Fast neutron fluence estimation by measurement of 93m Nb and 93 Mo

. The possibility for estimation of fast neutron fluence from 93m Nb produced from Nb contained in structural component materials of nuclear power plants was investigated as an evaluation tool for extending plant operating lifetime. First, to establish a measurement method for 93m Nb in structural component materials, chemical separations and measurements of 93m Nb were carried out using specimens of fast neutron-irradiated XM-19 and X-750 alloy; these are used as structural component materials and contain Nb as a component. The 93m Nb activities obtained from the measurements of both XM-19 and X-750 alloy agreed well with the calculated values of 93m Nb, which were simply calculated from the evaluated fast neutron fluence. On the other hand, in the case of type 316L stainless steel, which contains Mo, Mo derived 93m Nb presents a problem, so an approach was used that estimates the amount of Mo derived 93m Nb based on 93 Mo measurements with the third specimen, neutron-irradiated type 316L stainless steel. The difference between the measured and calculated 93m Nb values was about two-fold, indicating that the estimation method for 93m Nb produced from Mo needs to be improved.


Introduction
One of the approaches for improving the estimation accuracy of fast neutron fluence experienced by nuclear power plant structural component materials is to estimate the fast neutron fluence based on the measurement of the activity of the activated nuclides in the materials. This means that, the results of measurements made after chemical separations of nuclides produced by the reaction between elements contained in the structural materials and fast neutrons are reflected in the estimation of the fast neutron fluence. 93m Nb produced by the 93 Nb(n, n') 93m Nb reaction between Nb and fast neutrons has a relatively long half-life of 16.13 years and is used to estimate long-term fast neutron fluence. Types 304 and 316L stainless steels (SSs) are mainly used for structural components such as core shrouds and upper grid plates in BWRs, and it has been reported that type 304 SS contains several ppm to several tens of ppm of Nb [1], which may allow estimation of fast neutron fluence by the n') 93m Nb reaction have been made using Nb present in structural components on the order of several tens of mass ppm [2][3][4]. On the other hand, in the case of a material containing Mo such as type 316L SS, 93m Nb is also produced from Mo in that structural material [3,4], so the amount of 93m Nb derived from Mo must be estimated. 93m Nb from Mo is produced by the electron capture decay of 93 Mo that was produced from the thermal neutron capture reaction of 92 Mo, which is one of the stable isotopes of Mo. Therefore, in the case of a material containing Mo, 93m Nb produced from both Mo and Nb is included, and if the activity of 93m Nb produced from Mo is not negligible, a correction is necessary to subtract the activity of 93m Nb produced from Mo from the 93m Nb measured activity. (Of course, if the activity of 93m Nb produced from Mo is overwhelmingly greater than the activity of 93m Nb produced from Nb, the fast neutron flux from 93 Nb(n, n') 93m Nb cannot be estimated.) As a method for accurately estimating the produced amount of 93m Nb from Mo, an estimation from the measured value of 93 Mo is effective, as 93 Mo is an intermediate product in the production of 93m Nb from Mo.
In this study, the 93m Nb measurement method was established using irradiated specimens of XM-19 and X-750 alloy containing Nb as a constituent element whose amount is within the material specifications such as set by the ASTM. The validity of the measurement method was verified by comparing the measured value with the calculated value of 93m Nb estimated from the evaluated fast neutron fluence. The validated 93m Nb measurement method was used to measure 93m Nb for the third specimen, an irradiated type 316L SS specimen. To estimate the amount of 93m Nb produced from Mo, 93 Mo was also measured for the above three specimens and the amount of 93m Nb produced from Mo was estimated. The activity of 93m Nb and 93 Mo of each specimen were measured by X-ray measurements with a Ge semiconductor detector after dissolving each specimen and separating 93m Nb and 93 Mo from each other by an ion exchange separation method. The chemical separation between 93m Nb and 93 Mo is necessary, because both are measured by X-ray measurements and both are measured at the same X-ray energy of 16.6 keV.

Specimens
Irradiated XM-19, X-750 alloy and type 316L SS, which are used as structural component materials, were used as specimens and there was one test piece for each. XM-19 and X-750 alloy were used as specimens containing Nb as a component to establish the 93m Nb measurement method. Table 1 summarizes their compositions listed on the respective mill test reports. In the absence of Nb or Mo concentration data on the mill test reports, the concentration was determined by measuring the solution of each specimen by inductively coupled plasma mass spectrometry (ICP-MS). The concentration measurements were performed after confirming sufficient accuracy in advance.
All specimens were irradiated in the Japan Materials Testing Reactor (JMTR) under the conditions shown in Table 2. The fast neutron fluence integrated by the specimens is equivalent to the cumulative irradiation in the core shrouds of Japanese BWRs during a period of 60 years.

