Analysis of Mo Isotopes Originating from the Reactor Core of the Fukushima Daiichi Nuclear Power Station Using Extraction Chromatographic Separation and ICP-MS/MS Measurement with an Oxygen Dynamic Reaction

Abstract

Oxygen dynamic reactions with Zr, Mo, and Ru were elucidated to analyze Mo isotopes originating from the reactor core of the Fukushima Daiichi Nuclear Power Station. The main products were ZrO⁺, MoO₂ ⁺, and Ru⁺, respectively. The measured Mo isotope ratio in a mixed solution containing 1 ng/mL each of Zr, Mo, and Ru, using ICP-MS/MS with a 0.30 mL/min flow rate of the oxygen reaction cell, agreed within 4% with that of the Mo solution, indicating that MoO₂ ⁺ was successfully separated from ZrO⁺ and Ru⁺. The developed method was applied to the analysis of Mo isotopes in smear samples collected in reactor buildings at the Fukushima Daiichi Nuclear Power Station following dissolution and extraction chromatographic separation. A clearly elevated Mo/¹³⁷Cs ratio was observed in the smear sample collected on the second floor of the fuel handling machine room on the operating floor in Unit 2. In contrast, the ratios for smear samples collected from other floors of the Unit 2 reactor building were comparable to those for the composition in the reactor core.

Introduction

Stable Mo isotopes have high neutron-induced fission yields (cumulative fission yields of ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo, and ¹⁰⁰Mo from ²³⁵U under thermal neutron conditions are 0.06497, 5.432 × 10^–6, 0.06045, 0.05701, and 0.06789). Because the boiling points of Mo and MoO₃ and melting point of UO₂ are 4610, 1155, and 2878 °C, respectively, MoO₃ is easily evaporated, while Mo is hardly evaporated during the melt progression of the reactor core. If core melt progression occurs under an oxidizing atmosphere with water vapor, the chemical form of Mo is likely to be MoO₃ which can evaporate depending on the oxygen potential of the atmosphere. In this way, Mo isotopes originating from the reactor core serve as indicators for estimating atmospheric conditions during the core melt progression. Therefore, smear samples have been collected inside the reactor buildings of the Fukushima Daiichi Nuclear Power Station (1F) to analyze nuclides, including Mo isotopes. In addition, one of the most important fission products in severe accidents, ¹³⁷Cs, can potentially form Mo compounds such as Cs₂MoO₄. −

Previously, Mo isotopes in drain water from the exhaust stack shared by Units 1 and 2 of 1F were analyzed using sector field ICP-MS after chemical separation with a TEVA resin column − due to its small mass discrimination effects (ca. 1.5%). In contrast, significant mass discrimination effects were observed when using a quadrupole type of ICP-MS measurement. Typically, mass discrimination effects are corrected using the relationship between mass difference and the ratio of the true isotope ratio to the measured value. , In this study, mass discrimination effects were eliminated by preparing calibration curves for each Mo isotope as all target Mo isotopes were included in the Mo standard solution.

Isobaric interfering nuclides in the measurement of stable Mo isotopes using ICP-MS include ^92,94,96Zr and ^96,98,100Ru. Although the chemical separation of Mo from Zr and Ru is performed using a TEVA resin column, non-negligible amounts of Zr and Ru isotopes may still be introduced from the experimental environment, as the concentrations of the Mo isotope in smear samples are extremely low (10^–12–10^–10 g/mL in measurement solutions). Recently, the dynamic reaction cell has become a powerful tool for eliminating isobaric interferences in ICP-MS/MS measurements. − For example, ⁹⁰Sr⁺ can be separated from ⁹⁰Zr⁺ through the selective reaction of Zr with O₂-gas by changing ⁹⁰Zr⁺ to ⁹⁰Zr¹⁶O⁺; the m/z of ⁹⁰Zr shifts from 90 to 106. − In this study, the reactions of Zr, Mo, and Ru with O₂-gas were elucidated, and MoO₂ ⁺ was successfully separated from ZrO⁺ and Ru⁺. Furthermore, the developed conditions were applied to measure Mo isotopes in smear samples collected inside the reactor building of 1F.

