Muneo Handa

Fabrication of (U, Pu)N fuel pellets

Abstract Uranium-plutonium mixed nitride, (U, Pu)N, was fabricated by the carbothermic reduction in a N 2 -8% H 2 mixed gas stream from UO 2 and PuO 2 . The influence of the sintering conditions, such as milling time, compacting pressure, sintering temperature, sintering atmosphere and pore former content, on the characteristics of (U, Pn)N pellets was systematically investigated. High density (~ 95% TD) pellets were obtained by sintering at 2023 K in an Ar-8% H 2 or Ar stream without loss of plutonium using the highly activated powder. On the other hand, low density (~ 85% TD) pellets with excellent thermal stability were fabricated by sintering with pore former particles. The difference in the microstructure or composition was discussed in connection with the sintering conditions. Certain increase in the lattice parameter of (U, Pu)N was observed during sintering stage.

Thermal conductivity of near-stoichiometric (U, Pu, Nd)O2 and (U, Pu, Eu)O2 solid solutions

Abstract The thermal conductivities of near-stoichiometric [(U0.8Pu0.2),R]O2 solid solutions containing RO1.5(R = Nd or Eu) up to 10 mol% were determined by the laser flash method in the temperature range 700–1900 K. The thermal conductivities for the solid solutions up to about 1550 K. satisfied the phonon conduction equation K = (A + BT) −1 . The thermal conductivity decreased gradually with the increase of the rare earth content. This decrease was mainly caused by the lattice defect thermal resistance. The measured defect thermal resistivities ( = A ) were in good agreement with the calculated results based on the lattice defect model in which U4+, U5+, Pu4+ and R3+ ions were considered as phonon scattering centers. The lattice strain parameter ϵ = 97 and 103 were obtained for (U, Pu, Nd)O2 and (U, Pu, Eu)O2 solid solutions, respectively. The lattice strain effect on the thermal resistivity was about 15 times larger than the mass difference one.

Thermal conductivity of near-stoichiometric (U, Nd)O2, (U, Sm)O2 and (U, Eu)O2 solid solutions

Abstract The thermal conductivities of near-stoichiometric (U, R)O 2 solid solutions (R = Nd, Sm and Eu) containing RO 1.5 up to 15 mol% were determined in the temperature range 700–2000 K by the measurement of thermal diffusivity. The thermal conductivities satisfied the phonon conduction equation K = ( A + BT ) −1 within ± 5%. The constant A corresponding to the lattice defect thermal resistivity increased linearly with the rare earth element content, while the temperature coefficient B was almost independent of it. The change in A with the rare earth element content increased in order of (U, Eu)O 2 , (U, Sm)O 2 and (U, Nd)O 2 solid solutions. The increase of A was explained reasonably by the lattice defect model considering U 4+ , U 5+ and R 3+ ions in the solid solutions as phonon scattering centers, using a common value for the strain parameter ( ϵ = 110). For all solid solutions, the lattice strain effect on the lattice defect thermal resistivities was much larger than the mass effect. In addition, the effect of U 5+ ions on the lattice defect thermal resistivity caused by the lattice strain effect was larger than that of R 3+ ions.

Thermal conductivity of stoichiometric (Pu, Nd)O2 and (Pu, Y)O2 solid solutions

Abstract The thermal conductivities of stoichiometric (Pu, R)O 2 solid solutions containing RO 1.5 (R = Nd and Y) up to 10 mol% were determined by the laser flash method in the temperature range 700–1400 K. The thermal conductivities for all solid solutions satisfied the phonon conduction equation K = ( A + BT ) −1 within ± 6%. The lattice defect thermal resistivities (= A ) increased gradually with the neodymium or yttrium content, while the intrinsic lattice thermal resistivities (= BT ) were nearly equal to that measured for PuO 2 . The measured thermal conductivities were consistent with the results of the lattice defect model calculation in which Pu 4+ , Pu 5+ and R 3+ ions were considered as phonon scattering centers. The lattice strain parameters ϵ = 85 and 93 were obtained for (Pu, Nd)O 2 and (Pu, Y)O 2 solid solutions, respectively.

