Norio Masaki

The crystal structure of La6UO12

The phase relations in RE-U-O systems (RE = rare-earth element) have been studied extensively. An outstanding feature in these systems is the existence of rhombohedral compounds, Re/sub 6/UO/sub 12/ which form by heating the mixture of uranium oxides and rare-earth oxides in a ratio U:RE = 1.6 (where RE = La, Pr, Nd, Sm-Ho, Tm-Lu, and Y) above 1000/sup 0/C. Bartram has determined the crystal structure of Y/sub 6/UO/sub 12/ and Lu/sub 6/UO/sub 12/. In the present paper, the crystal structure of La/sub 6/UO/sub 12/ was studied by the X-ray powder diffraction technique. The crystal system of La/sub 6/UO/sub 12/ was known to rhombohedral. Atom positional parameters are reported.

X-ray and density studies of nonstoichiometric uranium sesquinitride

Abstract The lattice parameters of nonstoichiometric α-U2N3 were determined in the composition range of UN1.40 to UN1.76. The single phase region was from UN1.54 to UN1.75. Beyond the N/U atom ratio of 1·75, the structure was transformed from an Mn2O3-type b.c.c. to a CaF2-type pseudo f.c.c. Using the estimated lattice parameter for UN2, two separate linear relationships for the lattic parameter as a function of nitrogen content were obtained; one is characteristic of U2N3 and the other of UN2. The densities were also determined; the values obtained were consistent with a nitrogen interstitial model for U2N3+x.

Distributions of radionuclides on and in spent nuclear fuel claddings of pressurized water reactors

Abstract Radioactivities in Zircaloy-4 cladding of PWR spent nuclear fuel have been examined as a function of fuel burnup in the region of about 7000–40000 MWd/t. The fission products 137 Cs and 106 Ru in the cladding increased linearly with fuel burnup, whereas the radionuclides 134 Cs and 154 Eu, which were produced by neutron capture of fission products, increased with the second power of fuel burnup. Tritium and the activation products of alloy constituents and impurities increased in proportion with fuel burnup. The gross activities of α emitters on the inner and outer surfaces of the cladding increased with 3.1 and 1.3 powers of fuel burnup, respectively. The distribution of radionuclides in the cladding has been examined by radiochemical analysis combined with stepwise etching from the inner surface of the cladding. More than 98% of fission products were present within a 10 μm depth from the inner surface of the cladding, while activation products such as 60 Co were distributed homogeneously in the interior of the cladding except on the outer surface. The distribution of tritium tended to become heterogeneous with increasing fuel burnup.

Spectroscopic study of lithium oxide irradiated by fast neutrons

Abstract Lithium oxide (Li 2 O) is a candidate material for solid breeder blankets in d-t fusion reactors. Radiation damage in Li 2 O was investigated in IEA BEATRIX-II phase 1 irradiation tests using the Fast Flux Test Facility (FFTF). Li 2 O single crystal specimens with various 6 Li concentrations, 6 Li/( 6 Li + 7 Li), were irradiated at about 650 K for 300 effective full power days in FFTF by fast neutrons (the fast neutron fluence) ( > 0.1 MeV): 3.9 × 10 26 n/m 2 ). After the neutron-irradiation, measurements of electron-spin resonance (ESR) and optical absorption were carried out for the specimens at room temperature. From the measurements, colloidal lithium metal was found to be formed in Li 2 O irradiated with fast neutrons.

Irradiation effects on lithium oxide

Lattice parameter change of Li2O irradiated with thermal neutron reactors and RTNS-II (14 MeV neutron) was measured. 0.15 % lattice expansion and a curious change of lattice parameter were observed in Li2O sintered pellets irradiated to 2 × 1023 thermal neutrons/m2 and Li2O single crystals irradiated up to 2.3 × 1021n/m2 by 14 MeV neutrons, respectively. In Li2O single crystals and sintered pellets irradiated by thermal neutrons and high energetic oxygen ions, F+-centers were found to be induced as predominant paramagnetic irradiation defects. The inducing rate of the F+-centers by oxygen ion irradiation was very higher than that by the thermal neutron reactors.

Electronic properties in La2CuO4 at low temperatures

Abstract Dielectric constants at 9 GHz and dc resistivity were measured on La 2 CuO 4 in the temperature range 5–150 K. The conductivity behavior of the variable-range hopping was observed in the low temperature region (5–25 K) and the Debye type dielectric relaxation with the activation energy 0.024 eV was observed in the high temperature region (40–100 K). The experimental results indicate that holes are localized by the disorder potential (Anderson localization) at low temperatures and are localized by a unique potential at high temperature.

Diffraction study of α U2N3+x

Abstract X-ray and neutron powder diffraction studies were made upon nonstoichiometric α U 2 N 3+ x within the composition range 0.20 ⩽ x ⩽ 0.52. It was found that in the region 0.20 ⩽ x ⩽ 0.48 the compounds had a cubic symmetry whose structure was essentially bixbyite-type, U 2 N 3.52 , however had a rhombohedral symmetry [ a′ = (6.50 ± 0.3) A , α = (99.7 ± 0.6)° ], while its structure was very similar to a fluorite-type one. The results show that the phase relation in the U 2 N 3 UN 2 system is not a simple extension of that in the cubic cell, but analogous to that of some lanthanide oxide systems.

Preparation of source and sealed absorber holder for237Np and238U Mössbauer measurements

Preparation of source and sample holder is described for the Mössbauer studies of237Np and238U. For the Mössbauer measurement of237Np, a source assembly with small sources of241Am metal was developed taking account of the transport regulations for radioactive substances. The source assembly of241Am showed a sufficient activity enough to measure the Mössbauer spectra of237Np. In order to handle237Np compounds safely, trebly sealed holders were designed which could encapsulate237Np samples without the seepage of liquid helium. A source for238U Mössbauer measurement was also developed from a highly pure242PuO2.

Non-ohmic resistive state in ion-irradiated YBa2Cu3O7−x

Abstract We have studied the effect of 120 MeV 16 O ion irradiation on the non-ohmic electrical resistive state at 77.3 K in YBa 2 Cu 3 O 7− x . For small measuring currents, the voltage V varies as a power of the current I and the ion-fluence Ω, i.e. V ∼ I n and V∼Ω m , where the exponents n and m are functions of the ion-fluence and the current, respectively. For larger currents, the voltage obeys another power-law: V ∼ I n , where n ′ is lower than n at a given ion-fluence. Analysis of the experimental data usinga model of current-induced unbinding of thermally excited vortex pairs shows that ion-irradiations cause the enhancement of dissociation of the bound vortex-antivortex pairs.

Measurement of RF Superconductivity in YBa2Cu3O7-x with the Standing-Wave Method at 9 GHz

RF properties of superconducting YBa2Cu3O7-x were measured with the standing-wave method in the microwave region. The power absorption (Joule loss) of the powder specimen, calculated from the data of the voltage standing-wave ratio (VSWR), changes markedly at the transition temperature, and decreases with decreasing temperature in the superconducting state. In a lower temperature region, it tends to the formula A=17.6 exp (- 0.031 eV/ kT). Microwaves were found to penetrate considerably into the superconducting specimen.