Experimental Section

Optimization of Reaction Cell Conditions

HNO₃ solutions (1 mol/L) containing 1 ng/mL Zr, Mo, or Ru individually were prepared to study their reactions with O₂-gas during ICP-MS/MS measurement (Agilent 8900, Agilent Technologies, Inc., Japan). The flow rate of the O₂-gas was varied from 0 to 0.80 mL/min in 0.05 mL/min increments. The selected m/z values to measure Zr⁺, Mo⁺, and Ru⁺ were 90, 95, and 101, respectively, because these mass numbers are unique to each element. In addition, m/z values of +16, +32, +48, and +64 were measured to analyze oxidation products MO⁺ MO₂ ⁺, MO₃ ⁺, and MO₄ ⁺ (M = Zr, Mo, Ru). The abundance ratio was calculated by dividing the count of each individual m/z by the sum of the counts for M⁺, MO⁺ MO₂ ⁺, MO₃ ⁺, and MO₄ ⁺ at each O₂-gas flow rate.

A molybdenum solution containing 1 ng/mL Mo and a mixed solution containing 1 ng/mL Zr, Mo, and Ru were used for Mo isotope analysis. The analyzed m/z values were 92, 94, 95, 96, 97, 98, and 100 in the no-gas mode. For the ICP-MS/MS measurement of Mo in O₂-gas mode, the first quadrupole mass filter was set to 92, 94, 95, 96, 97, 98, and 100, and the second quadrupole mass filter was set to 124, 126, 127, 128, 129, 130, and 132, corresponding to the reaction with O₂-gas. The O₂-gas flow rate was set to 0.30 and 0.6 mL/min.

Treatment of Smear Samples

The sampling points of smear samples and sample IDs are summarized in Table .

1. Sampling Points of Smear Samples and Sample IDs.

ID	sampling point
U2RB-1FW	Unit 2, 1st floor, wall
U2RB-2FW	Unit 2, 2nd floor, wall
U2RB-3FW	Unit 2, 3rd floor, wall
U2RB-4FW	Unit 2, 4th floor, wall
U2RB-FHM-2FF	Unit 2, 5th floor, fuel handling machine room 2F, floor
U3RB-2FW-1	Unit 3, 2nd floor, wall

The samples were cut into five pieces, and γ-ray emitting nuclides in each piece were measured using a high-purity Ge semiconductor detector (HPGe, BE3830, Mirion Technologies Canberra). The distance between each piece of smear sample and the detector was set at 10 cm to reduce the effects of the configuration. The efficiency and energy of γ-rays were corrected using a mixed activity standard source (421 type, Japan Radioisotope Association). One piece of smear sample was placed in a PTFE beaker, and 0.1 mL standard solutions of Cs, Sr, and Re (Fujifilm Wako Pure Chemical Co., 1 mg/mL) were added as carriers. Next, 5 mL of HNO₃ (68%, TAMAPURE-AA-10 grade, Tama Chemical Co.) and 1 mL of H₂O₂ (30–35%, guaranteed reagent, Fujifilm Wako Pure Chemical Co.) were added to the beaker and heated on a hot plate to evaporate to dryness. This digestion using HNO₃ and H₂O₂ was repeated one more time. Then, 3 mL of HNO₃ and 2 mL of HF (38%, TAMAPURE-AA-100 grade, Tama Chemical Co.) were added to the residue and heated to evaporate to dryness. This digestion using HNO₃ and HF was repeated once. To convert the residue into the nitrate form, 1 mL of HNO₃ was added and evaporated to dryness. The residue was dissolved in 5 mL of 1 mol/mL HNO₃, and to ensure dissolution, the solution was left to stand overnight, and then ultrasonic irradiation was conducted. The solution was filtered (0.1 μm, resolve filter, Eichrom) into a 15 mL tube (Digitube, GL Sciences), and 1 mol/mL HNO₃ was added to the filtrate to bring the total volume to 10 mL. This solution is termed the dissolved solution. The γ-ray emitting nuclides in the dissolved solution were also measured using HPGe. A known activity of ¹³⁴Cs, ¹³⁷Cs, and ⁶⁰Co standard solutions (Japan Radioisotope Association) was added into a 15 mL tube, and 1 mol/mL HNO₃ was added to adjust the total volume to 10 mL, creating a standard solution source with the same configuration as the dissolved solution. The efficiency of the dissolved solution was corrected by using this standard solution source. The recovery rate of ¹³⁷Cs during the dissolution procedure was calculated using the ¹³⁷Cs radioactivity of the piece of the smear sample and the dissolved solution.

Aliquots of the dissolved solution were transferred into two vials, and a known amount of Mo solution was added to one dissolved solution. These solutions were evaporated to dryness, and these residues were dissolved in 2 mL of 4 mol/L HCl solutions to prepare Mo-spiked and nonspiked samples.

