Radiochemical analysis of the drain water sampled at the exhaust stack shared by Units 1 and 2 of the Fukushima Daiichi Nuclear Power Station

Radioactive gas of Unit 1 of the Fukushima Daiichi Nuclear Power Station was released from the exhaust stack shared by Units 1 and 2 through the venting line on March 12th, 2011. In the present study, radiochemical analysis of drain water sampled at the drain pit of the exhaust stack was conducted to study radionuclides released during venting of the Unit 1. Not only volatile 129I, 134Cs and 137Cs but also 60Co, 90Sr, 125Sb and Unit 1-originated stable Mo isotopes were detected. Although Unit 1-originated stable Mo isotopes were clearly detected, their amounts were quite low compared to Cs, suggesting that the formation of Cs2MoO4 was suppressed under the accident condition. Approximately 90% of iodine existed as I− and 10% as IO3− in November 2020. Furthermore, larger amount of 129I than 137Cs was observed, suggesting major chemical form of 131I was molecular iodine rather than CsI at the accident time. The 134Cs/137Cs radioactivity ratio decay-corrected to March 11th, 2011 was 0.86, supported the results that Unit 1 originated radiocesium in environment has smaller 134Cs/137Cs radioactivity ratio than Unit 2 and 3 originated radiocesium.


Materials and analysis method
Drain water. The sampling point was illustrated in Fig. 1. In the vent operation, the gas in the contaminant vessel was passed through the suppression chamber to reduce the radioactivity, and then, released into environment via stand by gas treatment system pipe and exhaust stack. The exhaust stack was shared by Units 1 and 2. It is considered that the vent of Unit 2 was failed since outflow side of the rupture disk in the vent line of Unit 2 was not contaminated, and the inside of the exhaust stack was mainly contaminated by vent of Unit 1. It is presumed that dew condensation water and rainwater in the stack were accumulated in the drain pit. Drain water was sampled in September 12, 2016. Salinity and pH of the drain water were preliminary measured using salinometer and pH test paper, and they were 0.01% and neutrality.
Analysis of γ-emitting nuclides. γ-Ray spectra were measured by a Ge semiconductor detector (GC1020S-CJT-F-1/S-2002C, Mirion Technologies Canberra). Energy and efficiency of the γ-ray spectrum were calibrated using a mixed standard source of 60 Co, 133 Ba, and 137 Cs, which was prepared as a filter paper (Amersham plc, UK). Therefore, the sample for quantitative analysis of drain water was prepared by transfusing 30 μl of sample solution into a filter paper to be the same sample form. The filter paper was doubly sealed in plastic bags. First, measurement of 134 Cs and 137 Cs was conducted on the drain water as it was. After that, the measurement of the nuclides excluding 134 Cs and 137 Cs, such as 60 Co, 106 Rh (daughter nuclide of 106 Ru), 125 Sb, and 152 Eu, was conducted after Cs was eliminated using ammonium phosphomolybdate (AMP) because γ-rays from the other nuclides were considered to be hidden by the higher level of radioactivity of 134 Cs and 137 Cs in the drain water. The amount of AMP, HNO 3 concentration and standing time were optimized to eliminate Cs and to remain the other elements before analysis of the drain water. Sample solutions for the optimization were prepared by diluting the standard solutions of B, Na, Mg, Al, Ca, Mn, Fe, Co, Ni, Se, Rb, Sr, Y, Zr, Mo, Ru, Rh, Ag, Sn, Sb, Te, Cs, Ba, Eu, Re (1 mg/ml solutions purchased from Fujifirm Wako Chemical CO.INC., Kanto Chemical CO.,INC., and Merck) and of Nb (0.1 mg/ml solution purchased from Accu Standard). Based on the optimization, 5 μl of standard solutions of Co, Se, Sr, Ru, Rh, Sb, Cs, Eu and Re (1 mg/ml) were added to 5 ml of drain water and HNO 3 concentration was adjusted to 2 M. The drain water sample solution was stood for overnight. After the addition of 0.02 g of AMP to the drain water sample solution, it was stirred for 1 h. AMP was filtered using a syringe and a membrane filter unit of which pore size was 0.45 μm. The filtrate was assigned as Cs-eliminated drain water, and quantitative analysis of γ-ray emitting nuclides was conducted. Furthermore, γ-ray spectra of the Cs-eliminated drain water in a bottle (approximately 5.7 ml) were recorded for the qualitative analysis. www.nature.com/scientificreports/ Analysis of 90 Sr. Five μl of Cs standard solution was added to the Cs-eliminated drain water, and then, the Cs elimination procedure using AMP was repeated one more time with the same manner of the first time. The obtained filtrate was assigned as the twice Cs-eliminated drain water. 