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Feasibility study of large-scale production of iodine-125 at the high temperature engineering test reactor
Applied Radiation and Isotopes 140 (2018) 209–214
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Feasibility study of large-scale production of iodine-125 at the high temperature engineering test reactor
T
⁎
Hai Quan Hoa, , Yuki Hondab, Shimpei Hamamotoa, Toshiaki Ishiia, Nozomu Fujimotob, Etsuo Ishitsukaa a
Department of HTTR, Oarai Research and Development Center, Japan Atomic Energy Agency, 4002, Narita-cho, Oarai-machi, Higashi-Ibaraki-gun, Ibaraki 311-1393, Japan b Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka-shi, Fukuoka 819-0395, Japan
H I GH L IG H T S
HTGRs have large space available for thermal neutron irradiation applications. • The feasibility of a large-scale I production at the HTGRs was investigated. • The Runge-Kutta method was used to calculated quantities of various isotopes. • The days of irradiation is the optimal irradiation planning of I at the HTTR. • Four • The optimal HTTR design could produce 1.8 × 10 GBq/y of I activity. 125
125
5
125
A R T I C LE I N FO
A B S T R A C T
Keywords: HTGR HTTR radioisotope production Radioisotope Iodine-125 production Xenon gas Irradiation planning, Runge-Kutta method
The feasibility of a large-scale iodine-125 production from natural xenon gas at high-temperature gas-cooled reactors (HTGRs) was investigated. A high-temperature engineering test reactor (HTTR), which is located in Japan at Oarai-machi Research and Development Center, was used as a reference HTGR reactor in this study. First, a computer code based on a Runge-Kutta method was developed to calculate the quantities of isotopes arising from the neutron irradiation of natural xenon gas target. This code was verified with a good agreement with a reference result. Next, optimization of irradiation planning was carried out. As results, with 4 days of irradiation and 8 days of decay, the 125I production could be maximized and the 126I contamination was within an acceptable level. The preliminary design of irradiation channels at the HTTR was also optimized. The case with 3 irradiation channels and 20-cm diameter was determined as the optimal design, which could produce approximately 1.8 × 105GBq/y of 125I production.
1. Introduction Iodine-125 (125I) is an attractive radioisotope which can be used as a tracer due to a low gamma energy of 35 keV and a long half-life of 60 days (Herod et al., 2014). 125I is also used for the in-vitro diagnosis or a radiation source in the brachytherapy (Metyko et al., 2016; Saxena et al., 2009). Although the demand for 125I as a medical radioisotope is increasing in the recent year, the 125I production is still limited due to the fact that there is a small number of commercial 125I facilities and that their irradiation planning somehow is not steady. Therefore, development of an irradiation facility for the establishment of stable supply of large-scale 125I production is being pursued in order to meet the global demand.
⁎
125
I production is generally produced by neutron irradiation of natural xenon gas containing xenon-124 (124Xe), which becomes the unstable isotope 125Xe, followed by the 125I through the beta decay. Since the natural abundance of 124Xe is only 0.096%, it requires a large volume of natural xenon gas target to obtain an appropriated amount of 125 I production. However, the target volume is considerably restricted by the size of irradiation channel. One of the options for expanding the 125 I production is to irradiate the enriched 124Xe gas target. In this method the irradiation time of enriched 124Xe target should be short to minimize the 126I formation, and thereby the decay time. Only a small amount of 124Xe transfers to 125Xe during a short time irradiation. For example, after one day irradiation in the HTTR, only 0.06% of enriched 124 Xe is burned (Section 2.1). Thus, a facility to recover the remaining
Corresponding author. E-mail address: [email protected] (H.Q. Ho).
https://doi.org/10.1016/j.apradiso.2018.07.024 Received 18 April 2018; Received in revised form 3 July 2018; Accepted 18 July 2018 Available online 20 July 2018 0969-8043/ © 2018 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 140 (2018) 209–214
H.Q. Ho et al. 124 Xe gas must be installed because the cost of enriched 124Xe gas is relatively expensive. A proposed facility of 125I production from enriched 124Xe gas could be found in the US patent US5867546A (Hassal, 1997). High-temperature gas-cooled reactors (HTGRs) is one of the most promising reactors in the generation IV initiative. This type of reactor has excellent passive safety feature because of its graphite moderator, helium gas coolant, and tristructural-isotropic (TRISO) fuel particles (Kadak, 2005). Besides the electricity generation and hydrogen production, HTGRs also show the advantages for commercial irradiation applications such as great stable operation in long-term and large space available for irradiation at the reflector region. The HTGRs with a large size of irradiation channel is desired because it can be expected to improve the amount of 125I production. In Japan, the high-temperature engineering test reactor (HTTR) has been constructed to establish and upgrade the basic technologies for the HTGRs. Development of the multi-purpose use will be an important key for future commercialization of the HTGRs. Many irradiation regions are reserved in the HTTR to be served as a potential tool for an irradiation test reactor in order to promote innovative basic researches such as materials, fusion reactor technology, and radiation chemistry and so on (Saito et al., 1994). The top of reactor pressure vessel (RPV) is formed with 31 standpipes, including control rod standpipes, irradiation standpipes, and standpipes for other instrumentation. The irradiation standpipes are utilized to introduce a closed system including experimental equipment and specimens into the core without opening the RPV. Therefore, loading and unloading the sample under high temperature and high pressure during operation are possible in the HTTR. In a previous study (Ho et al., 2018), the HTTR showed the potential of using neutron transmutation doping (NTD) method to produce an n-type spherical silicon solar cell, which has low fabrication cost and high conversion efficiency. This study shows a new irradiation application using HTTR, in which the feasibility of large-scale 125I production from natural xenon gas was investigated. The optimizations of configuration of irradiation channel as well as irradiation planning of natural xenon gas were also carried out in order to produce as large an amount of 125I as possible without significant contaminations of unexpected radioisotopes.
