Preliminary Tube Manufacturing of Oxide Dispersion Strengthened Ferritic Steels with Recrystallized Structure

The effect of cold wall temperature on separative performances for ¹⁴N¹⁵N-¹⁴N₂ separation was studied with the thermal diffusion column of an effective height of 950 mm, a hot wire of 0.15 mm in radius and a cold wall of 5.0 mm in radius. The column was maintained at a constant pressure 0.1 MPa with feed flow rate of 10 cm³.min^-1 (at 0.1 MPa and 298.15 K). The temperature of the cold wall was changed from 223 to 283 K by 20 K keeping at the same temperature difference 235 K between the hot and cold walls. The total separation factor became larger as the cold wall temperature decreased, but had almost the same value at 223 K and 243 K. The experimentally obtained separation factors were compared with the results of the numerical analyses of a convection-diffusion equation. The solutions of numerical analyses with thermal diffusion factor incorporated with inelastic effect gave a good reproduction of dependence of separation factors on cut and cold wall temperature rather than those without inelastic effect. The discrepancy between experimental and analytical results, however, became larger as the cold wall temperature decreased.     

Effects of Grain Morphology and Texture on High Temperature Deformation in Oxide Dispersion Strengthened Ferritic Steels

Oxide Dispersion Strengthened (ODs) ferritic steel is the promising candidate alloy for long-life core materials of fast reactor. A series of experiments, such as tensile tests, creep rupture tests, texture measurements and microstructure observations, are performed for the fabricated sheets of ODs ferritic steel with simulating type of morphology and also for the cladding tube in order to clarify the origin of the peculiar strength anisotropy of the cladding tube: degraded creep rupture strength in hoop direction. From these experiments, effects of grain morphology and texture on deformation of ODs ferritic steels are evaluated.

The sheets and the cladding tube have strong texture of {001}?110? and {111}?110?, respectively. In longitudinal and transverse directions of the sheets, strength level is significantly different from each other, but crystallographic orientation is almost equivalent. From that finding, it is considered that strength anisotropy of the cladding tubes is not attributed to the texture. From the results of micro structure analysis, it is concluded that origin of the degraded creep rupture strength in transverse hoop direction of the cladding tube comes from the grain boundary sliding at the large tilt angles.     

Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation

For use as fuel cladding of liquid metal fast reactors, Fe-0.12C-9Cr-2W ODS martensitic steel claddings were developed by cold-rolling under the softened ferrite phase induced by slow cooling from austenite phase, subsequently by ferrite to austenite phase transformation to break up substantially elongated grains produced by cold-rolling at the final heat-treatment. The produced claddings showed noticeable improvement in tensile and creep rupture strength that are considerably superior to PNC-FMS and even austenitic PNC316 at higher temperature and extended time to rupture. The strength improvement is mainly attributed to titanium addition in ODS martensitic steels through its reduction of Y₂O₃ particle size and shortening inter-particles spacing. The behavior of oxide particle size reduction is associated with stoichiometry between Y₂O₃ and TiO₂.     

Reduction of Plastic Anisotropy of Zircaloy Cladding by Neutron Irradiation, (II)

Initial plastic deformation behavior of zirconium alloy fuel cladding was described quantitatively by the deformation system of single crystal of α-zirconium, and a model was proposed to simulate the yield behavior of polycrystalline material. Based on the model, effects of crystallographic texture and stress state on the plastic deformation of the cladding were evaluated. Conclusions obtained from this investigation are:

(1) The proposed model shows good agreement with the von Mises' yield criteria for a material with isotropic properties.

(2) Plastic anisotropy of the cladding decreases when neutron irradiation affects prism slip more strongly than the other deformation systems.

(3) Dominant deformation systems for axial tension or internal pressurization of the cladding are predicted to be prism slip or tensile twin, respectively, when the stress state of the cladding reaches the yield condition.