Separation of Mo and Nb
The separation of Nb and Mo was performed by applying an ion exchange separation method using a hydrofluoric acid system, with reference to the 93m Nb separation method of Serén and Kekki [4] and the report of Fujimoto and Shimura [5] about element adsorption behaviors on an anion exchange resin in hydrofluoric acid and hydrochloric acid systems. Approximately 20 mg of each specimen was dissolved in a mixture of nitric acid (HNO3), hydrochloric acid (HCl), and hydrofluoric acid (HF) while heating on a hot plate. Then, the acidic solutions of dissolved specimens were evaporated to dryness by further hot plate heating. The residues were dissolved in 2 mol/L HF, and passed through an anion exchange column filled with DOWEX 1X-8 200-400 mesh. In the next step, 2 mol/L HF was passed through the column. This step flushed out the main structural material elements such as Fe, and the major radionuclides such as 60 Co, from the column. Next, Mo was eluted from the column using a mixture of 8 mol/L HF and 4 mol/L HCl. Finally, Nb was eluted from the column using 1 mol/L HCl.

Measurement of 93 Mo
The eluant of 8 mol/L HF and 4 mol/L HCl containing Mo was evaporated to dryness by heating on a hot plate and the residue was dissolved in 1 mol/L HNO3. Then, 1 mg of lanthanum was added to the solution, and lanthanum hydroxide precipitate was formed by adding ammonia water. In this case, Mo remained in the liquid phase, while 60 Co, which could not be removed by ion exchange separation, was included on the precipitate side. After a vacuum filtration, 100 μg of Mo was added to the collected filtrate, which was then adjusted to a slightly acidic condition as confirmed by pH paper testing by adding HCl. Then bromine water and an ethanolic solution of α-benzoin oxime were added to get Mo precipitate. The precipitate was collected on a membrane filter (47 mm diameter; 0.45 μm pore size). The activity of 93 Mo in the precipitate was determined with a high-purity germanium low energy photon spectrometer (LEPS) by measuring the 16.6 keV radiation emitted by 93 Mo. To estimate the recovery, the amount of Mo in the precipitate was determined by X-ray fluorescence (XRF). The LEPS and XRF apparatus were calibrated using standard samples.

Measurement of 93m Nb
To remove HF, the 1 mol/L HCl solution containing Nb was evaporated to dryness by heating on a hot plate and the residue was dissolved with 1 mol/L HCl. After 500 μg of lanthanum was added to the solution, ammonia water was added to precipitate lanthanum hydroxide, and Nb was coprecipitated. The precipitate was collected using a membrane filter (47 mm diameter; 0.45 μm pore size). The activity of 93m Nb in the precipitate was determined with the LEPS by measuring the 16.6 keV radiation emitted by 93m Nb. To estimate the recovery of Nb, the activity of 94 Nb in the precipitate was determined with a germanium semiconductor detector. The LEPS and germanium semiconductor detectors were calibrated using standard samples.

Activity of 93m Nb from 93 Nb
The activity of 93m Nb produced from 93 Nb (ANb93m(Nb)) is calculated as follows: where, NNb93 is the number of atoms of 93 Nb in the specimen, σNb93 is the cross section of the reaction 93 Nb(n, n') 93m Nb, φf is the fast neutron flux, λNb93m is the decay constant of 93m Nb, t is the irradiation time, and tm is the time from the end of irradiation to the measurement.
In the calculations of this study, the cross section of 93 Nb(n, n') 93m Nb and the fast neutron flux were assumed to be constant. For the fast neutron flux values, the values shown in Table  2 were used. For the cross section values, the values corresponding to 2 MeV, the average energy of the fission spectrum, were used. The activity of 93m Nb calculated from equation (1) was compared with the measured activity of 93m Nb for each specimen to confirm the validity of the measured values.