2. Optimization of irradiation planning
Additionally, 125I may absorb neutron with a large neutron absorption cross-section of about 890 barn, resulting in a formation of the 126I radionuclide. The existence of 126I, as well as 128I, contaminates the 125I product as they give some harmfulness for medical uses. In order to mitigate the contamination of 126I and 128I, the irradiation time and decay time must be optimized. Therefore, it is necessary to calculate the activity of these radioisotopes during and after irradiation. In the literature, several studies developed computer codes to optimize the irradiation planning for 125I production (Joshi et al., 2012; Martinho et al., 1984). However, these codes do not open for general use and the optimizations of irradiation planning were only for the production of small quantities of 125I for research purposes. For the large-scale 125I production at the HTTR, an optimization of irradiation planning is necessary. In this study, a computer program based on the Runge-Kutta method (Cheney and Kincaid, 2004) was developed to calculate the quantities of isotopes in Fig. 1. This computer program was verified by comparing with the reference results published in IAEATechnical Document No.1340, which shows the irradiation planning of the natural and enriched xenon gas targets. For the comparison, the initial conditions such as neutron flux, mass of xenon gas target, irradiation time, and decay time were set to the same values as those of the reference cases. Tables 1 and 2 show a good agreement between the results from this study and the results from the reference for the cases of natural and enriched xenon gas targets, respectively. For instance, in the case of 15 g natural xenon gas target, as shown in Table 1, 125I activity and 126I contamination calculated in this study (case 2) were 28.2GBq and 0.91%, respectively, comparing to 29.2GBq and 0.77% in the reference (case 1). In the case of 0.4 g enriched xenon gas target, as shown in Table 2, case 6 in this study gave 53.2GBq of 125I activity and 0.02% of 126 I contamination, comparing to 50.7GBq and 0.02%, respectively, in case 5 from the reference. The difference in results between this study and the reference is expected to the differences in nuclear data library and the calculation method, which were not mentioned in the IAEATechnical Document No.1340. It can be seen in Table 2 that the irradiation time of enriched xenon gas is short to prevent the formation of 126I. After one day irradiation in the HTTR (case 8), only 0.06% of 124Xenon transfers to 125Xe. Therefore, a facility to recover the expensive enriched 124Xe gas should be installed if the enriched 124Xe gas target is used.
2.1. Irradiation of natural xenon gas
2.2. Optimization of irradiation time and decay time
A block diagram of various nuclear reactions during and after irradiation of 124Xe is shown in Fig. 1. After being created by the decay of 125 Xe, 125I is changed to a stable tellurium-125 (125Te) isotope.
It should be noted that the unexpected gamma rays emitted from I and 128I may cause problems for patients during the radiotherapy or diagnosis process. The ratio of 126I and 128I activity to 125I activity 126
Fig. 1. Formation of isotopes of interest during irradiation of natural xenon gas (Martinho et al., 1984). 210
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Table 1 Verification of calculated Case Thermal flux (cm−2 s−1) Target Mass of target, g Irradiation time, hours Cooling time, days 125 I activity, GBq 126 I contamination, %
Table 2 Verification of calculated Case Thermal flux (cm−2 s−1) Target Mass of target, g Irradiation time, hours Cooling time, days 125 I activity, GBq 126 I contamination, %
125
I in cases of natural xenon target.
1 (IAEA) 5 × 1013 Natural 15 200 40 29.2 0.77
125
2 (this study)
3 (IAEA)
4 (this study)
200 40 28.2 0.91
300 45 40.3 0.89
300 45 38.4 1.10
I in cases of enriched xenon target.
5 (IAEA) 5 × 1013 Enriched 0.4 10 20 50.7 0.02
6 (this study) 124
7 (IAEA)
8 (this study)
24 20 121.2 0.11
24 20 126.6 0.10
Xe (99.99%)
10 20 53.2 0.02
Fig. 3. Calculation results in case of 10-days irradiation scheme.
should be controlled less than a limit value. The calculation results showed that 128I activity is about four orders of magnitude lower than that of the 126I activity. In addition, the half-life of 128I is only 25 min in comparison to 13 days of 126I. Therefore, the contamination of 128I was considered to be omitted. In this section, the optimization of irradiation planning was implemented to obtain the maximum 125I activity while keeping the contamination ratio of 126I less than 1.0% (IAEA, 2003; Baker and Gerrard, 1972). It was assumed that 1 g natural xenon gas target was irradiated in this section. The calculated thermal neutron flux at the irradiation channels of the HTTR was about 3.2 × 1013 cm−2 s−1. The irradiation time varied from 6 h to 30 days, while the decay time was prolonged until the contamination ratio of 126I was below 1.0%. The calculation results of 125I production in the cases of 12 h and 10 days irradiation are shown in Figs. 2 and 3, respectively. It can be seen that the 126I/125I activity ratio increased as the increase of irradiation time, reaching the maximum values at the end of irradiation period. After irradiation, the contamination ratio decreased because of the lower half-life of 126I (13 days) than that of 125I (60 days). In case of 12 h irradiation as shown in Fig. 2, the contamination ratio is less than the limit value of 1%, so that the 125I could be used directly in subsequent fabrication process without decay period. In case of 10 days irradiations, as shown in Fig. 3, the contamination ratio after irradiation is about 4.6%. Therefore, it is needed to wait for about 35 days to
Fig. 4. Total time.
125
I activity per year with different irradiation time and decay
guarantee the decrease of 126I contamination. The total 125I activity produced in a year with different irradiation time and different decay time, in which the 126I contamination radio was within the limit value of 1.0%, is shown in Fig. 4. It can be seen that if the irradiation time was 2 days or less, further decay process was not necessary. The total activity of 125I increased from about 10–55 GBq/g (natural Xe)/year when increasing the irradiation time from 6 h to 2 days. The amount of 125I reached the maximum value of about 85GBq/g (natural Xe)/year in the case of 4 days irradiation. In this case, 8 days of decay period is required to reduce the contamination of 126I less than 1.0%. According to Fig. 4, if the irradiation time was larger than 4 days, the amount of 125I decreased even if the irradiation time was increased. For example, the total 125I production reduced from 85 to 40GBq/g (natural Xe)/year when the irradiation time increased from 4 to 30 days. This is because increasing the irradiation time reduces the number of batches per year, leading to a reduction in the annual 125I production. The optimal results showed that, in order to obtain the maximum 125 I yield and to keep the minimum contamination of 126I, 4 days of irradiation with 8 days of decay was considered as the optimal irradiation planning for 125I production at the HTTR.
