Project 4 Chemical and electronic properties of Actinide compounds and their applications (R3P4)

概要

PR4 INTRODUCTION:
Actinide compounds shows a unique chemical and elec- tronic nature due to the partial and insufficient shield of 5f orbital electrons as inner transition elements. We have a deep interest in the aspect of the electronic properties of the actinide compounds and formed the group consisting of three major fields: (1) inorganic and coordination chem- istry, (2) electronic properties and (3) theoretical chemistry and its users. These studies will be also useful for applica- tion research area such as the handling of the 1F debris and developments of nuclear medicines.
Such research environments to handle actinides elements are extremely difficult to build in ordinary university insti- tutes. The hot laboratory of the KURNS offers unique op- portunities for the above-mentioned characteristic re- search activities.

PR4-1 INTRODUCTION:
The magnetic structure of the ura- nium compound UIr2Ge2 has been investigated for the first time by resonant X-ray scattering experiments using a sin- gle crystal sample. This compound has a tetragonal CaBe2Ge2-type structure (space group: P4/nmm, D4h7, No. 129) [1,2], which is characterized by an inversion-sym- metric pairing of two U ions occupying a site lacking spa- tial inversion symmetry in the unit cell. When an antifer- romagnetic order with order wavenumber Q = 0 is induced in such a U ion, the global spatial inversion symmetry of the system is broken and cross-correlated responses such as current-induced magnetization are predicted. For this reason, CaBe2Ge2-type magnetic materials have attracted much attention in recent years because they may provide the simplest example of a system in which odd-parity mag- netic multipoles can be active.
On the basis of bulk property measurements on single- crystal sample of UIr2Ge2, we have confirmed that the sys- tem exhibits a phase transition suggestive of antiferromag- netic ordering at 18.3 K (≡ TN), strong magnetic anisot- ropy with the c-axis as the easy magnetization axis [3]. In the present study, we performed resonant X-ray scattering experiments to identify the magnetic structure of this order.

PR4-2 NTRODUCTION:
Actinium chelation chemistry has been drawn attention from not only chemists but also med- ical workers, who are relevant to nuclear medicine. Be- 225 Ac (T1/2 = 10 d) which decays through a chain of cause four a-emissions and two b-emissions to the stable 209Bi, are of great interest for a targeted alpha therapy nuclide due to its ability to kill considerably high efficiency of tu- mor cell [1]. However, g-ray spectrometry, which is a con- venient method for identification of radionuclides, has an uncertainty about the application to 225Ac due to its low emission ratios. In contrast, 228Ac (T1/2 = 6.15 h, daughter nuclide of 228Ra) has potential value because it emits well- defined and intense g-rays that are easily resolved from the daughter nuclides. Havelka reported the preparation method of 228Ra standard solution from natural thorium ni- trate salt [2]. Aldrich et al. also reported the preparation method of 228Ac generator from natural thorium salts in 2020 [3]. Their method was composed of two parts with the balk thorium separation by the precipitation of thorium hydroxide, and purification of 228Ra by the column chro- matography using ion-exchange resins. In the handling of thorium hydroxide, the voluminous slurry precipitate com- plicates the convenient separation from the daughter nu- clides. Hence, we studied precipitation behavior of thorium hy- droxide by titration method.

PR4-3 INTRODUCTION:
Development of highly selective compounds for actinyl ions in aqueous media (extractants, precipitants, resins, etc.) has been important. We have been focusing on monoamide compounds (Fig. 1) as promising candidates for nitric acid media, considering the possibility of complete incineration of waste com- pounds (so-called “CHON principle”[1]). For mono- amide resins, not a few have been newly synthesized, and adsorptivity to metal ions has been examined, where ura- nium(VI) was used as the representative of actinyl ions. Fig. 1. Chemical structure of monoamide compounds. (R, R’, R” : hydrocarbon group) .
Effective elution of the adsorbed metal ions is neces- sary for recycle use of resins. Many of the monoamide resins are found to adsorb more U(VI) species with in- creasing concentration of HNO3. Selective U(VI) re- covery would be, therefore, achieved by adsorbing U(VI) under relatively higher concentration of HNO3 (e.g. 3 to 6 mol/dm3 (=M)) followed by elution of the adsorbed U(VI) by using H2O or diluted HNO3 (e.g. 0.1 M). On the other hand, it has been revealed that some metal ions except U(VI) are also adsorbed to monoamide resins.
Adsorptivity for many of the ions are lower than that of U(VI), but some ions are adsorbed in almost all HNO3 concentration range. In this case, the adsorbed metal ions can’t be eluted only by changing the concentration of HNO3, and use of chelating agents is necessary.
In the present study, a monoamide resin consisting of 1-(4-vinylbenzyl)pyrrolidin-2-one (VBPR) was taken. As can be seen in Fig. 2, it has a cyclic monoamide structure with a long spacer between the functional mon- oamide group and the main polymer chain. VBPR resin shows adsorptivity to Re(VII) (simulant of Tc(VII)) in all HNO3 concentration range and the distribution ratio, Kd, is higher under lower concentration of HNO3. Based on the above, elution properties Re(VII) by some chelating agents were investigated.