Mo was purified using a previously reported method. Briefly, 0.5 mL of TEVA resin was packed into columns and conditioned with 5 mL of 1 mol/L HNO₃, 3.4 mL of ultrapure water, and 2 mL of 4 mol/L HCl. The Mo-spiked sample and nonspiked samples were loaded onto the columns. After the columns were rinsed with 4 mol/L HCl followed by ultrapure water, the extracted Mo was recovered using 1 mol/L HNO₃. The Mo isotope concentrations (92, 94, 95, 96, 97, 98, and 100) in the recovered solution were measured using ICP-MS/MS in the O₂-gas mode with a +32 m/z shift.

The recovery (%) of the column separation procedure was calculated using eq :

$$\mathrm{recovery}=\frac{{[\mathrm{Mo}]}_{\mathrm{Mo}-\mathrm{spiked}}-{[\mathrm{Mo}]}_{\mathrm{non}-\mathrm{spiked}}}{{[\mathrm{Mo}]}_{\mathrm{spiked}}}\times 100$$ 1

where [Mo]_Mo‑spiked and [Mo]_nonspiked refer to the measured amounts of ⁹²Mo in the Mo recovery fractions of the Mo-spiked and nonspiked samples, respectively, and [Mo]_spiked refers to the spiked amount of ⁹²Mo.

Mo originating from the 1F reactor core is mixed with natural Mo from the surrounding sampling and experimental environment. Therefore, the concentrations of Mo isotopes originating from the 1F reactor core were calculated using a previously reported method. The isotope ratio of Mo originating from the reactor core differs from that of natural Mo. Although the natural abundance of ⁹²Mo is relatively high, 14.649%, ⁹²Mo is scarcely produced through nuclear fission. Accordingly, the amounts of natural Mo isotopes were estimated from the ⁹²Mo content and its natural abundance and subtracted from the observed amounts of Mo isotopes to calculate those originating from the 1F reactor core. The total Mo concentration originating from the 1F reactor core (Mo_1F) was obtained by summing ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo, and ¹⁰⁰Mo originating from the 1F reactor core.

Results and Discussions

Investigation of the Generated Ion Species of Zr, Mo, and Ru in the O₂-Gas Reaction Cell

The dependency of the abundance ratios of M⁺, MO⁺, MO₂ ⁺, MO₃ ⁺, and MO₄ ⁺ (M = Zr, Mo, and Ru) on the O₂-gas flow rate is shown in Figure a–c. The abundance ratio of Zr⁺ decreased rapidly with increasing O₂-gas flow rate, and ZrO⁺ became the dominant species over 0.1 mL/min of the O₂-gas flow rate. Although the lower rate of ZrO₂ ⁺ was generated in the range of 0.1 to 0.5 mL/min, its formation became negligible above 0.5 mL/min. Less than 10% of ZrO₃ ⁺ and ZrO₄ ⁺ were generated even with an increased O₂-gas flow rate. In the case of Mo, the dominant species was MoO₂ ⁺, with the other species remaining below 1% when the O₂-gas flow rate exceeded 0.3 mL/min. By contrast, Ru produced few oxidation products; the dominant species was Ru⁺; and the abundance ratios of RuO⁺, RuO₂ ⁺, RuO₃ ⁺, and RuO₄ ⁺ were less than 6.8, 1.7, 1.1, and 0.03%, respectively. In this way, dominant oxide ions differ among the Zr, Mo, and Ru. Therefore, Mo can be separated from Zr and Ru using an O₂-gas reaction cell.

1.

Dependency of abundance ratios of M⁺, MO⁺, MO₂ ⁺, MO₃ ⁺, and MO₄ ⁺ on the O₂-gas flow rate. (a) M = Zr, (b) M = Mo, and (c) M = Ru.