Fig. 2. Calculation results in case of 12 h irradiation scheme. 211
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Fig. 5. Designs of the HTTR.
3. Optimization of irradiation channels
optimization was carried out while keeping the depression of reactivity less than a tentative value of 0.5%Δk/k. The temperature of xenon gas target was 800 K, which is the temperature of reflector region of the HTTR. The pressure of xenon gas target was set to 3.9 atm, which was less than 4.0 atm of helium gas coolant of the HTTR in normal operation. The lower pressure is to ensure that the helium coolant cannot be contaminated by a leak of irradiated xenon gas in accident scenarios.
3.1. HTTR modeling Design characteristic of the HTTR, a prismatic block type of HTGRs, is shown in Fig. 5. The reactor core consists of fuel blocks, control rod guide blocks, irradiation blocks, and replaceable graphite blocks. All of the hexagonal blocks are formed in the same prismatic shape with 36 cm in across flat and 58 cm in height. There are 30 fuel columns in the active core, surrounding by various types of reflector blocks. The outer most of reactor core is a permanent reflector made of graphite. Each fuel block contains 31 or 33 fuel rods and 2 burnable poison rods. Each fuel rod comprises axially 14 fuel compacts, in which about 13,000 CFPs are stochastically embedded in an annular graphite matrix. The criticality calculations were carried out using the Monte-Carlo MVP code (Nagaya et al., 2005) and the JENDL-4.0 nuclear data library (Shibata et al., 2011). Calculation condition was the same as the normal operation condition at full power of the HTTR. The number of neutron histories per batch and the number of batches were 50,000 and 2000 (excluding 50 skip batches), respectively. As can be seen in Fig. 5, there are 12 replaceable blocks in the reflector region, which could be used for the purpose of investigating various types of irradiation applications. This section optimized the design of irradiation channels at these replaceable blocks. The HTTR configuration with a different number of irradiation channels is shown in Fig. 6. The irradiation channels were arranged uniformly to keep the change of neutron flux identical in the radial direction. The number of irradiation channels changed from 3 to 12 while the diameter of each channel was adjusted from 2 to 25 cm. The xenon gas target is usually loaded into a capsule or pipe made of zirconium alloy zircaloy-2, which is a very nonreactive alloy transparent to neutrons. Therefore, is was assumed that the neutronic effect of capsule or pipe material was omitted in the criticality calculations. It should be noted that the existence of natural xenon gas makes keff decrease because of its high neutron absorption cross-section. The
3.2. Optimization of irradiation channels The reactivity insertion when changing the number of irradiation channels and the diameter can be observed in Fig. 7. It can be seen that increasing the diameter at the same number of irradiation channels leaded to decrease the negative reactivity insertion. Also, the more number of irradiation channels made the keff decrease deeper. This is because increasing the diameter increases the amount of xenon gas target, leading to depress the neutron flux and therefore the keff decreases. This study tentatively set the maximum depression of reactivity larger than − 0.5%Δk/k so that only the cases in the upper region of the red line in Fig. 7 were acceptable. Table 3 shows the optimal design of HTTR with different irradiation channels. It can be seen that in order to keep the reactivity insertion less than the limitation, the diameter has to decrease when adding more number of irradiation channels. For example, the diameter decreased from 20 to 10 cm when increasing the number of irradiation channels from 3 to 12. Section 2.3 calculated optimal 125I activity when 1 g natural xenon gas target was irradiated with the thermal neutron flux of 3.2 × 1013 cm−2 s−1. The neutronic calculation results also showed that the thermal neutron flux at reflector region of HTTR did not change much even if the number of irradiation channels was changed from 3 to 12. Therefore, the total 125I activity produced in one-time irradiation at the optimal irradiation channels in Table 3 could be easily estimated. It can be seen that the 125I activity was almost the same for all cases, with about 1.9 × 105GBq/y being the maximum value in the case of 9 irradiation channels. However, from the viewpoint of economic issues, 212
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Fig. 6. Arrangement of irradiation channels.
Ram et al., 2013), they will be investigated in more detail in future study. In contrast, 126I contamination is chemically inseparable from 125 I and thus its level is only be mitigated through decay of the irradiated natural xenon gaseous. Optimization of the irradiation and decay time at the specific HTTR conditions showed that with 4 days irradiation and 8 days decay, the annual production of 125I could be maximized. In this case, the contamination of 126I (< 1.0%) was also within the permissible level. The irradiation channels for irradiating natural xenon gas target were optimized by changing the design of replaceable reflector blocks at the HTTR. It was found that 9 irradiation channels with a diameter of 12 cm could give the maximum 125I production of about 1.9 × 105GBq/ y. However, from the viewpoint of economic issues, three irradiation channels with a diameter of 20 cm was considered as the optimal design, even as the construction cost is significantly reduced. In this optimal design, the amount of 125I produced in a year is about 1.8 × 105GBq in comparison with 3.0 × 103GBq of total 125I supplied in Japan in 2016. It should be noted that this study tentatively set the negative reactivity insertion limit to − 0.5%Δk/k. This value could be enlarged, for example to − 1.0%Δk/k, depending on some operating conditions of the HTTR, resulting in a double increase in the 125I production. It was assumed that the capsule or pipe are made of zircaloy-2, which is widely used in the LWR. The zircaloy-2 may not be used in the HTTR, where the temperature and pressure may reach 800 K and 4 MPa, respectively, in nominal operation. One of the alloy candidates is nickel-based alloy 600, which is existing in the irradiation hole of the HTTR as a guide tube covering the neutron and gamma detectors. The alloy 600 has a greater effect on the keff than that of the zirconium alloy because it has a higher neutron absorption cross section than zirconium alloy. Therefore, the diameter of irradiation channel and the amount of 125 I should be reduced to compensate for the negative reactivity insertion of the alloy 600. Table 4 shows the optimal design of irradiation channels when alloy 600 is used. It can be seen that the diameter of the optimal design (3 holes) reduces from 20 cm to 14 cm, and the amount of 125I also decreases to 8.8 × 104GBq/y. It should be noted that even the alloy 600 can be stable under abnormal or accident scenarios, the integrity of capsule or pipe is also important. Further study will be carried out to design the capsule or pipe to prevent gas leakage during irradiation as well as accidents.
