solution and methanol and then Japanese Industrial Standard (JIS 0102-35.3) consequently, the 6000 mL of 4 M HCl, in that order. NaOH (granular solid) was concentration range of Cl– was from 16000 to 18000 mg/L. gradually added to a 4 M HCl fraction, and the pH was adjusted Those concentrations depended on the sampling locations. to 10 or more, and then 100 g of Na2CO3 was added; the mixture After sampling, concentrated HCl was added into the seawaters was thoroughly stirred and then heated. After allowing it to (the 0.1 v/v% HCl aq. sol. was prepared). The seawater was cool, the precipitate formed was separated by centrifugation allowed to stand for at least 24 h, and after precipitation of (3000 rpm, 10 min) and dissolved with HCl. This solution was impurities such as sand and seaweed, the supernatant was used heated on a hot plate for full evaporation; 200 mL of 0.5 M HCl as a sample. The collection period was 10 months from June was then added and the solid matter was completely dissolved. 2013 to March 2014, the sampling frequency was 1 to 2 times/ This sample solution was passed through a column (f30 mm, month, and the total number of samples was 60. Note: The L = 260 mm) filled with an anion-exchange resin (Dowex 50W- typical radioactivity concentrations of 90Sr in seawater near and X8 manufactured by Muromachi Technos Co., Ltd.) at a flow around Fukushima Daiichi NPP at April, 2018 ranged from 0.8 rate of 20 mL/min. to 1.8 mBq/L.22

Next, 1000 mL of a 1:1 mixture of a 2 M aqueous CH3COONH4 solution and methanol was flowed through the ion-exchange resin holding the adsorbed sample; Ca was eluted, and the Sr Results and Discussion was isolated by passing 600 mL of 2 M aqueous CH3COONH4 solution. The flow rate at this time was 20 mL/min. The sample Relationship between the total β radioactivity treated by the Fe- solution with isolated Sr was heated to evaporate the liquid, and Ba co-precipitation method and the 90Sr radioactivity 90 then a small amount of nitric acid was added to form SrNO3. Figure 2 shows the relationship between Sr and the total β 1280 ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34

Fig. 1 Map of the locations for seawater samples. Detailed information is shown in Table 1. The center of the circle in this figure is the intermediate point between plant 2 and 3 of the Fukushima nuclear power plant: latitude-N: 37.421598, longitude-E: 141.032720.