0.1 ml of the twice Cs-eliminated drain water was taken into a bottle, and HNO 3 and pure water were added to prepare 2 ml of 8 M HNO 3 sample solution. This was assigned as DW-Sr sample. 0.5 ml of the Sr resin (Eichrom Technology, LLC) swollen with ultrapure water was packed into a column and conditioned by passing through 5 ml of 8 M HNO 3 . Next, the DW-Sr sample was loaded into the column. The bottle which had contained DW-Sr sample was rinsed with 0.5 ml of 8 M HNO 3 4 times and all rinse solutions were loaded into the column. After that, 2.5 ml of 8 M HNO 3 was passed through the column 4 times. The extracted Sr was recovered by passing through 3 ml of 0.01 M HNO 3 . 0.2 ml of the recovery solution was sampled and mixed with 1.8 ml of 1 M HNO 3 to measure Sr concentration with inductively coupled plasma mass spectrometer, ICP-MS, (Agilent 7700, Agilent Technologies Japan, Lts), and the recovery rate during chemical separation was evaluated. Ultima Gold LLT (PerkinElmer) was added to the remaining recovery solution, and β-ray spectrum was measured with liquid scintillation counter (PerkinRlmer Tri-Carb 3110TR) after reaching radioactive equilibrium between 90 Sr and 90 Y. Efficiency was corrected by a calibration curve obtained from the measurement of standard samples containing 20 Bq of 90 Sr and 90 Y, various concentration of HNO 3 and Ultima Gold LLT.
Analysis of 99 Tc. Aliquot of the twice Cs-eliminated drain water was adjusted to approximately 5 ml of 1 M HNO 3 solution (DW-Tc). 0.5 ml of the TEVA resin (Eichrom Technology, LLC) swollen with ultrapure water was packed into a column and conditioned with 1 M HNO 3 . The DW-Tc was loaded into the TEVA resin column. The bottle which had contained DW-Tc sample was rinsed with ultrapure water 3 times and all rinse solutions were loaded into the column. After that, 2.5 ml of ultrapure water was passed through the column 4 times. The extracted Tc was recovered by passing through 1.5 ml of 8 M HNO 3 . The recovery solution was appropriately diluted to measure Re with ICP-MS. The remaining recovery solution was also appropriately diluted and mixed with Ultima Gold LLT to measure β-ray of 99 Tc with liquid scintillation counter (PerkinElmer Tri-Carb 3110 TR).

Analysis of Mo isotopes.
Molybdenum isotopes (mass number 92, 94, 95, 96, 97, 98 and 100) in a Mo standard solution (0.5 ng/mL, this solution was prepared by diluting a standard solution of 1 mg/ml of Mo, Fujifilm Wako Chemical Co. INC.) was measured using ICP-MS (Agilent 7700) and Sector field ICP-MS (Element 2, Thermo Fisher Scientific Co.) to study the effect of mass discrimination on the isotope ratios obtained by ICP-MS measurement. Reference values of the isotope ratio to 98 Mo, R true , were divided by those of measured, R meas , and plotted against the mass difference of the isotope of interest from 98 Mo, Δm.
Previously, separation method for Mo using a TEVA resin was developed to purify Mo from rubble samples 35 . Because the dissolved solution of the rubble sample contained large amount of Fe 3+ and Cr 6+ , which compete with Mo extraction on the TEVA resin, ascorbic acid was added to the sample solution to decrease their extraction by reducing to Fe 2+ and Cr 3+ . In addition, HF was added to the sample solution to avoid precipitation of Nb. However, influence of Fe, Cr, and Nb in the drain water is negligible due to lower concentration of them. Therefore, sample solution of the drain water to load the TEVA resin was adjusted to 4 M HCl using ultrapure grade HCl solution (31%, Kanto Chemical Co.,INC.), and the solution was assigned as DW-Mo sample. 0.5 ml of the TEVA resin swollen with ultrapure water was packed into a column, followed by washing with 5 ml of 1 M HNO 3 , 3.4 ml of ultrapure water, and 2 ml of 4 M HCl. Then, 5 ml of the DW-Mo sample was loaded into the column. The bottle which had contained DW-Mo sample was rinsed with 0.5 ml of 4 M HCl 3 times and all rinse solutions were loaded into the column. Next, the column was rinsed with 3.4 ml of 4 M HCl and 0.5 ml of ultrapure water. Finally, extracted Mo was recovered by passing through 2.5 ml of 1 M HNO 3 . As an operation blank solution, 4 M HCl solution was prepared and the same separation operation was carried out. Recovered Mo isotopes were measured with sector field ICP-MS. Net count rates of Mo isotopes for drain water were obtained by subtracting the measured count rates of Mo isotopes for the operation blank solution from those for the drain water. Whereas, net count rates of Mo isotopes for Mo standard solution, 0.5 ng/ml, were obtained by subtracting the measured count rates for 1 M HNO 3 solution from those for the Mo standard solution.