Fig. 7. Reactivity insertion with various irradiation channel schemes. Table 3 Optimal HTTR design with various number of irradiation channels. Number of irradiation channels
Optimal diameter (cm)
Mass of natural xenon gas (g)
Reactivity insertion (%∆k/k )
125 I activity (GBq/y)
3 6 9 12
20.0 14.0 12.0 10.0
2.1 × 103 2.1 × 103 2.3 × 103 2.1 × 103
−0.5 −0.5 −0.5 −0.4
1.8 × 105 1.8 × 105 1.9 × 105 1.8 × 105
the case of 3 irradiation channels with a diameter of 20 cm was considered as the optimal design as it requires the lowest construction cost. 4. Discussion Although the demand for 125I as a medical radioisotope continues to increase, the production of 125I is restricted. One of the reasons is the low natural atomic abundance of 124Xe. It is of interest that the HTGRs, with large space available for irradiation purposes, can compensate for the low abundance of 124Xe in natural xenon gas. As a result, a stable and large-scale 125I production at the HTGRs could be expected. There are various isotopes arising from the irradiation of natural xenon gas target, resulting in a difficulty to calculate the activity of some isotopes such as 125I and 126I. A computer code based on the Runge-Kutta method was developed to estimate the change of these isotopes during and after irradiation. The computer code was verified with a good agreement between the calculation results and the reference data from the IAEA-Technical Document No.1340. The formation of various radioisotopes such as 134Cs, 137Cs, and 126I contaminates the final 125I product. These cesium contaminations could be separated by using the reference chemical process (Joshi et al., 2012;
Table 4 Optimal design of irradiation channels and amount of used.
213
125
I when alloy 600 is
Number of irradiation channels
Optimal diameter (cm)
Mass of natural xenon gas (g)
Reactivity insertion (%∆k/k )
125
3 6 9 12
14.0 8.0 5.5 –
1.0 × 103 6.7 × 102 4.8 × 102 –
−0.5 −0.5 −0.5 –
8.8 × 104 5.7 × 104 4.1 × 104 –
I activity (GBq/y)
Applied Radiation and Isotopes 140 (2018) 209–214
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References
This study emphasized the optimization of the irradiation channels and the irradiation planning. A chemical process to separate 125I from the irradiated gaseous is important for quality of the final product. Several separation methods were introduced such as batch process, circulating loop process, and batch-opened loop process (Joshi et al., 2012; Baker and Gerrard, 1972). The methods to end up a large amount of natural xenon gas after irradiation is also important because it contains various radioisotopes. In this study, the inexpensive natural xenon gas is used. So, the simplest way is to keep the irradiated xenon gas in a high-pressure gas cylinder. It is easy to be stored in a radio-waste repository. Another idea is to release the irradiated xenon gas directly to the air after separating various radioisotopes. This method may not attractive because it requires a complicated facility to remove many types of radioisotopes. In order to select an appropriated method for large-scale 125I production at the HTTR, these methods will be investigated more detailed in the further study.
Baker, P.S., Gerrard, M., Feb. 1972. (ORNL-IIC-3) Iodine-125. Contract W-7405-eng-26. 34 p. Dep. NTIS. 26:20556. Cheney, W., Kincaid, D., 2004. Numerical Mathematics and Computing, 4th ed., Brooks/ Cole Thomson Learning, Belmont, CA. Hassal, S.B., 1997. inventors. Method and apparatus for production of radioactive iodine. US patent 5,867,546. Apr, 8. Herod, M.N., et al., 2014. Extraction of 129I and 127I via combustion from organic rich samples using 125I as a quantitative tracer. J. Environ. Radioact. 138, 323–330. Ho, H.Q., et al., 2018. Proposal of a neutron transmutation doping facility for n-type spherical silicon solar cell at high-temperature engineering test reactor. Appl. Radiat. Isot. 135, 12–18. IAEA-Technical Document No. 1340, 2003. Manual for Reactor Produced Radioisotopes. International Atomic Energy Agency, Vienna. Joshi, P.V., et al., 2012. Production of 125I from neutron irradiation of natural Xe gas and a wet distillation process for radiopharmaceutical applications. Ind. Eng. Chem. Res. 51, 8575–8582. Kadak, A.C., 2005. A future for nuclear energy: pebble bed reactors. Int. J. Crit. Infrastruct. 1 (4), 330–345. Martinho, E., et al., 1984. 125I production: neutron irradiation planning. Int. J. Appl. Radiat. Isot. 35 (10), 933–938. Metyko, J., et al., 2016. Verification of I-125 brachytherapy source strength for use in radioactive seed localization procedures. Appl. Radiat. Isot. 112, 62–68. Nagaya, Y., et al., 2005. MVP/GMVP II: General Purpose Monte Carlo Codes for Neutron and Photon Transport Calculations Based on Continuous Energy and Multigroup Methods. JAERI 1348, Japan Atomic Energy Research Institute. Ram, R., et al., 2013. Usefulness of nano-zirconia for purification and concentration of 125I solution for medical applications. Appl. Radiat. Isot. 82, 351–358. Saxena, S.K., et al., 2009. Development of a new design 125I-brachytherapy seed for its application in the treatment of eye and prostate cancer. Appl. Radiat. Isot. 67, 1421–1425. Saito, S., et al., 1994. Design of High Temperature Engineering Test Reactor (HTTR). Japan Atomic Energy Research Institute, pp. 1332. Shibata, K., et al., 2011. JENDL-4.0: a new library for Nuclear Science and Engineering. J. Nucl. Sci. Technol. 48, 1–30.
5. Conclusion This paper demonstrated the feasibility study of the stable and large-scale 125I production at the HTTR. The Runge-Kutta method was used to solve the system of differential equations and thus the density of isotopes arising from the neutron irradiation of natural xenon gas. The irradiation planning at the HTTR conditions was optimized so as to achieve the maximum 125I production while keeping the contamination of 126I less than 1.0%. In this case, the irradiation time is 4 days along with 8 day of decay period. Results of the optimal HTTR design for irradiation of the natural xenon gas showed that we could design three irradiation channels with a diameter of 20 cm that could produce approximately 1.8 × 105GBq of 125I per year.
