Noriko Shirasu

Extraction Behavior of Metal Ions by TODGA, DOODA, MIDOA, and NTAamide Extractants from HNO3 to n-Dodecane

Four novel diamide and triamide extractants developed in our group, TODGA, DOODA, MIDOA, and NTAamide, were examined to see the extractability trends of up to 74 metal ions from nitric acid into n-dodecane. TODGA and DOODA have one or two etheric and two amidic oxygen donor atoms, and MIDOA and NTAamide have a nitrogen donor atom centered in the backbones and two or three diamidic O atoms. The former two extractants are expected to have high extractability for metal ions classified as hard acids and the latter ones have higher extractability for soft acids. TODGA and DOODA have high distribution ratios, D, for metal ions in the 2A-4A groups. On the other hand, MIDOA and NTAamide have high D values for group 5A-7A and 8 metal ions, which follows HSAB theory. The positive relation between the slope values of the extractant dependence and the extraction constant, α, is found, which suggests that a high order of successive formation of metal complexes with extractants gives high extractability.

Novel Extractant, NTAamide, and its Combination with TEDGA for Mutual Separation of Am/Cm/Ln

A novel extractant, N,N,N′,N′,N′′,N′′-hexaalkyl-nitrilotriacetamide (NTAamide), was developed for the extraction and separation of actinides (An) and lanthanide (Ln). NTAamide has a nitrogen atom in the center of a backbone and is bonded to three acetamide groups; the nitrogen and three amide oxygen atoms can work as the donor atoms. NTAamide extracted Am and Cm with a low distribution ratio of Ln in diluted HNO3, and the separation factor (SF) for An/Ln was equal to or higher than 23.6 using 0.5 M NTAamaide/n-dodecane and 0.2 M HNO3. The addition of N,N,N′,N′-tetraethyldiglycolamide as a masking agent to the NTAamide extraction system was effective for separating Am from Cm; the maximum SF was 6.5, one of the highest SF values obtained using the HNO3 and n-dodecane system.

Characterization of solidified melt among materials of UO2 fuel and B4C control blade

To predict the fundamental phase relationships in the solidified core melt of the Fukushima Daiichi Nuclear Power Plant, solidified melt samples of the various core materials [B4C, stainless steel, Zr, ZrO2, (U,Zr)O2] were prepared by arc melting. Phases and compositions in the samples were determined by means of X-ray diffraction, microscopy, and elemental analysis. With various compositions, the only oxide phase formed was (U,Zr)O2. After annealing, the stable metallic phases were an Fe-Cr-Ni alloy and an Fe2Zr-type (Fe,Cr,Ni)2(Zr,U) intermetallic compound. The borides, ZrB2 and Fe2B-type (Fe,Cr,Ni)2B, were solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere (Ar-0.1%O2) resulted in the oxidation of U and Zr in the alloy and in ZrB2, and consequently the (Fe,Cr,Ni)2B and Fe-Cr-Ni alloy became dominant in the metallic part. The experimental phase relationships in the metallic part agreed reasonably with the thermodynamic evaluation of equilibrium phases in a simplified B4C–Fe–Zr system. The metallic Zr content in the melt was found to be a key factor in determining the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides, especially ZrB2, showed notably higher hardness than any other oxide or metallic phases.

Utilization of rock-like oxide fuel in the phase-out scenario

Utilization of rock-like oxide (ROX) fuel in light water reactors for plutonium (Pu) burning was studied by nuclear material balance (NMB) analysis for a case of Japanese phase-out scenario under investigation after the Fukushima accident. For the analysis, the NMB code was developed with features of accurate burn-up calculation, flexible combination of reactors and fuels, and an ability to estimate waste and repository. Three scenario groups of once-through Pu burning in mixed oxide (MOX) fuel and in ROX fuel were analyzed. Using two full-MOX or full-ROX reactors the Pu amount is reduced to about one-half and the isotopic vector of Pu deteriorated for being used as a nuclear weapon, especially in terms of spontaneous fission neutron generation. Effects of ROX reactors are more significant than MOX reactors in terms of both reduction in the Pu amount and deterioration of the isotopic vector. Repository footprint and potential radiotoxicity are not reduced by the MOX and ROX reactors because the heat and toxicity of MOX and ROX spent fuels are considerably high.

Variation in the surface morphology of polycrystalline UO2 powder induced by helium precipitation

Abstract This report addresses the precipitation of helium in polycrystalline UO2, which deforms the morphology of the grains and their surfaces. The formation of intragranular gas bubbles by the diffused helium can adversely affect the sintering process of ceramic-type nuclear fuel pellets. Helium was injected into pulverized UO2 particles at 1473 K by hot isostatic pressing (HIP). The specific surface area measured by volumetric gas adsorption instrument implied that small pores should exist on the as-helium-treated sample surface. Field-emission scanning electron microscopy observations showed that numerous shallow basins (approximately 500 nm in radius) with hexagonal fringe were formed on the surface. The basin resembles a ruptured blister whose lid has been forced open. SEM observations showed a uniform polygonal-shaped section of the gas bubble on the fracture surface; this implies that precipitated helium forms a negative crystal in the grain. These interesting results suggest the possibility that the rupture of the negative crystal formed in the vicinity of the surface is related to the formation of the basin with a hexagonal fringe.

Fundamental experiments on phase stabilities of Fe–B–C ternary systems

To understanding the control blade degradation mechanism of a boiling-water reactor (BWR), a thermodynamic database for the fuel assembly materials is a useful tool. Although the iron, boron, and carbon ternary system is a dominant phase diagram, phase relation data are not sufficient for the region in which boron and carbon compositions are richer than the eutectic composition. The phase relations of three samples such as Fe0.68B0.06C0.26 (at%), Fe0.68B0.16C0.16 (at%), and Fe0.76B0.06C0.18 (at%) were analyzed by X-ray diffraction, scanning electron microscopy, and energy–dispersive X-ray spectrometry. The results indicate that the Fe3(B,C) phase exists only in the intermediate region at 1273 K and that the solidus temperature widely maintains at approximately 1400 K for all three samples; these results differ from the calculated data using the previous thermodynamic database. The difference might originate from the overestimation of the interaction parameter between boron and carbon in Fe3(B,C). Proper titling was performed using the present data.