seawater samples that were collected at different locations around the Fukushima Daiichi NPP. Generally, in the total β radioactivity measurement, about 30% of 90Sr and 99% of the rare earth 90Y are believed to co-precipitate with iron ions,19 and it was considered that the slope of 90Sr to the total β radioactivity should be 1 or larger. Plausible causes of the constant of proportionality becoming smaller than 1, include a loss of the precipitate at the time of preparing the measurement sample, decrease in recovery rate, due to insufficient co-precipitation of 90Y in the co-precipitation operation with iron ions, and inhomogeneity of the sample. The certain precipitation rate of Sr was approx. only 10% in this study and the mere quarter of the energy of 90Sr was counted among of the only 10% precipitation; therefore, it was considered that any interference of other radionuclides was difficult to be found. For another confirmation, the total β radioactivity in a standard sample (after Fig. 2 Relationship of the 90Sr radioactivity in seawater samples the treatment of this method) obtained by adding a fixed amount 90 between the Fe-Ba coprecipitation method (proposed method) and the of Sr standard solution to the ion exchanged water was public authorized analytical method.5 The public authorized analytical measured; the plotted results are shown in Fig. 3. In this graph, method for 90Sr in seawater5 is the low background gas-flow counting the ordinate represents the total β radioactivity concentration measurement with a milling process. and the abscissa represents the addition amount of the 90Sr standard solution. According to this graph, the constant of proportionality of the total β radioactivity to 90Sr concentration was almost 1 with a correlation coefficient of 1.00, consistent radioactivity (after treatment by the proposed method) of with the constant of proportionality of the seawater sample seawater (collected around the Fukushima Daiichi NPP) with shown in Fig. 2. This suggests that sedimentation loss, during the presented method. The 90Sr concentration in seawater was the test sample preparation and counting error of the measuring determined by a public-authorized analytical method,5 which device, did not have any remarkable impact. was described in the section of “90Sr analysis of seawater by the Next, to check whether 90Y co-precipitates with Fe ions at the official method of Japan (milking process-LBC)”. The total β theoretical value, a certain amount of 89Y was added as a tracer radioactivity means the measurement values using LBC after the to the sample. The 89Y concentration contained in the treatment of Fe-Ba co-precipitate. As a result, good correlation supernatant, when the settled precipitate was formed by the (R2 = 0.978) was found between the total β radioactivity and the analytical procedure described above, was measured by ICP-MS 90Sr radioactivity. The plotted points indicate the different and the recovery rate was determined. Table 2 summarizes the ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34 1281 results. In all samples, the recovery rate of 89Y was 99%. radionuclide in the co-precipitate is almost 90Y. The 90Sr was Thus, Y co-precipitated according to the theoretical value. not able to detect due to the lower β-ray energy than 90Y (approx. Furthermore, to confirm the interference by other radionuclides, 25%), and the abundance was quite low in the co-precipitate. similar tests were performed on samples intentionally added This consideration was indicated to be the same as the results with an excessive amount of radioactive Cs but also in that case, using the 90Sr reference materials. no change was observed in the recovery rate of 89Y. These results confirmed that Y, in the sample, was recovered in its The identification of radionuclides using decay time entirety, and the likelihood of insufficient co-precipitation of Y Since the traditional total β radioactivity measurement method with iron ions could be denied. is a simplified means to estimate all β-ray emitting radionuclides, From those results, it is considered that pure β-emitting the identification of radionuclides is difficult. Therefore, the authors tried to identify radionuclides (after the treatment of presented method) by measuring the decay time to prevent any false recognition of the target nuclide. Using the short half-life of 90Y (HLT 64 h1), identification was attempted by checking the attenuation of β-rays emitted from the sample. Figure 4(A) shows the time course of the total β radioactivity of seawater samples. Here, to clearly confirm the attenuation of the sample, the results of samples with a comparatively large count value were used. Samples were then taken at the same location with different collection dates, and the measurement interval was 8 h. As a result, it was confirmed that the count values decreased at a fixed rate over time in all measured values. In addition, the data were converted to the equation of the radioactive decay due to identification of the nuclide as follows:

t ⎛ 1 ⎞ T A = A0exp(–λt) = A0 , (2) ⎝ 2 ⎠

Fig. 3 Spike of 90Sr and its detection test using ion-exchanged water where, the A and A0 represent the current and initial radioactivity, (solid line). The dotted line is a theoretical detection line. respectively; λ, t and T (= ln 2/λ) are the decay constant, the elapsed time, and the half life time (HLT) of the radionuclide, respectively. 90 Table 2 Recovery of yttrium in seawater samples Here, since we aim to make the precipitate of Y using the Fe-Ba co-precipitate reaction, the HLT value of 90Y is 64 h. The Sample Composition 89Y/ Recovery RSD, equation can be changed as follows: –1 89 No. 90Sr/Bq 89Y, ppm 137Cs/Bq μg L of Y, % % A = exp(−λt), Blank 1 0 1 100 2.5 99.8 8.5 A0 Blank 2 0 1 0 8.7 99.1 7.3 (3) ⎛ A ⎞ −2 1 0.5 1 100 4.3 99.6 47.5 ln⎜ ⎟ = −λt = −1.08 ×10 t. 2 1.0 1 100 5.3 99.5 6.2 ⎝ A0 ⎠ 3 2.5 1 100 2.6 99.7 5.0 4 5.0 1 100 3.1 99.7 14.2 The data concerning ln(A/A ) vs. time was plotted, as shown in 5 10.0 1 100 3.8 99.6 6.2 0 Fig. 4(B). The slopes of both samples corresponded to