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Feasibility study of large-scale production of iodine-125 at the high temperature engineering test reactor
T
⁎
Hai Quan Hoa, , Yuki Hondab, Shimpei Hamamotoa, Toshiaki Ishiia, Nozomu Fujimotob, Etsuo Ishitsukaa a
Department of HTTR, Oarai Research and Development Center, Japan Atomic Energy Agency, 4002, Narita-cho, Oarai-machi, Higashi-Ibaraki-gun, Ibaraki 311-1393, Japan b Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka-shi, Fukuoka 819-0395, Japan
H I GH L IG H T S
HTGRs have large space available for thermal neutron irradiation applications. • The feasibility of a large-scale I production at the HTGRs was investigated. • The Runge-Kutta method was used to calculated quantities of various isotopes. • The days of irradiation is the optimal irradiation planning of I at the HTTR. • Four • The optimal HTTR design could produce 1.8 × 10 GBq/y of I activity. 125
125
5
125
A R T I C LE I N FO
A B S T R A C T
Keywords: HTGR HTTR radioisotope production Radioisotope Iodine-125 production Xenon gas Irradiation planning, Runge-Kutta method
The feasibility of a large-scale iodine-125 production from natural xenon gas at high-temperature gas-cooled reactors (HTGRs) was investigated. A high-temperature engineering test reactor (HTTR), which is located in Japan at Oarai-machi Research and Development Center, was used as a reference HTGR reactor in this study. First, a computer code based on a Runge-Kutta method was developed to calculate the quantities of isotopes arising from the neutron irradiation of natural xenon gas target. This code was verified with a good agreement with a reference result. Next, optimization of irradiation planning was carried out. As results, with 4 days of irradiation and 8 days of decay, the 125I production could be maximized and the 126I contamination was within an acceptable level. The preliminary design of irradiation channels at the HTTR was also optimized. The case with 3 irradiation channels and 20-cm diameter was determined as the optimal design, which could produce approximately 1.8 × 105GBq/y of 125I production.
1. Introduction Iodine-125 (125I) is an attractive radioisotope which can be used as a tracer due to a low gamma energy of 35 keV and a long half-life of 60 days (Herod et al., 2014). 125I is also used for the in-vitro diagnosis or a radiation source in the brachytherapy (Metyko et al., 2016; Saxena et al., 2009). Although the demand for 125I as a medical radioisotope is increasing in the recent year, the 125I production is still limited due to the fact that there is a small number of commercial 125I facilities and that their irradiation planning somehow is not steady. Therefore, development of an irradiation facility for the establishment of stable supply of large-scale 125I production is being pursued in order to meet the global demand.
⁎
125
I production is generally produced by neutron irradiation of natural xenon gas containing xenon-124 (124Xe), which becomes the unstable isotope 125Xe, followed by the 125I through the beta decay. Since the natural abundance of 124Xe is only 0.096%, it requires a large volume of natural xenon gas target to obtain an appropriated amount of 125 I production. However, the target volume is considerably restricted by the size of irradiation channel. One of the options for expanding the 125 I production is to irradiate the enriched 124Xe gas target. In this method the irradiation time of enriched 124Xe target should be short to minimize the 126I formation, and thereby the decay time. Only a small amount of 124Xe transfers to 125Xe during a short time irradiation. For example, after one day irradiation in the HTTR, only 0.06% of enriched 124 Xe is burned (Section 2.1). Thus, a facility to recover the remaining
Corresponding author. E-mail address: [email protected] (H.Q. Ho).
https://doi.org/10.1016/j.apradiso.2018.07.024 Received 18 April 2018; Received in revised form 3 July 2018; Accepted 18 July 2018 Available online 20 July 2018 0969-8043/ © 2018 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 140 (2018) 209–214
H.Q. Ho et al. 124 Xe gas must be installed because the cost of enriched 124Xe gas is relatively expensive. A proposed facility of 125I production from enriched 124Xe gas could be found in the US patent US5867546A (Hassal, 1997). High-temperature gas-cooled reactors (HTGRs) is one of the most promising reactors in the generation IV initiative. This type of reactor has excellent passive safety feature because of its graphite moderator, helium gas coolant, and tristructural-isotropic (TRISO) fuel particles (Kadak, 2005). Besides the electricity generation and hydrogen production, HTGRs also show the advantages for commercial irradiation applications such as great stable operation in long-term and large space available for irradiation at the reflector region. The HTGRs with a large size of irradiation channel is desired because it can be expected to improve the amount of 125I production. In Japan, the high-temperature engineering test reactor (HTTR) has been constructed to establish and upgrade the basic technologies for the HTGRs. Development of the multi-purpose use will be an important key for future commercialization of the HTGRs. Many irradiation regions are reserved in the HTTR to be served as a potential tool for an irradiation test reactor in order to promote innovative basic researches such as materials, fusion reactor technology, and radiation chemistry and so on (Saito et al., 1994). The top of reactor pressure vessel (RPV) is formed with 31 standpipes, including control rod standpipes, irradiation standpipes, and standpipes for other instrumentation. The irradiation standpipes are utilized to introduce a closed system including experimental equipment and specimens into the core without opening the RPV. Therefore, loading and unloading the sample under high temperature and high pressure during operation are possible in the HTTR. In a previous study (Ho et al., 2018), the HTTR showed the potential of using neutron transmutation doping (NTD) method to produce an n-type spherical silicon solar cell, which has low fabrication cost and high conversion efficiency. This study shows a new irradiation application using HTTR, in which the feasibility of large-scale 125I production from natural xenon gas was investigated. The optimizations of configuration of irradiation channel as well as irradiation planning of natural xenon gas were also carried out in order to produce as large an amount of 125I as possible without significant contaminations of unexpected radioisotopes.