Analysis of 129 I − and 129 IO 3
− . It was reported that I − was extracted on the solid extraction disk, Anion-SR (Empore disk, 3 M Co.), from 3 M NaOH solution, and I − and IO 3 − were extracted on the Anion-SR from 0.01 M HCl containing 0.1 M NaHSO 3 because IO 3 − was reduced to I − by NaHSO 3 in diluted HCl solution 36,37 . In the present study, 0.5 ml of drain water was adjusted to 5 ml of 2.5 M NaOH solution to analyze I − and to 5 ml of 0.01 M HCl solution containing 0.01 M K 2 S 2 O 5 to analyze the sum of I − and IO 3 − . Instead of NaHSO 3 , K 2 S 2 O 5 was used because NaHSO 3 is unstable in a solid state, and formula number of Na 2 SO 3 , 126, is close to mass number of stable iodine, 127. As spike samples, a known amount (approximately 50 pg, "a" in the Eqs. (1) and (2)) of 127 I − or 127 IO 3 − was added to the drain water in the same solution conditions. In addition, 2.5 M NaOH solution and 0.01 M HCl solution containing 0.01 M K 2 S 2 O 5 were also prepared as blank samples. The I − or the sum of I − and IO 3 − were purified by Anion-SR using the reported method 36,37 . Briefly, Anion-SR was sequentially conditioned with 10 ml of acetone, 10 ml of methanol, 10 ml of pure water, 10 ml of 1 M HNO 3 and 30 ml of pure water. Next, the sample solution was loaded to the Anion-SR, and then, it was rinsed with 5 ml of pure water 6 times. Finally, extracted I − was recovered 9.5 ml of 1 M HNO 3  www.nature.com/scientificreports/ and 129 I ("y" in Eq. (2)) were measured by using ICP-MS (Agilent 7700). The amounts of 127 I and 129 I were calculated by using Eqs. (1) and (2), respectively.
where A and B were the measured amounts of 127 I and 129 I in a sample solution, respectively, A' and B' were the measured amounts of 127 I and 129 I in the spike sample solutions, respectively, and a was spiked amount of 127 I. The amounts of x and y were obtained for chemical species of I − and the sum of I − and IO 3 − , and the amount of IO 3 − was calculated by subtracting the amount of I − from that of the sum of I − and IO 3 − .

Results
Analysis of γ-emitting nuclides. Before the Cs elimination from the drain water using AMP, the solution condition and AMP amount were optimized to remove Cs so that the other elements remained (Supporting Table 1 and 2), 99.9% of Cs was removed with 0.01 g of AMP from 10 ml of 1 M HNO 3 containing 1 μg/ml of B, Na, Mg, Al, Ca, Mn, Fe, Co, Ni, Se, Rb, Sr, Y, Zr, Mo, Ru, Rh, Ag, Sn, Sb, Te, Cs, Ba, Eu and Re, and 0.1 μg/ml of Nb. Most elements remained in the solution although the rates of remaining Rb and Ag were low and those of Zr, and Nb decreased with increasing AMP amount. The concentration of Mo in the solution was increased to dozens of μg/ml because Mo in the AMP was dissolved to the solution. Remaining rates of metal ions were also affected by HNO 3 concentration. Increment of HNO 3 concentration tends to enhance remaining rates of metal ions. Although Zr and Nb hardly remained in 0.1 M HNO 3 , 94% and 67% of Zr and Nb, respectively, remained in 2 M HNO 3 . Therefore, 2 M HNO 3 condition was selected so that larger amount of Zr and Nb remain in the solution. Furthermore, the rate of remaining Sb depended on the standing time after preparation of sample solution. Although Sb existed as SbCl 3 in the standard solution, it could be changed to Sb 5+ in the oxidizing solution, 2 M HNO 3 , to remain in the filtrate. It seemed that a part of Sb 3+ was captured by AMP but Sb 5+ remained in the solution. Therefore, drain water was stood for overnight after adding acid and carrier to wait for the oxidization of Sb 3+ to Sb 5+ . Figure 2 shows the γ-ray spectra for (a) quantitative analysis of drain water, (b) quantitative analysis of Cseliminated drain water, and (c) qualitative analysis of Cs-eliminated drain water. Only peaks of 134 Cs and 137 Cs were observed in spectrum (a) except those of natural background nuclides and backscattering. The concentrations of 134 Cs and 137 Cs were 2.4 × 10 3 and 4.8 × 10 4 Bq/ml, respectively. When the radioactivity was decaycorrected to March 11, 2011, they were (5.1 ± 0.02) × 10 4 and (5.9 ± 0.007) × 10 4 Bq/ml, respectively, and the 134 Cs/ 137 Cs radioactivity ratio was 0.86 ± 0.01.
