1. Research for isotope separation
2. Research for isotope production
1. Research for isotope separation
1.1 Generalized technique of isotope separation with crown ether immobilized resin
Crown ether, a ring compound, can be synthesized with a ring of varied sizes. The purpose is to develop a generalized technique of isotope separation using with a crown ether immobilized resin which has an adequate size of ring for a compound including target isotopes. We succeeded in separating 15N by displacement chromatography using with 18-crown-6 immobilized resin.
Nitrogen isotope separation by displacement chromatography was performed with ammonium salt methanol solution at 293 K of the column. Ammonium ion with 15N was enriched in the resin phase.
Enrichment factors (ε) obtained by experiments were compared with calculated values by Gaussian03. Calculated values agreed with experimental values and calculation simulated well that 15N was enriched in the resin phase.
Fig.1 Experimental apparatus of displacement chromatography.
Table1 Comparison of enrichment factors
1.2 Isotope separation by supercritical fluid chromatography
Lithium isotope separation by supercritical fluid chromatography was investigated using with supercritical carbon dioxide and LiCl methanol solution. Supercritical carbon dioxide is a hopeful solvent of the 21st century and reduces the effects on the environment.
Lithium isotope separation by displacement chromatography was performed with a resin packed column of 0.8 cm inner diameter and 100 cm length at 293 K under the pressure of 10, 12, 15 and 18 kPa. Lithium-7 and lithium-6 were enriched in the solution phase and in the resin phase, respectively. Enrichment factor became minimum at 15 MPa, though the difference between the values under each pressure was not large. Enrichment factor was obtained as 0.002-0.012.
Fig.2 Experimental apparatus of supercritical fluid chromatography.
Fig.3 Correlation between pressure and enrichment factor.
This is world first experiment of displacement chromatography using with supercritical carbon dioxide and we developed a technique of isotope separation which reduces the effects on the environment.
- "Recovery of Alkali Salt by Supercritical Fluid Leaching Method using
Carbon Dioxide," T. Watanabe, S. Tsushima, I. Yamamoto, O. Tomioka,
Y. Meguro, M. Nakashima, R. Wada, Y. Nagase and R. Fukuzato, Proc. 2nd
International Symposium on Supercritical Fluid Technology for Energy and
Environment Application, 363-366 (2004).
- "Use of supercritical carbon dioxide for chromatographic separation
of lithium isotopes," Y. Enokida, T. Watanabe and I. Yamamoto, Proc.
8th Workshop on Separation Phenomena in Liquids and Gases (CD-ROM), 1-11
1.3 Application of carbon isotope enrichment for environmental or historical sciences
Existing techniques of radioactive carbon chronology can not identify an age back more than 60 thousand years because 14C isotope ratio is infinitesimal as 1.2×10-12 and its half life is 5,730 years long. A technique which provides a reproducible enrichment factor for 14C isotope separation enables us to extend the range of measurement. We examined 14C isotope separation with a thermal diffusion column which was compact and had a large separation factor.
The thermal diffusion column made by glass was 195 cm height. The radii of the hot wire and cold wall were 0.075 mm and 3.5 mm, respectively. Carbon monoxide was used as the gas. Enrichment factors were measured for various temperature of the hot wire under the condition of 0.1 MPa, 248 K of cold wall and total reflux mode.
Total separation factor was observed as increasing with temperature and was 2.00 at 888 K. We obtained reproducible value at 776 K with ±2 % deviation. A subtle curvature of the glass column might result in the difference between experiments and analyses. On the other hand, we obtained high accuracy of reproducibility and demonstrated that the present method is applicable to enrichment of the sample for age determination.
Fig.4 Thermal diffusion column for carbon isotope separation.
Fig.5 Correlation between temperature and total separation factor.
1.4 Preparation of hydrophobic platinum catalysts using a water-in-CO2 microemulsion
The hydrophobic platinum catalyst for chemical exchange reaction of hydrogen atoms between hydrogen gas and water was prepared using a water-in-CO2 microemulsion.
The surface of the stainless steel gauze was treated with chemical vapor deposition to make it superhydrophobic and used as a supported material. A reducing agent was injected into the nano-textured layer using a W/CO2 microemulsion. After that, the precursor of platinum dissolved in supercritical CO2 was contacted with the gauze to make platinum catalyst. This catalyst is inflammable and it decreases the pressure drop in the column.
Fig.6 Preparation of the hydrophobic platinum catalyst.
The activity of the catalyst developed in this study was measured using an isotopic exchange reaction of hydrogen atoms between hydrogen gas and water-vapor. Though the activity decreased gradually, the catalyst was active even in the present of the wet condition.
Fig.7 Evaluation of the catalyst activity.
- N. Sakuma, K. Sawada, M. Tone, Y. Enokida and I. Yamamoto, "Synthesis
of silver nanoparticles in a water in supercritical carbon dioxide microemulsion,"
Proc. 2nd International Symposium on Supercritical Fluid Technology for
Energy and Environment Application, 319-322 (2004).
- R. Shimizu, A. Nibe, K. Sawada, Y. Enokida, I. Yamamoto, "Isotopic
exchange reaction between hydrogen and deuterium by a platinum catalyst
synthesized in water-in-CO2 microemulsion," International Symposium on Isotope Science and Engineering
from Basics to Applications, (2005) Nagoya, 52.