2. Optimization of irradiation planning
Additionally, 125I may absorb neutron with a large neutron absorption cross-section of about 890 barn, resulting in a formation of the 126I radionuclide. The existence of 126I, as well as 128I, contaminates the 125I product as they give some harmfulness for medical uses. In order to mitigate the contamination of 126I and 128I, the irradiation time and decay time must be optimized. Therefore, it is necessary to calculate the activity of these radioisotopes during and after irradiation. In the literature, several studies developed computer codes to optimize the irradiation planning for 125I production (Joshi et al., 2012; Martinho et al., 1984). However, these codes do not open for general use and the optimizations of irradiation planning were only for the production of small quantities of 125I for research purposes. For the large-scale 125I production at the HTTR, an optimization of irradiation planning is necessary. In this study, a computer program based on the Runge-Kutta method (Cheney and Kincaid, 2004) was developed to calculate the quantities of isotopes in Fig. 1. This computer program was verified by comparing with the reference results published in IAEATechnical Document No.1340, which shows the irradiation planning of the natural and enriched xenon gas targets. For the comparison, the initial conditions such as neutron flux, mass of xenon gas target, irradiation time, and decay time were set to the same values as those of the reference cases. Tables 1 and 2 show a good agreement between the results from this study and the results from the reference for the cases of natural and enriched xenon gas targets, respectively. For instance, in the case of 15 g natural xenon gas target, as shown in Table 1, 125I activity and 126I contamination calculated in this study (case 2) were 28.2GBq and 0.91%, respectively, comparing to 29.2GBq and 0.77% in the reference (case 1). In the case of 0.4 g enriched xenon gas target, as shown in Table 2, case 6 in this study gave 53.2GBq of 125I activity and 0.02% of 126 I contamination, comparing to 50.7GBq and 0.02%, respectively, in case 5 from the reference. The difference in results between this study and the reference is expected to the differences in nuclear data library and the calculation method, which were not mentioned in the IAEATechnical Document No.1340. It can be seen in Table 2 that the irradiation time of enriched xenon gas is short to prevent the formation of 126I. After one day irradiation in the HTTR (case 8), only 0.06% of 124Xenon transfers to 125Xe. Therefore, a facility to recover the expensive enriched 124Xe gas should be installed if the enriched 124Xe gas target is used.
2.1. Irradiation of natural xenon gas
2.2. Optimization of irradiation time and decay time
A block diagram of various nuclear reactions during and after irradiation of 124Xe is shown in Fig. 1. After being created by the decay of 125 Xe, 125I is changed to a stable tellurium-125 (125Te) isotope.
It should be noted that the unexpected gamma rays emitted from I and 128I may cause problems for patients during the radiotherapy or diagnosis process. The ratio of 126I and 128I activity to 125I activity 126
Fig. 1. Formation of isotopes of interest during irradiation of natural xenon gas (Martinho et al., 1984). 210
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Table 1 Verification of calculated Case Thermal flux (cm−2 s−1) Target Mass of target, g Irradiation time, hours Cooling time, days 125 I activity, GBq 126 I contamination, %
Table 2 Verification of calculated Case Thermal flux (cm−2 s−1) Target Mass of target, g Irradiation time, hours Cooling time, days 125 I activity, GBq 126 I contamination, %
125
I in cases of natural xenon target.
1 (IAEA) 5 × 1013 Natural 15 200 40 29.2 0.77
125
2 (this study)
3 (IAEA)
4 (this study)
200 40 28.2 0.91
300 45 40.3 0.89
300 45 38.4 1.10
I in cases of enriched xenon target.
5 (IAEA) 5 × 1013 Enriched 0.4 10 20 50.7 0.02
6 (this study) 124
7 (IAEA)
8 (this study)
24 20 121.2 0.11
24 20 126.6 0.10
Xe (99.99%)
10 20 53.2 0.02
Fig. 3. Calculation results in case of 10-days irradiation scheme.
should be controlled less than a limit value. The calculation results showed that 128I activity is about four orders of magnitude lower than that of the 126I activity. In addition, the half-life of 128I is only 25 min in comparison to 13 days of 126I. Therefore, the contamination of 128I was considered to be omitted. In this section, the optimization of irradiation planning was implemented to obtain the maximum 125I activity while keeping the contamination ratio of 126I less than 1.0% (IAEA, 2003; Baker and Gerrard, 1972). It was assumed that 1 g natural xenon gas target was irradiated in this section. The calculated thermal neutron flux at the irradiation channels of the HTTR was about 3.2 × 1013 cm−2 s−1. The irradiation time varied from 6 h to 30 days, while the decay time was prolonged until the contamination ratio of 126I was below 1.0%. The calculation results of 125I production in the cases of 12 h and 10 days irradiation are shown in Figs. 2 and 3, respectively. It can be seen that the 126I/125I activity ratio increased as the increase of irradiation time, reaching the maximum values at the end of irradiation period. After irradiation, the contamination ratio decreased because of the lower half-life of 126I (13 days) than that of 125I (60 days). In case of 12 h irradiation as shown in Fig. 2, the contamination ratio is less than the limit value of 1%, so that the 125I could be used directly in subsequent fabrication process without decay period. In case of 10 days irradiations, as shown in Fig. 3, the contamination ratio after irradiation is about 4.6%. Therefore, it is needed to wait for about 35 days to
Fig. 4. Total time.
125
I activity per year with different irradiation time and decay
guarantee the decrease of 126I contamination. The total 125I activity produced in a year with different irradiation time and different decay time, in which the 126I contamination radio was within the limit value of 1.0%, is shown in Fig. 4. It can be seen that if the irradiation time was 2 days or less, further decay process was not necessary. The total activity of 125I increased from about 10–55 GBq/g (natural Xe)/year when increasing the irradiation time from 6 h to 2 days. The amount of 125I reached the maximum value of about 85GBq/g (natural Xe)/year in the case of 4 days irradiation. In this case, 8 days of decay period is required to reduce the contamination of 126I less than 1.0%. According to Fig. 4, if the irradiation time was larger than 4 days, the amount of 125I decreased even if the irradiation time was increased. For example, the total 125I production reduced from 85 to 40GBq/g (natural Xe)/year when the irradiation time increased from 4 to 30 days. This is because increasing the irradiation time reduces the number of batches per year, leading to a reduction in the annual 125I production. The optimal results showed that, in order to obtain the maximum 125 I yield and to keep the minimum contamination of 126I, 4 days of irradiation with 8 days of decay was considered as the optimal irradiation planning for 125I production at the HTTR.