The concentration of 137 Cs was decreased to 63 Bq/ml for the Cs-eliminated drain water, and peaks of 125 Sb showed up. The concentration of 125 Sb in the drain water was 14 Bq/ml. It is reported that Sb was released by the oxidation of LWR fuel cladding by steam 38 . While not only 125 Sb but also 60 Co were observed for the qualitative analysis of the Cs-eliminated drain water, the concentration of 60 Co were less than its detection limit for quantitative analysis, 0.7 Bq/ml, due to a small amount of sample, 30 μl. Figure 3 shows the liquid scintillation spectrum of the Sr measurement sample separated from DW-Sr sample. The spectrum consists of β-rays emitted from 90 Sr and 90 Y. The maximum energies were shifted to lower energy side because of chemical quenching due to HNO 3 . Recovery rate of Sr for chemical separation was (92 ± 8)%. The obtained 90 Sr concentration was (47 ± 4) Bq/ml at the measurement time, and (55 ± 5) Bq/ml at the time point of the accident at the FDNPS.

Analysis of β-emitting nuclides.
There is no stable Tc isotope. Because Re and Tc tend to form ReO 4 − and TcO 4 − , respectively, and show similar behavior, Re was used as a carrier of Tc. The recovery rate of Re after all chemical separation processes was 99.5%, and this result supports that the chemical separation procedure is suitable for the 99 Tc separation. However, the concentration of 99 Tc in the drain water was less than detection limit, 0.012 Bq/ml.  www.nature.com/scientificreports/ Analysis of iodine species. In this study, organic and molecular iodine were not measured because they seemed to be vaporized during long period until sampling and analysis on September 12th, 2016, and on November 17-18th, 2020, respectively. Furthermore, it was reported that the largest fraction of the radioiodine present in BWR condensate was found to be I −39 , and molecular I 2 was hydrolyzed and disproportionated in water to produce I − and IO 3 −40 . Therefore, these species were considered dominant species and measured. Table 1 shows the concentrations of iodine species in the drain water. As the total inorganic iodine, 20 ± 3 and 21 ± 2 ng/ml of 127 I and 129 I, respectively, were obtained and the values were the same order of the reported 129 I concentration in the accumulated water before treatment 36 . It was estimated that the amounts of 127 I and 129 I in Unit 1 core were 2.14 × 10 3 and 9.44 × 10 3 g/core, respectively 41 , whereas reported 127 I concentrations in the rainwater collected from Fukushima prefecture were a few ng/ml level 42 . Therefore, 127 I in the drain water is considered a mixture of Unit 1-originated 127 I and rainwater-originated 127 I. Although the chemical form might be changed until the analysis and measurement, most iodine was observed as I − in the present time; approximately 90% of iodine was I − and 10% was IO 3 − in both isotopes. In case of the seawater collected at the offshore Fukushima on June 3-17th, 2011, the I − /IO 3 − ratio for 129 I different from that for nature-originated 127 I was observed; the 129 I originated from FDNPS existed mainly as I − form but nature-originated 127 I mainly as IO 3 − form 43 . However, our result shows similar I − /IO 3 − ratio for both isotopes, indicating isotope exchange reaction reached equilibrium. . Therefore, Mo is an important element to consider chemical form of radiocesium. Mass discrimination effect was evaluated to measure Mo isotope ratios precisely. Figure 4 shows the relationship between (R true /R meas ) ratio and Δm for the measurement of Mo standard solution using Agilent 7700 and Element 2 with relational expression. Noticeable mass discrimination effect was observed for the measurement using Agilent 7700, suggesting that the isotope ratios should be corrected by the mass discrimination factor obtained from the slope of the relational expression. On the other hand, mass discrimination effect for Element 2 was relatively small, and their dispersion was less than 1.5%. Therefore, Mo isotopes in the drain water sample was measured by Element 2 and the obtained isotope ratio was used without correction in this study. Table 2 shows the Mo isotope ratios in the Mo standard solution and the drain water after