Tsuyoshi Nishi

NMR Evidence for the 8.5 K Phase Transition in Americium Dioxide

We report here the first NMR study of americium dioxide (AmO 2 ). More than 30 years ago, a phase transition was suggested to occur in this compound at 8.5 K based on magnetic susceptibility data, while no evidence had been obtained from microscopic measurements. We have prepared a powder sample of 243 AmO 2 containing 90 at. % 17 O and have performed 17 O NMR at temperatures ranging from 1.5 to 200 K. After a sudden drop of the 17 O NMR signal intensity below 8.5 K, at 1.5 K we have observed an extremely broad spectrum covering a range of ∼14 kOe in applied field. These data provide the first microscopic evidence for a phase transition as a bulk property in this system. In addition, the 17 O NMR spectrum has been found to split into two peaks in the paramagnetic state, an effect which has not been reported for actinide dioxides studied up to now. We suggest that the splitting is induced by self-radiation damage from the alpha decay of 243 Am.

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.

Thermal Conductivities of Zr-based Transuranium Nitride Solid Solutions

In this study, the authors prepared sintered samples of (Zr0:58Pu0:21Am0:21)N, (Zr0:80Pu0:10Am0:10)N, and (Zr0:70Np0:06Pu0:15Am0:075Cm0:015)N solid solutions. The thermal diffusivities and heat capacities of these Zr-based transuranium nitride solid solutions were measured using a laser flash method and drop calorimetry, respectively. Thermal conductivities from 473 to 1,473K were determined from the measured thermal diffusivity, heat capacity, and bulk density. The thermal conductivities of Zr-based transuranium nitride solid solutions were found to be higher than that of (Pu0:5Am0:5)N due to the high thermal conductivity of ZrN as the principal component, although they were lower than that of ZrN due to the impurifying effect of the transuranium elements. At 873 K, the thermal conductivities of ZrN, (Zr0:58Pu0:21Am0:21)N, (Zr0:80Pu0:10Am0:10)N, (Zr0:70Np0:06Pu0:15Am0:075Cm0:015)N, and (Pu0:5Am0:5)N were 63.9, 15.8, 25.5, 20.6, and 8.9Wm−1K1, respectively. In these results, the thermal conductivities of the Zr-based transuranium nitride solid solutions increased with increasing ZrN concentration.

Thermal conductivities of (Np,Am)N and (Pu,Am)N solid solutions

The thermal diffusivities and heat capacities of transuranium nitride solid solutions, (Np,Am)N and (Pu,Am)N, were measured by using a laser flash method and a drop calorimetry, respectively. The thermal conductivities of these samples were determined from the measured thermal diffusivities, heat capacities and bulk densities. The thermal conductivities of (Np,Am)N and (Pu,Am)N increased with temperature over the temperature range investigated. The increases in the thermal conductivities were probably due to the increases of electrical components. In addition, the thermal conductivities of (Np,Am)N and (Pu,Am)N decreased with increasing Am contents. It could be considered that the decreases in the thermal conductivities with increasing Am contents correspond to the lowering of electronic contribution.

Thermodynamic properties of neptunium nitride: a first principles study

The thermal and mechanical properties of neptunium nitride (NpN) were investigated by first principles calculations. From the Helmholtz free energy equilibrium lattice constants, thermal expansion coefficients, bulk moduli and specific heat capacities were calculated for temperatures up to 1500 K. The electronic specific heat capacity was also calculated from the electronic density of states at the Fermi energy. The obtained specific heat capacity reproduced the experimental data well. It was thus clarified that the specific heat capacity of NpN consists of the lattice and electronic specific heat capacities and the contribution of the lattice dilatation to the specific heat capacity.

Development of in-house fast X-ray diffraction apparatus and its application to the supercooled liquid Pd40Ni10Cu30P20 alloy

Abstract A fast X-ray diffraction apparatus has been developed for obtainingthe local structures of bulk metallic glasses at the supercooled liquid state by applying the Debye–Scherrer camera geometry in combination with a curved position sensitive proportional counter. This arrangement makes it possible to eliminate the time loss due to angular motion of a counter and to do a very short time X-ray measurement when using the conventional in-house X-ray source. The usefulness of this new apparatus was confirmed by obtaining the radial distribution functions of silicon powder and SiO2 glass sample within one hundred seconds. Then, the short-range ordering structure of the supercooled liquid Pd40Ni10Cu30P20 alloy was observed as a function of time at 583 K.

Thermal Conductivity of Borosilicate Melt

Borosilicate glass is used in mixture of high-level radioactive waste (HLW) generated by nuclear spent fuel. Thermophysical property data of borosilicate melts in molten state during processing of HLW were studied to investigate a possibility of a simpler and flexible process for immobilisation of high level wastes. Especially, these data are indispensable information to optimize the process of the temperature distribution in the glass melting furnace. In this study, the thermal effusivity of B2O3-CaO-SiO2 melt was measured using a front heating-front detection laser flash method. The thermal conductivity was evaluated by combining the measured thermal effusivity data with specific heat capacity and density. In these results, the thermal conductivity of CaO-B2O3-SiO2 melt was increased with temperature.

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.