Shoichi Nasu

Irradiation induced lattice defects in UO2

Abstract Two kinds of sintered UO 2 having 2.5 and 5 μm grain sizes were irradiated below 150°C to fission doses between 1.14 × 10 14 and 2.92 × 10 18 fissions/cm 3 . The changes of lattice parameter and lattice strain with fission dose were studied. Three stages appeared in the dose dependence of lattice parameter and lattice strain which depended on grain size. In the beginning of the first stage the lattice strain decreased to some extent. The recovery of the lattice parameter within each stage was studied and defects produced in each stage were also discussed. The effective volumes per fission event were calculated at each stage based on kinetic consideration of defects.

Diffusion of tritium in single crystal Li2O

The release of tritium from neutron irradiated spherical samples of single crystal Li2O was measured by isothermal annealing experiments. The release is shown to be controlled by diffusion of tritium in the solid under appropriate experimental conditions. Deviations from solely diffusion controlled release were observed when traces of water were present in the He-purge gas used in the experiments. The diffusivity of tritium in single crystal Li2O is given by In(D/cm2s−1) = − (5.93 ± 0.48) − (81.73 ± 4.24)103J/RTfor 850 K < T < 1200 K. (R = 8.314 JK−1mol−1).

Tritium release from neutron-irradiated li2o; constant rate heating measurements

Abstract Based on the constant rate heating method, a study of tritium release from neutron irradiated Li 2 O has been-performed for sintered pellets and single crystals. About 8% of the tritiated species were released in gaseous forms and the other was released in condensible forms, both from sintered pellets (86% TD) and from single crystals. The rate limiting process of the release of condensible species from the sintered pellets was inferred to be due to the desorption mechanism. The apparent activation energies of the tritium release were determined from the maximum release temperatures of the release curves. The neutron fluence and flux effects on the tritium release were also examined.

POROSITY DEPENDENCE OF THERMAL CONDUCTIVITY OF URANIUM MONONITRIDE.

Abstract Thermal diffusivity of sintered uranium mononitrides of various densities 70–95% TD were measured over the temperature range from 300 to 1400 °C. The curve of thermal diffusivity versus porosity at each temperature exhibited a break at porosity near 0.1. Thermal conductivity obtained from thermal diffusivity was found to be well fitted to a set of the simplified Maxwell-Eucken equations. The coefficient β in the equation was obtained as 1.68–1.86 for the porosity range above 0.10 and that for the smaller porosity range varied from 1.38 at 300 °C to −0.09 at 1300 °C. The porosity dependence of thermal conductivity of the porous material is discussed, taking account of the effect of pore conduction in a similar way as conducted by Marino. The calculated result for the larger porosity is in agreement with the experimental ones, whereas that for the smaller porosity is not wholly consistent with the experiments.

Irradiation induced volume change in UN2

Two sets of uranium dioxide samples having grain sizes of 2.5 and 5 μm were irradiated with different fission doses in the range of 1.1 × 1014 to 2.9 × 1018 fissions/cm3; the volume change was measured. A steep volume expansion was observed up to a dose of about 5 × 1016 fissions/cm3 which corresponds to the reported dose for maximum elongation of stack length of UO2 fuel elements in the starting period of BWR. At higher doses, an abrupt volume shrinkage was noted. The concentrations of interstitials and vacancies at various doses were evaluated from the changes of the volume and the lattice parameter. Kinetic equations for the densification of fuels based on the concentration of vacancies are proposed.

Electrical resistivities of the uranium carbides

Abstract Electrical resistivities of arc-melted uranium carbides, UC, UC2, U2C3 and UC + UC2, were measured over the temperature range between 4 and 1900 K. The monocarbide, dicarbide and the mixture of carbides showed metallic character in their resistivity dependence on temperature above 30 K, while below this the resistivities were constant. U2C3 showed a knee in the resistivity curve at 54.0 K corresponding to a magnetic transition. Above 1100 K, the resistivity of U2C3 did not vary with temperature. The resistivity of the mixture (UC + UC2 with U2C3 precipitates) showed an arrest between 800 and 950 K.

Reaction of several iron and nickel based alloys with sintered Li2O Pellets

Abstract The reaction of type 316 stainless steel, Incoloy 800, Hastelloy X-R, Inconel 600 and pure Ni with sintered Li 2 O pellets has been studied between 800 and 1100°C under dynamic vacuum. The reaction products were analyzed by means of metallographic, microprobe and X-ray diffraction methods. The reactions proceeded measurably between 800 and 950°C and appreciably at 1000°C, being greatest with Incoloy 800 and least with Hastelloy X-R. Among the primary alloy constituents, chromium was exclusively attacked by lithium and oxygen diffusing from the Li 2 O into the alloys to form LiCrO 2 . This phase grew into a reaction zone (subscale) of uniform thickness beneath the surface of each alloy. Preferential growth of LiCrO 2 along the grain boundaries was observed only in the case of Inconel 600 below 950°C. On the other hand, iron diffused toward the Li 2 O pellets to form volatile Li 5 FeO 4 . However, any reaction product associated with Ni was not detected and Ni metal was little attacked by the Li 2 O pellet over the whole range of reaction temperature.

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.

Heat capacity and thermal decomposition of lithium peroxide

Abstract The heat capacity of Li 2 O 2 was measured by adiabatic scanning calorimetry from 301 to 566 K resulting in C p = 59.665 + 52.123 × 10 −3 T + 5.0848 × 10 5 T −2 (J/K · mol) . The thermal decomposition of Li 2 O 2 was studied by continuous calorimetric measurements. An endothermic decomposition was observed above 570 K. The enthalpy of the thermal decomposition was determined to be 25.8 kJ/mol. The effects of atmosphere and heating rate of the decomposition were also studied for the powder and the compact specimens.

Compatibility between several heat resistant alloys and sintered Li2O in static helium gas environment

The reaction of sintered Li2O discs with several commercial heat resistant alloys has been investigated under the conditions of 3.3 × 104Pa (13 atm) static He gas atmosphere in the temperature range of 500 and 750° C. Reaction products were identified by X-ray diffraction analysis to be two phases of Li5FeO4 and LiCrO2. The former was dominant below 650° C and the latter was dominant above 650° C. The activation energies of the reaction were determined by the Arrhenius plots for weight changes and sub-scale thickness measurements. The reactivity of the four Fe-Ni-Cr alloys decreased according to the order of Incoloy 800, 316 SS, Hastelloy X-R and Inconel 600. Grain boundary penetration was observed above 500° C for Incoloy 800, 550° C for 316 SS and 600° C for Inconel 600. There was no grain boundary penetration in Hastelloy X-R.