separation using the TEVA resin. In addition, the isotope ratios of Mo generated in the Unit 1 were estimated by ORIGEN code 41,44 . Isotope ratios for the Mo standard solution after separation using TEVA resin agreed with those of natural isotope ratios, indicating isotope effect is negligible during the chemical separation, whereas Mo isotope ratios for drain water was different from the natural isotope ratios and those originated from Unit 1. It is considered that nature-originated Mo and Unit 1-originated Mo were mixed in the drain water. Molybdenum-92 in the drain water is considered nature-originated isotope because the fission yield of 92 Mo is approximately 10 order magnitude lower than those of 95,97,98,100 Mo 44 . Then, count rates of nature-originated Mo isotopes in the drain water were calculated from the count rate of 92 Mo and natural isotope ratio. Next, count rates of Mo isotopes  www.nature.com/scientificreports/ originated from Unit 1 were calculated by subtracting the calculated count rates of nature-originated Mo in the drain water from the measured count rates of Mo isotopes for the drain water. Molybdenum isotope ratios originated from Unit 1 in the drain water were calculated and listed in Table 2. They were roughly agreed with those calculated by ORIGEN code, indicating that the drain water contains Unit 1-originated Mo isotopes. This is the first measurement result of the stable Mo isotopes released from FDNPS. The concentrations of the total Unit 1-originated stable Mo and nature-originated Mo in the drain water were 0.13 and 0.82 μg/L, respectively. Considering that it is reported that sub-μg/L quantities of Mo are expected in rainwater 46 , main source of natureoriginated Mo in the drain water is probably rainwater.

Analysis of Mo isotopes. Stable
Analysis of Th, U, Pu and Am. Thorium and U were not detected. The limit of detection of ICP-MS measurement for Th and U were 0.0006 and 0.0013 ng/mL, respectively, and this means that the concentrations of Th and U in the drain water were less than 0.12 and 0.25 ng/mL, respectively, because the drain water was diluted 200 times for the measurement. Also, Pu and Am were not detected in the drain water. Here, the recovery rates for Pu and Am obtained from the analytical results of 242 Pu or 243 Am-spiked drain water samples were 93% and 101%, respectively, and the limits of detection for 242 Pu and 243 Am were both 0.002 Bq.

Discussion
Mole ratios of determined isotopes. The measured radioactivity concentrations of target nuclides in the drain water were decay-corrected to the time point of the accident at FDNPS, March 11th, 2011, and then, the radioactivity ratios of the target nuclides to 137 Cs were calculated (see Table 3). TEPCO and some researchers reported radioactivity concentrations of 134 Cs and 137 Cs in stagnant water in the FDNPS [47][48][49] . The 134 Cs/ 137 Cs radioactivity ratios for stagnant water of Unit 1 decay-corrected to March 11th, 2011 were varied from 0.89 to 0.97 and the averages reported by Komori et al. and Nishihara et al. were 0.93 ± 0.03 and 0.92 ± 0.02, respectively. Furthermore, the radioactivity ratio reported by Komori et al. was 0.91 ± 0.2 for the stagnant water sampled before October, 22th 2011 to avoid mixture of stagnant water from Unit 2 due to transportation of stagnant water from Unit 1 to Unit 2. Our value, 0.86, is slightly smaller than these reported values. It is reported that neutron spectrum and flux are not uniform throughout a reactor core 50,51 . In consequence, variety occurs in the Cs isotopic composition as a function of location within the core: the upper side had smaller 134 Cs/ 137 Cs radioactivity ratio. In addition, Snow et al. suggested that (i) when power excursion and/or loss of coolant happened rapidly, Cs isotope ratio would be similar to total core ratio, ii) when coolant loss happened gradually over the course of minutes to hours, radiocesium component would be changed 51 . Compare to the variety of Cs isotope composition in reactor core, the difference in the obtained 134 Cs/ 137 Cs radioactivity ratios is small and close to the ratio in the Unit 1 core. Therefore, power excursion and/or loss of coolant is possibly happened rapidly.