PR4-4 INTRODUCTION:
Theoretical calculations of acti- nide compounds are very important to understand or pre- dict new phenomenon in these compounds. We have de- veloped computational methods based on relativistic quantum chemistry, which can describe electronic states of actinide compounds. In this work, we calculated equi- librium isotope fractionation coefficients () for 64 U species in U(VI), U(V), and U(IV) states with various ligands (e.g., H2O, CO32-, CH3COO-, Cl-, NO3-, etc.) us- ing accurate relativistic quantum chemical methods. The U isotope fractionations are important and widely dis- cussed in geochemistry [1]. We verify the accuracy of the computational methods by comparing to some experi- mental results of U isotope fractionations.

PR4-5 INTRODUCTION:
Partitioning and transmuting is presently regarded as an effective method to address the issue of high-level radioactive waste disposal [1]. We have proposed to transmute MA rather moderately, namely with a low fission reaction rate to avoid imposing severe engineering challenges on the system design [2].
This concept has considered MA mixed oxides (e.g., Np–Am–O) to be loaded into limited space in fusion re- actors. Whereas some studies have reported the phase diagram of U or Pu based MA oxides, no reports have provided the diagrams of the MA-only mixed oxides.
The objective of this study is to evaluate the ther- modynamic values and phase diagram of the MA-only mixed oxides. Measurement of the phase diagrams is not a straightforward task; we thus aim to develop a method to evaluate the phase diagrams.
In the current fiscal year, we have performed the electronic structure calculations of NpO2 and AmO2 mixed oxide based on the method obtained in the previ- ous year. Because there are no reported crystal structure data for the mixed oxide, crystal structures were prepared assuming appropriate atomic configurations, and struc- tural optimization calculations were performed on them.

PR4-6 INTRODUCTION:
Unusual physical properties aris- ing from geometrical characteristics of magnetic mo- ments are now attracting active research interests. In par- ticular, the situations where magnetic interactions are frustrating can result in highly unconventional ground states as demonstrated historically by the spin-glass or, more recently, skyrmions formations involving a number of spins. Here, we investigate uranium compounds where the uranium atoms form honeycomb layers. The target materials in this study are a series of compounds having Sr0.6Fe2Si4.9-type structure where uranium atoms are lo- cated at Sr position. Although earlier study suggested atomic disorder inside the uranium layer, our investiga- tion on a U-Pt-Ga ternary analogue showed that a hon- eycomb arrangement of uranium atoms is likely. The re- sulting formula should therefore be U2Pt6Ga15. In this study we further investigated uranium intermetallics with the same crystal structure.

PR4-7 INTRODUCTION:
We have been studying the solid adsorbents for separation and recovery of actinides [1], and actinide analysis [2]. Recently, the supply of -nuclides for -therapy is becoming increasingly im- portant. For the purpose, the actinides separation from the decay series is required. In this year, the adsorption be- haviors of actinides on polyvinylpolypyrrolidone (PVPP) and styrene (Sty)-divinylbenzene (DVB) type pyrrolidone resin were investigated. Especially, we obtained the ex- tended data about adsorption of thorium ion on PVPP, and investigated the cross-linkage effects on the adsorp- tion. In addition, we studied the dissolution methods of thoria.

PR4-8 INTRODUCTION:
Development of highly selective compounds for actinyl ions in aqueous media (extractants, precipitants, resins, etc.) has been important. We have been focusing on monoamide compounds (Fig. 1) as promising candidates for nitric acid media, considering the possibility of complete incineration of waste com- pounds (so-called “CHON principle”[1]). For the resins, we have taken two factors into accounts for selective in- teraction between functional monoamide groups and actinyl ions; one is “chelating effect” of the ring formed by polymer monoamides and actinyl ion(s), and the other is “flexibility” of monoamide. Our previous experi- mental results suggest that the contribution of “chelating effect” is predominant. This report describes our past one year’s activity in the above research field.

PR4-9 INTRODUCTION:
For decommissioning the Fuku- shima-Daiichi Nuclear Power Plant, it is necessary to understand the characteristics of fuel debris. Particularly, for safe and reliable removing of the debris, estimation of the aged deterioration of a debris with changes in envi- ronmental conditions could be required. Thus, this study is aiming to investigate the changes in chemical state and structure of a simulated fuel debris under controlled en- vironment simulating conditions inside and/or outside the reactor by XAFS method.

PR4-10 NTRODUCTION:
Effective separation of U from Th and other fission products in spent Th fuels is needed in Th fuel cycle. To enable the separation, thorium–uranium extraction (THOREX) process, like the plutoni- um–uranium redox extraction (PUREX) process, has been studied [1]. In the THOREX process, UO22+ is ex- tracted by tri-n-butyl phosphate (TBP) with the aid of Al(NO3)3. Some of the other extractant which consisted of C, H, O, N atoms (CHON principle) such as monoam- ide is recently reported for U/Th separation [2]. In this study, Phthalocyanine (Pc) was selected as the main structure of the extractant which also satisfy the CHON principle. To make the solubility of Pc in organic solvent higher, the Pc derivatization is ongoing. The Pc-metal complex in organic solvent itself is interesting to investi- gate. To obtain the complex with actinide, the purification technique with minimized waste production is highly desirable. In FY2021, some of the purification techniques were tested including chromatography, Solxlet extraction and sublimation. This year, some of the purified samples, mainly Pc-Zn compounds were characterized as well as the other samples.

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