Although an O₂-gas flow rate of 0.6 mL/min was the most effective for suppressing the production of ZrO₂ ⁺, the relative signal intensity of ⁹⁵MoO₂ ⁺ in the O₂-gas mode compared to Mo⁺ in the no-gas mode decreased to 13%. In contrast, the relative signal intensity was 49% when the O₂-gas flow rate was 0.3 mL/min, under which condition 96% of Mo was converted to MoO₂ ⁺. Therefore, 0.3 mL/min was selected to measure the Mo isotope ratio. The Mo solution and the mixed solution containing Zr, Mo, and Ru were measured using no-gas and O₂-gas modes to confirm the effect of the O₂-gas reaction cell. The m/z values of 92, 94, 95, 96, 97, 98, and 100 were measured in the no-gas mode. The first Q-mass was set to m/z of 92, 94, 95, 96, 97, 98, and 100, and the second Q-mass was set to 124, 126, 127, 128, 129, 130, and 132 for O₂-gas mode. Abundance was calculated as the count for each m/z divided by the total count multiplied by 100 (Table ). In the case of the no-gas mode, the abundance in the mixed solution differed from that of Mo due to the presence of Zr and Ru isotopes. In contrast, both the abundances for the Mo solution and the mixed solution agreed within 4% in the O₂-gas mode. This result indicates that counts from Zr and Ru were effectively eliminated and that MoO₂ ⁺ was selectively measured.

2. Measured Mo Isotope Composition (Weight Percent) in Mo and Mixed Solutions Using No-Gas and O₂-Gas Modes.

		no-gas mode		O2-gas mode
Mo isotopes	natural weight percent	Mo solution	mixed solution	Mo solution	mixed solution
92Mo	14.03	13.9 ± 0.1	19.5 ± 0.1	13.6 ± 0.1	13.9 ± 0.1
94Mo	8.99	9.2 ± 0.1	16.6 ± 0.1	8.8 ± 0.1	9.1 ± 0.1
95Mo	15.70	15.8 ± 0.1	9.2 ± 0.1	15.7 ± 0.1	15.5 ± 0.1
96Mo	16.67	16.5 ± 0.1	15.8 ± 0.1	17.0 ± 0.1	16.5 ± 0.1
97Mo	9.68	9.5 ± 0.1	6.2 ± 0.1	9.7 ± 0.1	9.5 ± 0.1
98Mo	24.79	24.9 ± 0.1	17.1 ± 0.1	24.8 ± 0.1	24.9 ± 0.1
100Mo	10.15	10.2 ± 0.1	15.0 ± 0.1	10.5 ± 0.1	10.4 ± 0.1

The measured abundance for the Mo solution in both modes slightly differed from the natural abundance due to mass discrimination effects, where heavier isotopes showed higher abundance. Therefore, calibration curves were prepared for individual Mo isotopes using the total Mo amount in the standard solutions and natural abundance. The slopes of the calibration curves were plotted against the m/z of MoO₂ ⁺ in Figure . Heavier Mo isotopes exhibited higher slopes, and a proportional relationship was obtained, with a correlation factor of 0.996. Since there is no radioactivity standard for ⁹³ Mo, which is an important nuclide for inventory evaluation of radioactive waste, its calibration curve is obtained by interpolating using this relationship.

2.

Relationship between the slope of the calibration curve of Mo isotopes and m/z of MoO₂ ⁺. Error bars indicate the standard deviations of slopes.

Analysis of Mo Isotopes in Smear Samples

After the dissolution of the smear samples, Mo was separated by using a TEVA resin column. The recovery of ¹³⁷Cs during dissolution and the recovery of Mo during chemical separation with the TEVA resin column are summarized in Table . The recovery of Mo during dissolution was 104 ± 3% (n = 3) when a known amount of Mo was added to a smear filter (blank sample) and subjected to the same dissolution procedure. Therefore, the Mo loss for smear samples during dissolution was considered negligible.

3. Recovery of ¹³⁷Cs during Dissolution and Recovery of Mo during Chemical Separation Using the TEVA Resin Column.

ID	recovery of 137Cs (%)	recovery of Mo (%)
U2RB-1FW	95 ± 3	96 ± 7
U2RB-2FW	98 ± 7	110 ± 11
U2RB-3FW	100 ± 2	94 ± 8
U2RB-4FW	100 ± 1	95 ± 6
U2RB-FHM-2FF	72 ± 1	67 ± 1
U3RB-2FW-1	78 ± 1	113 ± 1

The Mo isotope concentrations in the Mo fraction were measured by the ICP-MS/MS in O₂-gas mode. To evaluate the influence of Zr and Ru isotopes, m/z = 90 and 101, which correspond exclusively to Zr and Ru isotopes, respectively, for the no-gas mode and the +32 shift of them, 122 and 133, for the O₂-gas mode were monitored for the measurement of Mo fractions. Although an average signal of 4353 cps (2113–8462 cps, corresponding to ca. 0.00126–0.0231 ng/mL) was observed of ⁹⁰Zr for in the no-gas mode, the corresponding signal for the +32 shift of m/z = 90, that is 122, decreased to 142 cps (60–217 cps) in the O₂-gas mode, indicating that a small portion of Zr was contained in the Mo fraction but the influence of Zr isotopes was effectively eliminated due to the mass shift. By contrast, both signals of 101 for no-gas and 133 for the O₂-gas modes were background level (0.00017 ng/mL), indicating that Ru was negligible in these measured samples.