Fig. 4 Time-course curve of the beta-ray count rate values (A) and the correlation line between the radioactivity ratio vs. time (B). Samples #1 and #2 were seawater samples collected at Oct. 30th, 2013 and March 10th, 2014, respectively. 1282 ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34

Fig. 5 Typical radionuclides of decay curves that were released to nature, and the correlation of the measurement values obtained by the Fig. 6 90Sr detection in the presence of 100 Bq of radio Cs. The Cs proposed method. The sample was seawater samples collected at Oct. radioactivity was the total radioactivity of 134Cs and 137Cs. 30th, 2013. The two of 1 L of samples were treated by the proposed method and measured as n = 2.

for easily analyzing 90Sr, even when a large amount of radioactive –1.08 × 10–2 [h–1]; thus, the radiation source from the precipitate Cs is released, as in the case of this accident. From these was found to be 90Y. results, the actual Fukushima Daiichi nuclear accident the Moreover, we checked whether this attenuation was due to amount of radioactive Cs emitted into the environment was 90Y. For other radionuclides present in the test sample, with more than 10-times higher than that of radioactive Sr; this highly probability and natural radionuclides, the attenuation of method can use in the actual monitoring. As a result, it was the count value was calculated at the same time and plotted as found that the presence of radio Cs in the concentration range of shown in Fig. 5. As a result, the measured value of the sample several Bq/L in seawater did not influence the 90Sr quantification agreed with the theoretical value calculated for the attenuation using the presented method. of 90Y. The actual measurement value’s plots were described on Furthermore, the detection limit (DL, average value; n, 60) of the theoretical decay curve of 90Y until 70 h. After 80 h, the the 90Sr radioactivity was 0.03 Bq/L (uncertainty; 4.2%) in this plotted values slightly shifted from the theoretical line. It was presented method. The time required for the analysis was 12 h suggested that trace amounts of long-lived radionuclide survived at maximum speed, which was 1/40 compared to an analysis by in the precipitate. However, the trace nuclides were little the milking-LBC method.5 In addition, the DL was 10 to affected on the proposed method (the approx. 40 h 100-fold lower than other method, such as ICP-MS.6,7 measurements). In addition, an overshoot of the counting value due to the mixture of the short half-life nuclides was not observed. The decays of the natural radionuclides, e.g. 212Pb Conclusions (HLT: 10.6 h) was obviously different from the measurement dots. We investigated whether 90Sr contained in seawater can be From the above, in the total β radioactivity analysis, by the quantified by the total β Fe-Ba co-precipitation method. As a Fe-Ba co-precipitation method, by measuring the change over result, a correlation (correlation coefficient R2 = 0.9) was found time of the sample we were able to achieve an identification of between the data of the total β radioactivity analysis method the nuclide, which is a weak point of this analytical method. treated after the Fe-Ba co-precipitation method of seawater Furthermore, as the nuclide identification became possible in collected in the waters surrounding the Fukushima Daiichi NPP, this way, false identification of the nuclide can be prevented, Tokyo Electric Power Co., Inc. and the values of 90Sr and it seems that this analytical method can be very useful for concentration obtained by the milking-LBC method. screening tests in the case of an emergency. Furthermore, by measuring the decay time (in the measurement time was limited range from 0 to 70 h, typically 40 h), 90Y was Impact to radioactive Cs on this method identified. Thus, it was possible to avoid false recognition We investigated