- Y. Enokida, R. Shimizu, A. Nibe, K. Sawada, I. Yamamoto, "Platinum
catalyst synthesis on superhydrophobic surface for hydrogen isotope separation
using water-in-supercritical carbone dioxide microemulsion," The 4th
International Symposium on Supercritical Fluid Tachnology for Energy, Environment
and Electronics Applications, (2005) Taipei, OP-24.
- R. Shimizu, A. Nibe, K. Sawada, Y. Enokida, I. Yamamoto, "Preparation
of hydrophobic platinum catalyst in water-in-CO2 microemulsion for chemical exchange reaction between hydrogen and water,"
10th European Meeting on Supercritical Fluids, (2005) Colmar, Mc2.
- R. Shimizu, A. Nibe, K. Sawada, Y. Enokida and I. Yamamoto, "Preparation
of hydrophobic platinum catalysts using a water-in-CO2 microemulsion", J. Supercrit. Fluids (submitted).
2. Research for isotope production
2.1 Production of new isotopes
In this study, we aim to produce of neutron-rich nuclei including the new isotopes with nuclear fission and to study the decay properties of them. The nuclei of interest are produced with the 235U(n, f) and 238U(p, f) reaction, and separated with the on-line isotope separators (ISOL) from the fission products at Kyoto University Reactor (KUR) and Tandem accelerator at Japan Atomic Energy Agency (JAEA).
Fig.8 The schematic view of KUR-ISOL (Isotope Separator On-Line)
The 235UF4(50 mg) target is irradiated with thermal neutrons(φth=3×1012 n/cm2s). The produced nuclei are ionized by a surface ionization type ion sources, and separated with electromagnetic field.
Fig.9 Chart of the nuclei around A=160 rare-earth elements.
The red nuclei indicate new isotopes. The open circle indicates the isotopes whose β-decay energies were measured for the first time. Europium isotopes have low fission yields and less ionized. Using the newly developed thermal ion source with UC2 thick target, we successfully produced Eu new isotopes.
2.2 Measurements of -decay energies and half-lives of new isotopes
Since most of new isotopes far form the β-stability line have shorter half-lives and less fission yields, it is difficult to study their decay properties. Especially, measured β-decay energies are scarce. In this study, we succeeded in measurements of the β-decay energies with the following newly developed two total absorption detectors.
Fig.10 Total absorption type BGO detector.
Fig.11 Total absorption type HPGe detector.
Total absorption type BGO detector is composed of large volume (12cmφ × 10cmt) twin BGO scintillation detectors which are located at 180 degree geometry. It has high detection efficiency for β and γ-rays.
Total absorption type HPGe detector is composed of a large volume (8 cmφ × 9 cmt) Ge detector and anticompton BGO scintillation detector. The Ge detector has a through hole in the center to have large solid angel almost 100% for the radioactive sources.
Fig.12 Total absorption spectrum of A=162.
Fig.13 Total absorption spectrum of A=163.
2.3 Feasibility Study on the Medical Radioactive Isotopes Production using LWR
The radioactive isotopes, which are generally produced by irradiation of stable isotopes, are used in various fields such as fundamental science, medical science, industrial technology and so on. For example, in the case of medical isotopes, 131I are used for therapy of the malignancy of thyroid, 198Au, 192Ir, 137Cs and 60Co are used for the cancer therapy, 90Y is used as unsealed source. Nuclear reactors for irradiation and cyclotrons are commonly used to produce these radioactive isotopes. When reactors and cyclotrons are compared from the viewpoint of production device of radioactive isotopes, nuclear reactors would be potentially superior since it can provide higher and more stable neutron flux (1012 ~ 1014 n/cm2sec) than cyclotrons and thus it could create radioactive isotopes with lower cost. Therefore, in this study, we focused on the Pressurized Water Reactors (PWRs), which are widely used for power generation and are considered as a proven technology, rather than the dedicated irradiation reactors to enhance the cost merit of isotope productions in nuclear reactors. The target isotopes are 177Lu and 153Gd, which are expected to be used for the pain palliation of the metastatic bone tumor and the contrast agent of the magnetic resonance imaging, respectively.
Feasibility of the medical radioactive isotopes production is evaluated by irradiating natural occurring isotopes in the instrumentation or guide thimbles of a 17-by-17 fuel assembly for PWR as shown in Fig.14.The analysis results indicate that 177Lu and 153Gd can be produced using natural Hf and Eu, respectively, as shown in Figs. 15 and 16. Furthermore, since both Hf and Eu can be also used as burnable absorbers, they could achieve good balance between reactivity control of reactor cores and production of medical radioactive isotopes.
Fig.14 Cross-sectional view of PWR 17-by-17 fuel assembly
Fig.15 Production of 177Lu by irradiation of Hf
Fig.16 Production of 153Gd by irradiation of Eu
- T. Endo, A. Yamamoto, "Medical Isotope Production Using Pressurized Water Reactor," Proc. International Symposium on Isotope Science and Engineering from Basic to Applications, Nagoya, Japan, Sep. 21-23, 2005, O-5, (2005). [CD-ROM].