The calculated mole concentrations of Mo isotopes that originated from the 1F reactor core are summarized in Table . Samples were measured 10 times to evaluate counting statistics. The counting statistics of ⁹²Mo were propagated to estimate the amounts of natural Mo. Uncertainties in the Mo isotopes originating from the 1F reactor core were calculated by using counting statistics for measured values and the estimated natural Mo. The recovery rate of Mo by chemical separation was determined from the difference in ⁹²Mo amounts between spiked and nonspiked samples and the added amount. The concentrations of Mo isotopes were corrected using the recovery rate. However, when recovery rates exceeded 100%, correction using recovery rates was not performed. The uncertainties associated with chemical separation and dilution were not included in the uncertainties for the Mo concentrations originating from the 1F reactor core. The concentrations of ^{95+96+97+98+100}Mo were summed up to obtain the total Mo concentration originating from the 1F reactor core, Mo_1F. The isotope ratios for ^{95,96,97,98,100}Mo are summarized in Table . Nishihara et al. evaluated the inventory of nuclides in the irradiated uranium pellet and the activated cladding tube of zirconium alloy in the reactor cores of the 1F with the use of the ORIGEN2 code. Using the evaluated inventories of Mo isotopes, the Mo isotope ratios in reactor cores of Units 2 and 3 at the time of the accident were calculated. Similarly, natural Mo isotope ratios were calculated for ^{95,96,97,98,100}Mo, excluding ^92,94Mo. Weight percent values for the measured Mo isotopes are summarized in Table S1 of the Supporting Information. The weight percents of measured Mo isotopes for U2RB-1FW are nearly comparable to those of natural Mo isotopes. Therefore, the Mo isotopes originating from the 1F reactor were masked by the natural Mo isotopes, and reliable quantification was not possible for U2RB-1FW. In contrast, the weight percent values of measured Mo isotopes in the other samples differ from those of natural Mo isotopes, indicating the presence of Mo isotopes originating from the 1F reactor core. The calculated Mo isotope ratios originating from the 1F reactor core, summarized in Table , generally agree with those calculated from the results of ORGEN2 code calculation. It is considered that the mole concentrations of Mo isotopes originating from 1F were well estimated. Although Mo isotope ratios originating from the 1F reactor core range from 0.21 to 0.28 for ^95,97,98,100Mo, that for ⁹⁶Mo is 1 order of magnitude lower (0.010, corresponding to 1%). Because of the few percent of counting statistics inherent in ICP-MS/MS measurement, this 1% difference for ⁹⁶Mo was not statistically significant, and the calculated mole concentrations for ⁹⁶Mo have large uncertainties.

4. Mole Concentrations of Mo Isotopes and Total Mo, Mo_1F, Originating from the 1F Reactor Cores in Dissolved Solutions (mol/mL).

ID	95Mo	96Mo	97Mo	98Mo	100Mo	Mo1F
U2RB-1FW	ND	ND	ND	ND	ND	ND
U2RB-2FW	(1.7 ± 0.8) × 10–13	(1.3 ± 29.2) × 10–14	(2.0 ± 0.4) × 10–13	(2.1 ± 0.9) × 10–13	(2.2 ± 0.4) × 10–13	(8.1 ± 3.2) × 10–13
U2RB-3FW	(1.9 ± 1.0) × 10–13	(1.6 ± 46.7) × 10–14	(1.9 ± 0.6) × 10–13	(2.2 ± 1.3) × 10–13	(2.6 ± 0.6) × 10–13	(8.8 ± 5.0) × 10–13
U2RB-4FW	(5.0 ± 1.2) × 10–13	(4.5 ± 69.3) × 10–14	(5.5 ± 0.7) × 10–13	(6.7 ± 1.7) × 10–13	(6.4 ± 0.9) × 10–13	(2.4 ± 0.7) × 10–12
U2RB-FHM-2FF	(1.5 ± 0.1) × 10–11	(7.2 ± 2.9) × 10–13	(1.7 ± 0.1) × 10–11	(1.7 ± 0.1) × 10–11	(1.9 ± 0.1) × 10–11	(6.8 ± 0.1) × 10–11
U3RB-2FW-1	(4.7 ± 1.3) × 10–13	(2.1 ± 13.4) × 10–14	(5.9 ± 0.6) × 10–13	(5.2 ± 1.5) × 10–13	(5.7 ± 0.9) × 10–13	(2.2 ± 0.4) × 10–12