the influence of other radionuclides contained regarding 90Sr and other radionuclides. This method could in samples in the environment on the total β radioactivity. An expect to be used as an alternative radiometric quantification for excess amount of radioactive Cs (134Cs + 137Cs = 100 Bq) was radioactive Sr in limited environmental conditions; (a) two added to distilled water containing the 90Sr standard solution, weeks and more have passed since a nuclear disaster, and (b) the and the total β radioactivity (after the treatment of this method) total radioactivity concentration is sufficiently low to the was measured. The results are shown in Fig. 6. Even in the environmental level. sample to which radioactive Cs was added, similarly to the correlation between 90Sr and the total β radioactivity illustrated in Fig. 6, a very good correlation (R2 = 1.000) was found, and Acknowledgements also the calibration was substantially prepared. The graph also revealed that there was no change in the value of the total β The authors gratefully acknowledge funding by the Ministry of radioactivity, even in the presence of an excessive amount of Education, Culture, Sports, Science & Technology in Japan radioactive Cs. This demonstrates that the method is effective (MEXT), Human Resource Development and Research Program ANALYTICAL SCIENCES NOVEMBER 2018, VOL. 34 1283 for Decommissioning of Fukushima Daiichi Nuclear Power Methods, 2014, 6, 355. Station. In addition, we thank Mr. Makoto Matsueda (Japan 7. M. Furukawa and Y. Takagai, Anal. Chem., 2016, 88, 9397. Atomic Energy Agency), Mr. Hiroaki Ogata and Ms. Mizuki 8. A. Ayala and Y. Takagai, Anal. Sci., 2018, 34, 387. Odashima (Fukushima University) for experimental cooperation 9. M. Furukawa, M. Matsueda, and Y. Takagai, Anal. Sci., and useful discussions. 2018, 34, 471. 10. S. Scarpitta, J. Odin-McCabe, R. Gaschott, A. Meier, and E. Klug, Health Phys., 1999, 76. Supporting Information 11. Y. Takagai, M. Furukawa, Y. Kameo, M. Matsueda, and K. Suzuki, Bunseki Kagaku, 2017, 66, 223. This material is available free of charge on the Web at http:// 12. M. Furukawa, M. Matsueda, and Y. Takagai, Bunseki www.jsac.or.jp/analsci/. Kagaku, 2017, 66, 181. 13. C. Dueñas, M. C. Fernández, E. Gordo, S. Cañete, and M. Pérez, Atmos. Environ., 2011, 45, 1015. References 14. D. Zapata-García, M. Llauradó, and G. Rauret, Appl. Radiat. Isot., 2009, 67, 978. 1. R. B. Firestone and V. S. Shirley, “Table of Isotopes (CD- 15. M. Palomo, M. Villa, N. Casacuberta, A. Peñalver, F. ROM Ver.)”, 8th ed., 1996, John Wiley & Sons Inc. Borrull, and C. Aguilar, Appl. Radiat. Isot., 2011, 69, 1274. 2. Ministry of Health Labour and Welfare (Japan), Inspection 16. P. Thakur and G. P. Mulholland, Appl. Radiat. Isot., 2011, of Radiomaterials in Tap water, http://www.mhlw.go.jp/ 69, 1307. file/06-Seisakujouhou-10900000-Kenkoukyoku/01_housha 17. S. Wisser, E. Frenzel, and M. Dittmer, Appl. Radiat. Isot., sei_120328_m1.pdf. 2006, 64, 368. 3. Fukushima Prefecture Government, Results of Environmental 18. R. I. Kleinschmidt, Appl. Radiat. Isot., 2004, 61, 333. Radioactivity Monitoring, http://www.pref.fukushima.lg.jp/ 19. M. Bau, Geochim. Cosmochim. Acta, 1999, 63, 67. sec_file/monitoring/k-1/kaisui110516-110530.pdf. 20. C. Liu, Sci. China, Ser. D: Earth Sci., 2002, 45, 449. 4. Japan Atomic Energy Agency, Database for Radioactive 21. C.-K. D. Hsi and D. Langmuir, Geochim. Cosmochim. Substance Monitoring Data, http://emdb.jaea.go.jp/emdb/ Acta, 1985, 49, 1931. en/. 22. Nuclear Regulation Authority, Radioactivity Concentration 5. MEXT, Research and Development Bureau, Atomic Energy in the Seawater Near and Around Fukushima Daiichi NPP, Division, Analytical Methods of the Radioactive Strontium, http://radioactivity.nsr.go.jp/en/contents/13000/12796/24/ No. 2, Japan, 2000. 349_1_20180525.pdf. 6. Y. Takagai, M. Furukawa, Y. Kameo, and K. Suzuki, Anal.