92
Mo produces 93 Mo by thermal neutron capture, and 93 Mo disintegrates to 93m Nb. In this study, it was assumed that all the 92 Mo reacted with neutrons to become 93 Mo, and all the 93 Mo formed decayed to 93m Nb. The activity of 93m Nb produced from 93 Mo (ANb93m(Mo)) is calculated as follows: Mo93 (e − λMo93 m − e − λNb93m m ) + Nb93m e − λNb93m m (3) where, N'Nb93m is the number of atoms of 93m Nb produced from 92 Mo during the period between irradiation and measurement, λMo93 is the decay constant of 93 where, NMo92 is the initial number of atoms of 92 Mo in the specimen, σMo92 is the cross section of 92 Mo(n, γ) 93 Mo, and φth is the thermal neutron flux. Here, φth is the thermal neutron flux that contributes to the reaction of 92 Mo(n, γ) 93 Mo, and it was estimated from the measured activity of 93 Mo along with σMo92. NMo93 in equation (4) can be estimated from equation (6) based on the measured activity of 93 Mo (AMo93).
When NMo93 is obtained, σMo92 φth can also be obtained from equation (4). Furthermore, once σMo92 φth is obtained, NNb93m can also be obtained from equation (5). Therefore, the procedure to obtain the activity of 93m Nb produced from Mo based on the measured activity of 93 Mo is the following.
First, the number of atoms of 93 Mo at the end of irradiation, NMo93, is obtained from the measured activity of 93 Mo by equation (6). Next, after substituting the obtained NMo93 into equation (4), the product of the cross section and thermal neutron flux, σMo92 φth, is obtained. Then, by substituting σMo92 φth into equation (5), the number of atoms of 93m Nb produced from 92 Mo at the end of irradiation, NNb93m, is obtained. By substituting NMo93 and NNb93m into equation (3), the number of atoms of 93m Nb produced from 92 Mo during the period between irradiation and measurement, N'Nb93m, is obtained. Finally, N'Nb93m is converted to the activity of 93m Nb, ANb93m(Mo), by equation (2).
In this study, the activity of 93m Nb produced from Mo in each specimen was evaluated using the measured activity of 93 Mo in each specimen and the above procedure. Table 3 lists the concentrations of Nb and Mo and the activities of 93m Nb and 93 Mo for each specimen. The Nb/Mo ratio shown in Table 3 Table 4 allows a comparison of the measured activity of 93m Nb (M) and the calculated activity of 93m Nb produced from Nb (C). The ratios of the measured and calculated values of XM-19 and X-750 alloy are 1.1 and 1.2, which are in relatively good agreement, and the measured values of 93m Nb obtained by the measurement method in this study are reasonable.

Results and discussion
On the other hand, in type 316L SS, the ratio of measured to calculated values is 2.8, and the agreement is poor. Focusing on the Nb/Mo ratio in each specimen, the ratio of type 316L SS is lower than the ratios of the other two. Baers and Hasanen [3] reported that the Nb/Mo ratio must be greater than a certain value (Nb/Mo ratio > 0.01 in their study) for the 93m Nb produced from Nb to be dominant. This means that when the Nb/Mo ratio is low, the proportion of 93m Nb produced from Mo in the activity of 93m Nb in the specimen becomes large, and 93m Nb produced from Mo cannot be neglected. In type 316L SS, the Nb/Mo ratio is low and the measured activity is higher than the calculated activity. This difference between measured and calculated activities suggests that the actual activity (i.e., the measured activity) was affected by 93m Nb produced from Mo, resulting in a discrepancy between the measured and calculated values. Table 5 shows the results of estimating the activity of 93m Nb produced from 92 Mo based on the measured activity of 93 Mo shown in Table 3. The calculated activity of 93m Nb produced from Mo in XM-19 and X-750 alloy is sufficiently small compared to the measured activity of 93m Nb as shown in Table 4. On the other hand, in type 316L SS, the calculated activity of 93m Nb produced from Mo is only about one order of magnitude smaller than the measured value, which is a non-negligible amount. Table 6 is a comparison of the measured activity of 93m Nb after subtracting the activity of 93m Nb produced from 92 Mo (M') and the calculated activity of 93m Nb produced from Nb (C). Even after subtracting the activity of 93m Nb produced from Mo, the ratio of measured to calculated values in type 316L SS is 2.4, which is a slight improvement, but there is still a discrepancy compared to the XM-19 and X-750 alloy values. The reason for the discrepancy between the calculated and measured values may be due to uncertainty in the measured value of 93 Mo, uncertainty in the calculation model for the amount of 93m Nb produced from 92 Mo, or other factors. The calculation of 93m Nb produced from 92 Mo should be done in detail using an analysis code such as ORIGEN2 [6].

Conclusions
In this study, the measurement method of 93m Nb was confirmed using three irradiation specimens of XM-19, X-750 alloy and type 316L SS. In addition, as an estimation method of Mo derived 93m Nb when the influence of 93m Nb produced from Mo cannot be ignored, an estimation method of 93m Nb produced from Mo based on the measured 93 Mo was examined. The measured activity of 93m Nb for XM-19 and X-750 alloy agreed well with the calculated activity assuming that 93 Nb is the source, and it was confirmed that the method used in this study can obtain reasonable 93m Nb measurements.
On the other hand, in type 316 L SS with low Nb concentration relative to Mo concentration, compared to XM-19 and X-750 alloy, the measured activity of 93m Nb did not agree well with the calculated activity assuming that 93 Nb is the source, and the effect of 93m Nb from Mo was confirmed. Therefore, 93m Nb from Mo was evaluated based on 93 Mo measurements and subtracted from the 93m Nb measurements. However, the difference between the measured and calculated values of 93m Nb was still about two-fold, indicating that the estimation method for 93m Nb produced from Mo needs to be improved. It is planned to investigate the amount of 93m Nb produced from Mo in detail in the future.
In addition, type 304 SS, which was not treated in this study, does not contain Mo as a constituent element, and there is a possibility that 93m Nb produced from Mo can be neglected, and it is planned to study the feasibility of fast neutron fluence estimation from Nb contained in type 304 SS also.