Hirotatsu Kishimoto

The influences of irradiation temperature and helium production on the dimensional stability of silicon carbide

Isotropic volume expansion, or swelling, is a well-known irradiation-induced phenomenon for silicon carbide (SiC), as observed after neutron irradiation. In this work, the influences of irradiation temperature and helium production on fluence-dependent swelling behavior in cubic SiC were characterized following the establishment of an experimental technique to determine ion-irradiation-induced swelling within an accuracy of 200 °C. Measured swelling values yielded approximately at the lower edge of neutron-induced swelling data band at T∼<600 °C. A fusion-relevant helium-to-dpa condition significantly enhanced swelling at 400<∼T<∼800 °C. The temperature dependence of saturated swelling both with and without helium co-implantation suggested a transient in defect behavior between 800 and 1000 °C. The surviving defect production efficiency in heavy-ion irradiated SiC at 333 K was very roughly estimated to be 20% from low-dose swelling data.

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.

Properties and radiation effects in high-temperature pyrolyzed PIP-SiC/SiC

Abstract Polymer-derived silicon carbide has been regarded to be susceptible to radiation damage, because of severe radiation damage associated with radiation-induced crystallization of amorphous structure. However, recent advancement of highly heat-resistant silicon carbide fiber allows pyrolysis of matrix precursors at higher temperatures and expectedly the production of polymer impregnation and pyrolysis (PIP)–SiC/SiC composites with improved radiation resistance. In this work, characterization of microstructure, mechanical properties, and radiation stability of a developmental high-temperature pyrolyzed PIP–SiC/SiC composite was carried out. Although mechanical properties of Tyranno™-SA-reinforced composite pyrolyzed at 2023–2073 K were not very encouraging due to insufficient matrix densification, Tyranno-SA fiber appeared to retain its strength after repeated pyrolysis at temperatures ⩾2023 K. Carbon coating on the fiber fabric worked effectively as a fiber-matrix (F-M) interlayer in this system. Superior radiation stability was demonstrated by microstructural examination following dual-beam ion irradiation to 10 dpa at 873 K.

Microstructural stability of SiC and SiC/SiC composites under high temperature irradiation environment

Abstract Silicon carbide continuous fiber-reinforced silicon carbide matrix composites (SiC/SiC composite) are attractive as the structural material of advanced energy systems, including nuclear fusion. The irradiation may affect the fiber/matrix interphases which are responsible for the pseudo-ductile fracture behavior of SiC/SiC composites. In this work, the investigation of the microstructural evolution of SiC/SiC composites in a fusion environment is performed by the dual-ion irradiation method. Reinforcements were Tyranno™-SA and Hi-Nicalon™ Type-S. The displacement damage rate was up to 100 dpa. The irradiation temperature and He/dpa ratio were up to 1673 K and 60 appm, respectively. The microstructural modification induced by the dual-ion irradiation especially occurred in the interphase. The advanced SiC fiber did not shrink and the C/SiC multilayer interphase showed a superior microstructural stability against the dual-ion irradiation at high temperatures.

Radiation swelling of SiC under neutron irradiation

Ceramic materials produced on the basis of SiC and SiC/SiC composites are considered due to their high temperature strength, pseudo-ductile fracture behavior and low-induced radioactivity as candidate materials for fusion reactors. The radiation resistance of ceramic materials under neutron irradiation is the key problem which determines the use of these materials in fusion reactor environment. In the present paper the general physical mechanisms of radiation swelling of SiC are investigated. Recent experimental results concerning the effect of neutron and charged particle irradiation on radiation swelling of SiC are presented. A new theoretical model is suggested for the description of radiation swelling in ceramic materials. Point defects in ceramic materials can have an effective charge (e.g., an F+ center, vacancy with a single trapped electron). The theoretical model is based on kinetic consideration of charged point defect accumulation and kinetic growth of dislocation loops in the matrix taking into account the effect of internal electric field formed under irradiation in the matrix on diffusion processes of charged point defects. The theoretical results for radiation swelling are compared with the existing experimental data for irradiated SiC material.

