Takashi Namekawa

Molecular dynamics study of mixed oxide fuel

Abstract In order to develop new techniques to calculate the physicochemical properties of MOX fuel, molecular dynamics methods were applied to UO2, PUO2, and (U,Pu)O2. These methods enabled us to obtain the heat capacity and thermal conductivity from basic properties, viz., the lattice parameter, linear thermal expansion coefficient, and compressibility. Results for UO2 showed both the existence of a Bredig transition and a peak in the heat capacity at high temperature. The lattice parameter, heat capacity, and thermal conductivity of MOX fuel were calculated from basic properties of UO2 and PuO2. These results showed that molecular dynamics techniques can be usefully applied to determine physicochemical properties of MOX fuel.

Fabrication Technology for MOX Fuel Containing AmO2 by an In-cell Remote Process

An in-cell remote fabrication technique was developed for MOX fuel pellets containing 3 and 5% americium (Am-MOX fuel pellet). The fuel pellet was fabricated by means of conventional powder metallurgy. A series of fuel pellet fabrication apparatuses were systematically installed in the alpha-gamma cell (hot cell) to protect workers from a strong γ-ray exposure from 241 Am, and were remotely controlled from a panel in the operation room outside the hot cells as much as possible. From a preliminary UO2 pellet fabrication run, ball milling of powder for 4h, pressing at 4t/cm2 and sintering at 1,700°C for 2h were determined as a good fabrication, but the ball milling time was too short for the UO2 and Am-PuO2 powders of different morphologies to be uniformly mixed. Then, the 5% Am-MOX fuel pellet of density more than 93% T.D. which is proper to the irradiation in FBR was successfully fabricated by extending the ball milling time for more than 10 h. It was, furthermore, found that the complete cleanup of the powder feeder was necessary in the transition of fabrication runs to prevent the formation of uranium and plutonium spots in the pellets.

Molecular Dynamics Studies of Minor Actinide Dioxides

The molecular dynamics (MD) calculation was performed for minor actinide (MA: Np and Am) dioxides in the temperature range from 300 to 2,500K to evaluate the thermophysical properties viz., the lattice parameter, thermal expansion coefficient, compressibility, heat capacity, and thermal conductivity. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined by fitting to the experimental data of the lattice parameter. The usefulness and applicability of the MD method to evaluate the thermophysical properties of minor actinide dioxides were discussed.

Mechanical properties of (U,Ce)O2 with and without Nd or Zr

Abstract The mechanical properties of (U 0.8− x Ce 0.2 M x )O 2 [M: Nd (0≤ x ≤0.13) or Zr (0≤ x ≤0.06)] were studied. The longitudinal and shear sound velocities were measured by an ultrasonic pulse–echo method at room temperature, which enabled us to evaluate the elastic properties and Debye temperature. The microhardness measurement was performed at room temperature using a micro-Vickers hardness tester. The elastic moduli such as Young’s modulus and shear modulus decrease with increasing M atoms content. The values of micro-Vickers hardness and yield stress also decrease with increasing M atoms content.

Thermal conductivity of (U,Ce)O2 with and without Nd or Zr

Abstract The thermal conductivities of (U 0.8− x Ce 0.2 M x )O 2 [M: Nd (0⩽ x ⩽0.13) or Zr (0⩽ x ⩽0.06)] were evaluated from the thermal diffusivity measured by the laser flash method. The thermal conductivities of (U 0.8− Ce 0.2 M x )O 2 indicated a systematic decrease with increasing M-atom content at all temperatures from 200 to 1500 K. In addition, the thermal conductivities of all samples decreased with increasing temperature up to about 1000 K. Up to about 1000 K, the thermal conductivity of (U 0.8− x Ce 0.2 M x )O 2 could be expressed as a function of M-element content by the phonon conduction equation.

Phase equilibria in the BaUO3-BaZrO3-BaMoO3 system

The phase equilibria in the pseudo-ternary BaUO3-BaZrO3-BaMoO3 system were studied to understand the thermochemical properties of the perovskite type gray oxide phase in high burnup MOX fuel. Thermodynamic equilibrium calculation for the system was performed by using a ChemSage program under the various oxygen potentials. Solid solutions existing in the system were treated by an ideal solution model. The present calculation results well agreed with the previous reported post irradiation examination results, showing that BaMoO3 was scarcely included in the gray oxide phase.

Analysis of curium isotopes in mixed oxide fuel irradiated in fast reactor

Curium is one of the key elements in recycling and transmutation of minor actinides (MA) because of its high ra- diotoxicity and difficulty of transmutation. In order to make isotopic analysis of curium in heavily irradiated fuel, the isolation technique of curium was developed by adopting anion exchange chromatography in nitric acid-methanol mixed media. The technique was successfully applied to the analysis of curium in mixed oxide (MOX) fuel irradiated in the experimental fast reactor “JOYO”. The transmutation behaviors of curium in fast reactor are discussed on the basis of observed isotopic ratio of curium.

Thermal conductivity modeling of high burnup MOX fuel

Abstract The thermal conductivity of MOX fuel containing FP precipitates such as oxide and metallic inclusions was evaluated by using a finite element method. (U0.8Pu0.2)O2 and (M0.94Nd0.06)O2 (M: U0.8Pu0.2) were employed as the fuel matrix, and BaUO3 and Mo-Ru-Rh-Pd alloy as the inclusions. The dispersed phase size and concentration were determined from post irradiation examination results of about 10 at.% burnup MOX fuel. 2.5 vol.% of the spherical inclusions with the diameter of 25 μm were dispersed in the fuel matrix cube with a size of 400×400×400 μm. The effect of the inclusions on the thermal conductivity of the fuel pellet was calculated to be at most several percents, which is about one order lower than that of the dissolved FPs.