Fig. 2. Calculation results in case of 12 h irradiation scheme. 211
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Fig. 5. Designs of the HTTR.
3. Optimization of irradiation channels
optimization was carried out while keeping the depression of reactivity less than a tentative value of 0.5%Δk/k. The temperature of xenon gas target was 800 K, which is the temperature of reflector region of the HTTR. The pressure of xenon gas target was set to 3.9 atm, which was less than 4.0 atm of helium gas coolant of the HTTR in normal operation. The lower pressure is to ensure that the helium coolant cannot be contaminated by a leak of irradiated xenon gas in accident scenarios.
3.1. HTTR modeling Design characteristic of the HTTR, a prismatic block type of HTGRs, is shown in Fig. 5. The reactor core consists of fuel blocks, control rod guide blocks, irradiation blocks, and replaceable graphite blocks. All of the hexagonal blocks are formed in the same prismatic shape with 36 cm in across flat and 58 cm in height. There are 30 fuel columns in the active core, surrounding by various types of reflector blocks. The outer most of reactor core is a permanent reflector made of graphite. Each fuel block contains 31 or 33 fuel rods and 2 burnable poison rods. Each fuel rod comprises axially 14 fuel compacts, in which about 13,000 CFPs are stochastically embedded in an annular graphite matrix. The criticality calculations were carried out using the Monte-Carlo MVP code (Nagaya et al., 2005) and the JENDL-4.0 nuclear data library (Shibata et al., 2011). Calculation condition was the same as the normal operation condition at full power of the HTTR. The number of neutron histories per batch and the number of batches were 50,000 and 2000 (excluding 50 skip batches), respectively. As can be seen in Fig. 5, there are 12 replaceable blocks in the reflector region, which could be used for the purpose of investigating various types of irradiation applications. This section optimized the design of irradiation channels at these replaceable blocks. The HTTR configuration with a different number of irradiation channels is shown in Fig. 6. The irradiation channels were arranged uniformly to keep the change of neutron flux identical in the radial direction. The number of irradiation channels changed from 3 to 12 while the diameter of each channel was adjusted from 2 to 25 cm. The xenon gas target is usually loaded into a capsule or pipe made of zirconium alloy zircaloy-2, which is a very nonreactive alloy transparent to neutrons. Therefore, is was assumed that the neutronic effect of capsule or pipe material was omitted in the criticality calculations. It should be noted that the existence of natural xenon gas makes keff decrease because of its high neutron absorption cross-section. The
3.2. Optimization of irradiation channels The reactivity insertion when changing the number of irradiation channels and the diameter can be observed in Fig. 7. It can be seen that increasing the diameter at the same number of irradiation channels leaded to decrease the negative reactivity insertion. Also, the more number of irradiation channels made the keff decrease deeper. This is because increasing the diameter increases the amount of xenon gas target, leading to depress the neutron flux and therefore the keff decreases. This study tentatively set the maximum depression of reactivity larger than − 0.5%Δk/k so that only the cases in the upper region of the red line in Fig. 7 were acceptable. Table 3 shows the optimal design of HTTR with different irradiation channels. It can be seen that in order to keep the reactivity insertion less than the limitation, the diameter has to decrease when adding more number of irradiation channels. For example, the diameter decreased from 20 to 10 cm when increasing the number of irradiation channels from 3 to 12. Section 2.3 calculated optimal 125I activity when 1 g natural xenon gas target was irradiated with the thermal neutron flux of 3.2 × 1013 cm−2 s−1. The neutronic calculation results also showed that the thermal neutron flux at reflector region of HTTR did not change much even if the number of irradiation channels was changed from 3 to 12. Therefore, the total 125I activity produced in one-time irradiation at the optimal irradiation channels in Table 3 could be easily estimated. It can be seen that the 125I activity was almost the same for all cases, with about 1.9 × 105GBq/y being the maximum value in the case of 9 irradiation channels. However, from the viewpoint of economic issues, 212
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Fig. 6. Arrangement of irradiation channels.
Ram et al., 2013), they will be investigated in more detail in future study. In contrast, 126I contamination is chemically inseparable from 125 I and thus its level is only be mitigated through decay of the irradiated natural xenon gaseous. Optimization of the irradiation and decay time at the specific HTTR conditions showed that with 4 days irradiation and 8 days decay, the annual production of 125I could be maximized. In this case, the contamination of 126I (< 1.0%) was also within the permissible level. The irradiation channels for irradiating natural xenon gas target were optimized by changing the design of replaceable reflector blocks at the HTTR. It was found that 9 irradiation channels with a diameter of 12 cm could give the maximum 125I production of about 1.9 × 105GBq/ y. However, from the viewpoint of economic issues, three irradiation channels with a diameter of 20 cm was considered as the optimal design, even as the construction cost is significantly reduced. In this optimal design, the amount of 125I produced in a year is about 1.8 × 105GBq in comparison with 3.0 × 103GBq of total 125I supplied in Japan in 2016. It should be noted that this study tentatively set the negative reactivity insertion limit to − 0.5%Δk/k. This value could be enlarged, for example to − 1.0%Δk/k, depending on some operating conditions of the HTTR, resulting in a double increase in the 125I production. It was assumed that the capsule or pipe are made of zircaloy-2, which is widely used in the LWR. The zircaloy-2 may not be used in the HTTR, where the temperature and pressure may reach 800 K and 4 MPa, respectively, in nominal operation. One of the alloy candidates is nickel-based alloy 600, which is existing in the irradiation hole of the HTTR as a guide tube covering the neutron and gamma detectors. The alloy 600 has a greater effect on the keff than that of the zirconium alloy because it has a higher neutron absorption cross section than zirconium alloy. Therefore, the diameter of irradiation channel and the amount of 125 I should be reduced to compensate for the negative reactivity insertion of the alloy 600. Table 4 shows the optimal design of irradiation channels when alloy 600 is used. It can be seen that the diameter of the optimal design (3 holes) reduces from 20 cm to 14 cm, and the amount of 125I also decreases to 8.8 × 104GBq/y. It should be noted that even the alloy 600 can be stable under abnormal or accident scenarios, the integrity of capsule or pipe is also important. Further study will be carried out to design the capsule or pipe to prevent gas leakage during irradiation as well as accidents.