Tensile, Compressive and In-Plane/Inter-Laminar Shear Failure Behavior of CVI- and NITE-SiC/SiC Composites

A SiC/SiC composite is an attractive candidate material but it is a challenge to apply it to the practical components because of the inherent brittle-like failure and structural anisotropy. This study aims to evaluate the failure behavior of SiC/SiC composites by various test modes. Comparison between tensile and compressive strengths revealed the clear axial anisotropy of failure strength. The in-plane shear strength by the off-axial tensile method is invalid unless considering the mixed failure modes. Alternatively, it was demonstrated that the in-plane shear strength can successfully be obtained by the Iosipescu method. The true inter-laminar shear strength can be identified by the diametral compression method.

Evaluation of dual-ion irradiated β-SiC by means of indentation methods

Dual-ion irradiation using the high-voltage accelerators and the micro- and/or nano-indentation scheme are useful techniques to evaluate the irradiation-induced mechanical property changes of fusion materials. In this study, polycrystalline β-SiC was irradiated by silicon ions with and without simultaneous injection of helium ions using DuET Multi-beam Facility. β-SiC irradiated with dual-beam was carefully tested by means of micro-indentation technique to establish hardness and fracture toughness for determining micro-mechanics. Simultaneous irradiation of Si and He ions clearly enhances radiation-induced hardening than that by single Si ion irradiation. Indentation fracture toughness of radiation-induced β-SiC increased, however, the enhancement decreased with increasing irradiation temperature and time.

Industrialization of Advanced SiC/SiC Composites and SiC Based Composites; Intensive activities at Muroran Institute of Technology under OASIS

Organization of Advanced Sustainability Initiative for Energy System/Material (OASIS), at Muroran Institute of Technology is now intensively enforcing its efforts for industrialization of advanced SiC/SiC composites and SiC based composites by fabrication system integration of NITE method. This paper provides the brief review of the activities at OASIS, where prototype production line of green sheets and prepreg sheets are under installation. The near-net shaped preforms with the NITE green-sheets and prepreg-sheets are made into near-net shape components for potential applications under consideration by HIP and Pseudo-HIP. Aiming at the near term utilization, SiC/SiC hybrid structures with metallic materials, such as steels and other refractory metals, were fabricated with promising results. Trial to reduce the production cost has been continued.

Superior Radiation Resistance of ODS Ferritic Steels

The oxide dispersion strengthened (ODS) ferritic steel and non-ODS reduced-activation ferritic (RAF) steel were irradiated at 773 K by means of a dual-beam ion irradiation technique to a dose of 0.4 dpa with simultaneous helium implantation up to 1000 appm. Microstructural changes were investigated by transmission electron microscopy. The RAF steel showed a preferential formation of cavities at grain boundaries, precipitate interfaces and dislocations. In contrast, the ODS ferritic steel showed a homogeneous and fine distribution of cavities in the matrix. This paper discusses the superior resistance of the ODS ferritic steel against development of cavities in terms of the effects of nano-oxide particles dispersed in the matrix.

In-situ Observation of Fracture Behavior on Nano Structure in NITE SiC/SiC Composite by HVEM

We have been successfully done in situ observation on the sequence of fracture event at the interface of NITE SiC/SiC composite examined by using miniaturized double notched shear specimen for TEM prepared by Focused Ion Beam method. In this study, we used nano-mechanics TEM experimental apparatus to investigate not only microstructure evolution and but also load and displacement curve at once in High Voltage Electron Microscope. Our results summarize as follows. Cracks were initiated at the interface between carbon coating layer on the SiC fiber and SiC matrices, and propagated along the interface. Load drop in the load and displacement curve during in-situ TEM was clearly observed at the crack initiation. The shear strength by using the miniaturized specimen is about ten times higher than that obtained by the standard testing.