Shigeharu Ukai

Perspective of ODS alloys application in nuclear environments

Abstract Oxide dispersion strengthened (ODS) steels are the most promising class of materials with a potential to be used at elevated temperature under severe neutron exposure environment. Leading technology development of ODS steels has been conducted at the Japan Nuclear Cycle Development Institute (JNC) particularly emphasizing fuel cladding application for fast reactors. This paper reviews the JNC’s activities on ODS steel development as ‘nano-composite materials’. Martensitic 9Cr-ODS and ferritic 12Cr-ODS steels have been successfully developed; Y2O3 oxide particles can be controlled on a nano-scale and high-temperature properties were noticeably improved through controlling the grain boundary structure on an atomic scale. The ODS-technology development achieved in the field of fast reactors should be effectively spun off to the fusion reactor first wall and blanket structural materials to allow for safe and economical reactor design.

Alloying design of oxide dispersion strengthened ferritic steel for long life FBRs core materials

Abstract Oxide dispersion strengthened (ODS) ferritic steels with excellent swelling resistance and superior high temperature strength are prospective cladding materials for advanced fast breeder reactors. The addition of Ti in 13Cr-3W ODS ferritic steels improved the high temperature strength remarkably by the formation of uniformly distributed ultra-fine oxide particles. ODS ferritic steels have a bamboo-like grain structure and a strong deformation texture. The decrease of creep rupture strength in the bi-axial direction compared to the uni-axial direction is attributed mainly to this unique bamboo grain structure. Nearly equivalent creep rupture strength for both bi-axial and uni-axial direction was successfully attained by introducing the α to γ transformation in ODS martensitic steel.

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.

Characterization of High Temperature Creep Properties in Recrystallized 12Cr-ODS Ferritic Steel Claddings

The high temperature strengthening mechanism of previously manufactured 12Cr-ODS ferritic steel claddings was clarified. In the recrystallized 12Cr-2W-0.3Ti-0.24Y2O3-ODS ferritic steel cladding, αY2TiO5 type complex oxide formation was responsible for the drastic reduction of oxide particle size and the resulting shortened distance between particles, which led to superior internal creep rupture strength at 973 K because of the high resistance to gliding dislocation. Internal creep deformation was considered to be controlled by the grain boundary sliding associated with grain morphology: the near Σ11, Σ and Σ19 coincidence boundaries with a (110) common axis.

Tube manufacturing and characterization of oxide dispersion strengthened ferritic steels

Abstract Oxide dispersion strengthened (ODS) ferritic steels have an advantage in radiation resistance and superior creep rupture strength at elevated temperature due to finely distributed Y2O3 particles in the ferritic matrix. Using a basic composition of low activation ferritic steel (Fe–12Cr–2W–0.05C), cladding tube manufacturing by means of pilger mill rolling and subsequent recrystallization heat-treatment was conducted while varying titanium and yttria contents. The recrystallization heat-treatment, to soften the tubes hardened due to cold-rolling and to subsequently improve the degraded mechanical properties, was demonstrated to be effective in the course of tube manufacturing. For a titanium content of 0.3 wt% and yttria of 0.25 wt%, improvement of the creep rupture strength can be attained for the manufactured cladding tubes. The ductility is also adequately maintained.

R&D of oxide dispersion strengthened ferritic martensitic steels for FBR

As prospective cladding material for the long-life core of a Fast Breeder Reactor (FBR), we developed oxide dispersion strengthened (ODS) ferritic/martensitic steels, which have more swelling resistance than austenitic steels and are expected to have a superior creep strength at elevated temperatures. In order to improve the inferior strength in the hoop direction of manufactured ODS cladding tubes, recrystallization and martensitic phase transformation techniques have been developed, and the strength anisotropy was successfully improved in laboratory scale tests. It is also demonstrated that cold rolling manufacturing for the ODS ferritic cladding was possible using the recrystallization technique.

Tube manufacturing and mechanical properties of oxide dispersion strengthened ferritic steel

Abstract In order to apply the ODS ferritic steels for the prospective cladding materials of advanced fast breeder reactors, fabrication tests of thin-walled cladding tubes were carried out from a viewpoint of future industrial manufacturing. The manufactured claddings within the specification limit exhibited a superior high temperature strength and sufficient Charpy impact properties. The degradation of creep rupture strength in the bi-axial direction, as compared with the uni-axial direction, is mainly attributed to the grain boundary fracture mode within the elongated bamboo grain structure.

Development of Oxide Dispersion Strengthened Ferritic Steels for FBR Core Application, (I): Improvement of Mechanical Properties by Recrystallization Processing

As to an oxide dispersion strengthened (ODs) ferritic steel cladding as the promising candidate for long-life core materials of the fast reactors, previously fabricated claddings had inferior internal creep rupture strength in hoop direction and inferior formability due to less ductility. Those unexpected features of ODs claddings are substantially ascribed to the needle-like grain structure excessively elongated along the forming direction. Controlling the grain morphology by applying the recrystallization method to ODs ferritic steel made possible to improve those inferior features. The ranges of Y2O3 and excessive oxygen contents for possibly cold-rolling and recrystallization were revealed, and the effects of extruded temperature and deformation texture on recrystallization characteristics were evaluated. The recrystallized ODs ferritic steel showed superior internal creep rupture strength and ductility. It was demonstrated from those results that cold-rolling manufacturing of ODs cladding at room tem...

Formation of nanoscale complex oxide particles in mechanically alloyed ferritic steel

Oxide-dispersion-strengthened (ODS) ferritic steel incorporating nanoscale oxide particles was produced by mechanical alloying, and the complex oxide particles, comprised of titanium and yttrium, characterized by high-resolution transmission electron microscopy (HRTEM) and characteristic X-ray nano-analyses. The interface between the ferritic matrix and the major Ti-Y complex oxide was incoherent. From HRTEM and characteristic X-ray nano-analyses, plus the extremely limited literature data on ODS ferritic steels, it was determined that the most stable complex oxide was cubic Y2Ti2O7, which formed by a spontaneous reaction between nano-sized ceramics with a low solubility limit in the metal.

High Burnup Fuel Cladding Materials R&D for Advanced Nuclear Systems: Nano-sized oxide dispersion strengthening steels

Cladding materials development is crucial to realize highly efficient and high-burnup operation over 100GWd/t of so called Generation IV nuclear energy systems, such as supercritical-water-cooled reactor (SCWR) and lead-cooled fast reactor (LFR). Oxide dispersion strengthening (ODS) ferritic/martensitic steels, which contain 9–12%Cr, show rather high resistance to neutron irradiation embrittlement and high strength at elevated temperatures. However, their corrosion resistance is not good enough in SCW and in lead at high temperatures. In order to improve corrosion resistance of the ODS steels in such environment, high-Cr ODS steels have been developed at Kyoto University. An increase in Cr content resulted in a drastic improvement of corrosion resistance in SCW and in lead, while it was expected to cause an enhancement of aging embrittlement as well as irradiation embrittlement. Anisotropy in tensile properties is another issue. In order to overwhelm these issues, surveillance tests of the material performance have been performed for high Cr-ODS steels produced by new processing technologies. It is demonstrated that high-Cr ODS steels have a high potential as fuel cladding materials for SCWR and LFR with high efficiency and high burnup.