Fig. 7. Reactivity insertion with various irradiation channel schemes. Table 3 Optimal HTTR design with various number of irradiation channels. Number of irradiation channels
Optimal diameter (cm)
Mass of natural xenon gas (g)
Reactivity insertion (%∆k/k )
125 I activity (GBq/y)
3 6 9 12
20.0 14.0 12.0 10.0
2.1 × 103 2.1 × 103 2.3 × 103 2.1 × 103
−0.5 −0.5 −0.5 −0.4
1.8 × 105 1.8 × 105 1.9 × 105 1.8 × 105
the case of 3 irradiation channels with a diameter of 20 cm was considered as the optimal design as it requires the lowest construction cost. 4. Discussion Although the demand for 125I as a medical radioisotope continues to increase, the production of 125I is restricted. One of the reasons is the low natural atomic abundance of 124Xe. It is of interest that the HTGRs, with large space available for irradiation purposes, can compensate for the low abundance of 124Xe in natural xenon gas. As a result, a stable and large-scale 125I production at the HTGRs could be expected. There are various isotopes arising from the irradiation of natural xenon gas target, resulting in a difficulty to calculate the activity of some isotopes such as 125I and 126I. A computer code based on the Runge-Kutta method was developed to estimate the change of these isotopes during and after irradiation. The computer code was verified with a good agreement between the calculation results and the reference data from the IAEA-Technical Document No.1340. The formation of various radioisotopes such as 134Cs, 137Cs, and 126I contaminates the final 125I product. These cesium contaminations could be separated by using the reference chemical process (Joshi et al., 2012;
Table 4 Optimal design of irradiation channels and amount of used.
213
125
I when alloy 600 is
Number of irradiation channels
Optimal diameter (cm)
Mass of natural xenon gas (g)
Reactivity insertion (%∆k/k )
125
3 6 9 12
14.0 8.0 5.5 –
1.0 × 103 6.7 × 102 4.8 × 102 –
−0.5 −0.5 −0.5 –
8.8 × 104 5.7 × 104 4.1 × 104 –
I activity (GBq/y)
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References
This study emphasized the optimization of the irradiation channels and the irradiation planning. A chemical process to separate 125I from the irradiated gaseous is important for quality of the final product. Several separation methods were introduced such as batch process, circulating loop process, and batch-opened loop process (Joshi et al., 2012; Baker and Gerrard, 1972). The methods to end up a large amount of natural xenon gas after irradiation is also important because it contains various radioisotopes. In this study, the inexpensive natural xenon gas is used. So, the simplest way is to keep the irradiated xenon gas in a high-pressure gas cylinder. It is easy to be stored in a radio-waste repository. Another idea is to release the irradiated xenon gas directly to the air after separating various radioisotopes. This method may not attractive because it requires a complicated facility to remove many types of radioisotopes. In order to select an appropriated method for large-scale 125I production at the HTTR, these methods will be investigated more detailed in the further study.
Baker, P.S., Gerrard, M., Feb. 1972. (ORNL-IIC-3) Iodine-125. Contract W-7405-eng-26. 34 p. Dep. NTIS. 26:20556. Cheney, W., Kincaid, D., 2004. Numerical Mathematics and Computing, 4th ed., Brooks/ Cole Thomson Learning, Belmont, CA. Hassal, S.B., 1997. inventors. Method and apparatus for production of radioactive iodine. US patent 5,867,546. Apr, 8. Herod, M.N., et al., 2014. Extraction of 129I and 127I via combustion from organic rich samples using 125I as a quantitative tracer. J. Environ. Radioact. 138, 323–330. Ho, H.Q., et al., 2018. Proposal of a neutron transmutation doping facility for n-type spherical silicon solar cell at high-temperature engineering test reactor. Appl. Radiat. Isot. 135, 12–18. IAEA-Technical Document No. 1340, 2003. Manual for Reactor Produced Radioisotopes. International Atomic Energy Agency, Vienna. Joshi, P.V., et al., 2012. Production of 125I from neutron irradiation of natural Xe gas and a wet distillation process for radiopharmaceutical applications. Ind. Eng. Chem. Res. 51, 8575–8582. Kadak, A.C., 2005. A future for nuclear energy: pebble bed reactors. Int. J. Crit. Infrastruct. 1 (4), 330–345. Martinho, E., et al., 1984. 125I production: neutron irradiation planning. Int. J. Appl. Radiat. Isot. 35 (10), 933–938. Metyko, J., et al., 2016. Verification of I-125 brachytherapy source strength for use in radioactive seed localization procedures. Appl. Radiat. Isot. 112, 62–68. Nagaya, Y., et al., 2005. MVP/GMVP II: General Purpose Monte Carlo Codes for Neutron and Photon Transport Calculations Based on Continuous Energy and Multigroup Methods. JAERI 1348, Japan Atomic Energy Research Institute. Ram, R., et al., 2013. Usefulness of nano-zirconia for purification and concentration of 125I solution for medical applications. Appl. Radiat. Isot. 82, 351–358. Saxena, S.K., et al., 2009. Development of a new design 125I-brachytherapy seed for its application in the treatment of eye and prostate cancer. Appl. Radiat. Isot. 67, 1421–1425. Saito, S., et al., 1994. Design of High Temperature Engineering Test Reactor (HTTR). Japan Atomic Energy Research Institute, pp. 1332. Shibata, K., et al., 2011. JENDL-4.0: a new library for Nuclear Science and Engineering. J. Nucl. Sci. Technol. 48, 1–30.
5. Conclusion This paper demonstrated the feasibility study of the stable and large-scale 125I production at the HTTR. The Runge-Kutta method was used to solve the system of differential equations and thus the density of isotopes arising from the neutron irradiation of natural xenon gas. The irradiation planning at the HTTR conditions was optimized so as to achieve the maximum 125I production while keeping the contamination of 126I less than 1.0%. In this case, the irradiation time is 4 days along with 8 day of decay period. Results of the optimal HTTR design for irradiation of the natural xenon gas showed that we could design three irradiation channels with a diameter of 20 cm that could produce approximately 1.8 × 105GBq of 125I per year.
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