The measured 134 Cs/ 137 Cs radioactivity ratio for stagnant water of Unit 1 was clearly smaller than those for the stagnant water of Units 2 and 3 (1.00 ± 0.03), and clearly smaller 134 Cs/ 137 Cs radioactivity ratio was observed at north-northwest site near FDNPS and Oshika Peninsula [27][28][29]48 . Although released amounts of radionuclides by vent operation of the Unit 1 was estimated smaller than those by explosion, the 134 Cs/ 137 Cs radioactivity ratio could become an index for the source term analysis.
The  and these values were close to our value. In addition, it is reported that 90 Sr concentration in the accumulated water increased gradually over time and reached almost comparable to 137 Cs concentration in early 2012 55 . Since transportation process of 90 Sr during accident and after accident would be different, the sampling location and time should be noted for consideration. This complicated transportation processes of Sr may cause wide variation of 90 Sr/ 137 Cs radioactivity ratio for environmental samples [56][57][58][59] .
In order to discuss the chemical form of radiocesium, mole ratio is important rather than radioactivity ratio. Therefore, radioactivity concentrations were converted to mole concentrations, and the mole concentrations of the target nuclides were divided by the mole concentration of 137 Cs to obtain their mole ratios to 137 Cs. It is considered that the radionuclides in the drain water were originated from vent operation of the Unit 1. Therefore, the amounts of radionuclides in the Unit 1 core were considered as original inventory. The mole ratios of the target nuclides to 137 Cs in the Unit 1 core were calculated by using the reported values of ORIGEN calculation and listed in Table 3. Furthermore, the mole ratios to 137 Cs obtained from the measurement were divided by the mole ratios to 137 Cs obtained from ORIGEN calculation (hereafter referred to as the ratio of drain water to Unit 1 core ratios). These results are summarized in Table 3. The order of ratios of the drain water to Unit 1 core ratios was 129 I > 137 Cs≈ 134 Cs > 125 Sb > Mo > 90 Sr. It is reported that Mo release increased under oxidizing conditions with sufficient water vapor inside reactor core through the formation of volatile species 60 . However, the amount of Mo was quite low compared to that of Cs in the case of this study, suggesting that the formation of Cs 2 MoO 4 had been suppressed under steam-starved conditions.
Based on the ORIGEN calculation, isotope mole ratio of 129 I to the total I and 137 Cs to the total Cs in Unit 1 core were calculated to be 78.69% and 40.27%, respectively, at the accident time 41 From the isotope mole ratio and measured concentration of 129 I and 137 Cs in the drain water, the concentration of the total I and the total Cs were calculated to be 2.1 × 10 −10 and 3.4 × 10 −10 mol/ml, respectively, and the ratio of the total I to the total Cs was 0.6 in the drain water. Compared to the ratio of the total I to the total Cs in the Unit1 core, 0.08, the ratio in the drain water was approximately one order magnitude larger. This trend implies that major chemical form of I was molecular I rather than CsI.

Conclusion
In order to study the behavior of radionuclides released during venting of Unit 1 at FDNPS, radiochemical analysis was conducted for the drain water sampled from the drain pit of the exhaust stack shared between Units 1 and 2.
Although γ-ray spectra of the drain water showed only peaks related to 134 Cs and 137 Cs, 60 Co and 125 Sb were detected after elimination of radiocesium using ammonium phosphomolybdate. The decay-corrected 134 Cs/ 137 Cs radioactivity ratio to March 11th, 2011 was 0.86.
In this study, Unit 1-originated stable Mo isotopes were clearly detected. Their amounts were quite low compared to Cs, suggesting that the formation of Cs 2 MoO 4 was suppressed under the accident condition.
Inorganic iodine chemical forms in samples collected at the FDNPS were analyzed, and it was found that approximately 90% of iodine existed as I − and 10% as IO 3 − in the samples. Although it is a big subject to www.nature.com/scientificreports/ extrapolate the present result to accident time, the information related to the iodine chemistry was obtained for the first time.
The ratios of the mole concentration ratios of nuclides to 137 Cs in the reactor core to those in the drain water was calculated: The order was 129 I > 137 Cs≈ 134 Cs > 125 Sb > Mo > 90 Sr. The larger ratio for 129 I implies that iodine was released as molecular iodine rather than CsI.
The results and information obtained from this study could be used as source terms to analyze contamination of environment, and would be applied for the accident analysis of Unit 1 at FDNPS to discuss chemical form of released radionuclides and reactor condition of the accident time.