Gibbs free energies of formation of molybdenum carbide and tungsten carbide from 1173 to 1573 K

The gas equilibrium method of CH4/H2 has been widely used for measuring carbon potential. However, it has been reported that this method is not applicable at high temperatures since the equilibrium between CH4 and H2 is disturbed by the reaction of CH4 with moisture in the system. Nevertheless, this method should be applicable theoretically at high temperatures below which CH4 decomposition can be neglected because the equilibrium between CH4 and H2 reaches constant ratio in spite of the reaction. Since the role of moisture is to oxidize the sample during the measurements under the oxygen potential determined byPh2o/ph2 ratio, the Gibbs free energies of formation of Mo2C and WC were successfully measured from 1173 to 1573 K by keeping the moisture level in the system low enough not to oxidize the sample. The experimental results are expressed by the following equations which were derived by least squares treatments of the data: Mo2C:ΔG = -68270 + 8.23T J mol-1 WC:ΔG = -52330 + 14.06T J mol-1 These values were in good agreement with those measured by M. Gleiseret al. for narrow tempareture ranges using the CO/CO2 gas equilibrium method.

Thermal conductivity of (Pu1−xNdx)O2−y and (Pu1−xYx)O2−y solid solutions

Abstract The thermal conductivities of (Pu1−xRx)O2−y solid solutions (R = Nd and Y) containing RO1.5 up to 10 mol% were determined in the temperature range 700–1450 K from thermal diffusivities measured by the laser flash method. The thermal conductivities satisfied the phonon conduction equation K = (A + BT)−1 within ± 7%. The values of A, corresponding to the lattice defect thermal resistivity, increased linearly with the neodymium or yttrium content, while those of B were nearly constant. The increasing rate of A for (Pu, Nd)O2−y solid solutions was slightly larger than that for (Pu, Y)O2−y. These increases were reasonably explained by the lattice defect model in wich Pu4+, R3+, O2− ions, and oxygen vacancy in the solid solutions were considered as phonon scattering centers. For both solid solutions, the lattice strain effects on the lattice defect thermal resitivities were in preference to the mass effects. In addition, the stoichiometry effects on the additional defect thermal resistivities were about 1.3 times larger than the cation effects.

Determination of nitrogen in UN, PuN and (U,Pu)N by oxidation in circulating oxygen and gas chromatographic measurement of the combustion gases

Abstract A simple and accurate method for the determination of nitrogen in uranium- and plutonium-bearing materials was developed. The loss of nitrogen by oxidation of the sample before analysis was prevented by pulverizing, weighing and packing the sample into a tin capsule in a glove-box with a high-purity argon atmosphere. Nitrogen was determined by oxidizing the nitrides in the tin capsule in circulating oxygen at 850 °C and analysing the combustion gases by gas chromatography with thermal conductivity detection. The relative standard deviation was about 0.7% and the time required to analyse one sample was about 10 min for successive analyses. Skilled techniques for glove-box work are not necessary. The method is applicable not only to the analysis of research samples but also to the quality control of nitride fuel production lines.

A new gas equilibration method for the measurement of carbon potential: Carbon potential in austenitic 316 stainless steel at 1000°C

Abstract A new method for the determination and control of the carbon potential of a sample material using CH 4 /H 2 gas equilibration was developed. The carbon potential of the sample could be controlled in a wide range with an accuracy of about ±0.1 kcal/mol by adopting several kinds of reference mixtures, in which the temperature of the reference material, metal/metal-carbide mixture set, was changed. The carbon potential in austenitic 316 stainless steel at 1000°C was measured successfully by this method.

Fission gas release from UO2-dispersed graphite during irradiation

Abstract In-pile release of 85mKr, 87Kr, 88Kr, 133Xe, 135Xe and 138Xe from natural graphite was measured over a temperature range of 100 to 950°C and that of 133I and 135I was estimated. The effects of fission rate and fission density on the fission gas release were examined. The β-decay process of iodine within and outside the graphite was observed to control the gross release of xenon. Possible mechanisms of an anomalous sink of 88Kr in the plot of log R B versus log λ were discussed. A new model for knock-out release was proposed on the basis of its temperature dependence found. These results were compared with those of post-irradiation experiments and explained by a mechanism whereby fission gas is trapped in defects created in graphite by fission and β-decay energy, and released through annealing of the defects.

Thermal conductivity of near-stoichiometric (U, Ce)C and (U, Pu, Ce)C solid solutions

Abstract The thermal conductivities of near-stoichiometric (U, Ce)C and (U, Pu, Ce)C solid solutions containing CeC up to 10 mol% were determined in the temperature range from 740 to 1600 K by the laser flash method. The thermal conductivity decreased with the cerium content in the solid solutions. The electrical resistivities were also measured for the purpose of analyzing the heat conduction mechanism. It was found that the decrease of electronic heat conduction caused by the addition of cerium resulted in decreasing the thermal conductivities of (U, Ce)C and (U, Pu, Ce)C compared with UC and (U, Pu)C.