5. Mo Isotope Ratios (Mole Fraction) Originating from the 1F Reactor Core and Mo_1F/¹³⁷Cs Mole Ratio .

ID	95Mo	96Mo	97Mo	98Mo	100Mo	Mo1F/137Cs
U2RB-2FW	0.21	0.016	0.24	0.26	0.27	6.8
U2RB-3FW	0.22	0.018	0.22	0.25	0.29	1.6
U2RB-4FW	0.21	0.019	0.23	0.28	0.27	1.4
U2RB-FHM-2FF	0.22	0.010	0.25	0.25	0.27	250
U3RB-2FW-1	0.22	0.0035	0.27	0.24	0.27	0.62
ORIGEN2 code Unit 2	0.22	0.010	0.25	0.25	0.28	3.8
ORIGEN2 code Unit 3	0.21	0.010	0.25	0.25	0.28	3.8
natural Mo	0.21	0.22	0.13	0.32	0.13

^aMo isotope ratios using burn-up calculation results obtained with the ORIGEN2 calculation code are also listed.

The mole ratios of Mo_1F/¹³⁷Cs from the second floor to the fourth floor of Unit 2 were of the same order of magnitude as those calculated from fuel burn-up calculation results using the ORIGEN2 code. In contrast, the ratio for U2RB-FHM-2FF was significantly higher than the others. The fuel handling machine room is located on the operation floor, which is the fifth floor and is directly above the reactor core. Radioactive gas from the reactor core is considered to reach the operation floor by passing through the shield plug, as the underside of the shield plug was highly contaminated by ¹³⁷Cs. The higher Mo_1F/¹³⁷Cs ratio suggests that the meltdown proceeded under oxidation conditions with vapor and that Mo likely forms MoO₃ or Cs₂MoO₄, which could then have evaporated. The Mo_1F/¹³⁷Cs of U3RB-2FW-1 from Unit 3 was an order of magnitude lower than that of U2RB-2FW from Unit 2. The reactor core conditions during the meltdown may have differed among the units, although additional analytical results are required for further discussion.

Conclusions

The main products of Zr, Mo, and Ru of the dynamic reaction cell using O₂-gas were ZrO⁺, MoO₂ ⁺, and Ru⁺, respectively. Therefore, Mo isotopes were effectively separated from the isobaric nuclides of ^92,94,96Zr and ^96,98,100Ru during mass-shift mode measurements using ICP-MS/MS.

Typically, mass discrimination effects are corrected by using the relationship between the mass difference and the ratio of the true isotope ratio to the measured ratio. In this study, individual calibration curves were prepared for ⁹²Mo, ⁹⁴Mo, ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo, and ¹⁰⁰Mo to eliminate mass discrimination effects. A proportional relationship between the slopes of the calibration curves and m/z values was observed. Therefore, the calibration curve for radioactive ⁹³ Mo, for which no standard solution exists, was obtained by interpolation.

Mo isotopes in smear samples collected inside the reactor building of 1F were analyzed by using the developed method. Because the measured concentrations of Mo isotopes were a mixture of Mo originating from natural and fission products, the concentrations of Mo isotopes originating from natural Mo were estimated based on ⁹²Mo, which is hardly produced by nuclear fission, and subtracted from their measured concentrations to obtain the concentrations of Mo isotopes originating from the reactor core. The abundance ratios of Mo isotopes originated from the reactor core agreed well with those calculated from the fuel burn-up results using the ORIGEN2 code, except for U2RB-1FW, which contained too much natural Mo to determine Mo concentrations reliably.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08680.

• Weight percent values for the measured Mo isotopes in smear samples (PDF)

Asako Shimada: corresponding author, conceptualization, methodology, investigation, writing–original draft. Yoshihisa Iida: resources, writing–review & editing. Yu Maruyama: resources, writing–review & editing, project admininstration.

This research was conducted as part of a project undertaken by the Nuclear Regulatory Agency.

The authors declare no competing financial interest.