R. H. Jones

Design and material issues for high performance SiCf/SiC-based fusion power cores

The SiCf/SiC composite is a promising structural material candidate for fusion power cores and has been considered internationally in several power plant studies. It offers safety advantages arising from its low induced radioactivity and afterheat, and the possibility of high performance through high temperature operation. However, its behavior and performance at high temperatures and under irradiation are still not well known and need to be better characterized. This paper summarizes the current SiCf/SiC design and R&D status. The latest SiCf/SiC-based power core design studies are summarized, and the key SiCf/SiC parameters affecting the performance of power core components are highlighted. The current status of the material R&D is discussed, with the focus on fabrication and joining, baseline properties and properties under irradiation, as well as the desirable evolution of these properties. In the light of this, the R&D plans are summarized and assessed. Finally, to help present-day design studies and in the expectation of future confirmatory R&D results, recommendations are provided on SiCf/SiC parameters and properties to be assumed for present design analysis of long term SiCf/SiC-based power plants.

Recent advances in the development of SiC/SiC as a fusion structural material

Abstract Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat characteristics coupled with their excellent high-temperature properties. These new materials will require optimization of their hermeticity, thermal conductivity, radiation stability, chemical compatibility with fusion relevant environments, and joining methodology before they can be utilized effectively in a fusion energy system. Recent advances have been demonstrated for advanced SiC fibers and fiber/matrix interfaces. Improved creep resistance and radiation stability have been demonstrated for Hi-Nicalon, Nicalon-S and DOW/Corning Sylramic fibers. Advanced fiber/matrix interfaces made with porous SiC and multi-layer SiC/C/SiC are being developed to minimize the presence of C in the interface for improved radiation and chemical stability of the composite materials. There have also been advances in evaluating He effects in SiC/SiC composites, the effects of 26Al on activation of SiC and joining methodology for SiC/SiC composites. These advances continue to support the promise of these advanced composite materials for fusion energy applications.

Helium-bubble formation behavior of SiCf/SiC composites after helium implantation

Abstract Helium-bubble formation behavior in SiC-fiber-reinforced SiC-matrix (SiCf/SiC) composites was studied using helium implantation. Microstructural observation of the He-implanted specimens was carried out after post-implantation annealing at 1673 K for 1 h. Microstructural observation revealed small cavities in the SiC matrix only. No cavities were observed in the SiC fibers or in the carbon coating layers or their interfaces.

Radiation response of SiC-based fibers

Abstract Loss of strength in irradiated fiber-reinforced SiC/SiC composite generally is related to degradation in the reinforcing fiber. To assess fiber degradation, the density and length changes were determined for four types of SiC-based fibers (Tyranno, Nicalon CG, Hi Nicalon and Dow X) after high temperature (up to 1000°C) and high dose (up to 80 dpa-SiC) irradiations. For the fibers with nonstoichiometric compositions (the first three types in the list), the fiber densities increased from 6% to 12%. In contrast, a slight decrease in density (

Low activation materials

Abstract Low or reduced activation materials are currently being developed and evaluated as structural materials for fusion energy systems. The goal of developing low activation materials is to provide fusion energy systems with a competitive edge over fission energy systems where high level waste issues abound. The primary low activation materials being developed by the international fusion materials community are: (1) ferritic/martensitic steels, (2) vanadium alloys and (3) SiC/SiC composites. These three materials offer a range of temperature and coolant design options and would likely be the optimum choices even without a low activation criteria. However, there are a number of activation, safety and disposal issues that must be solved to achieve an optimum blanket design.

Study of helium effects in SiC/SiC composites under fusion reactor environment

Helium release behavior from He-ion implanted SiC/SiC, monolithic β-SiC, and SiC fiber (Hi-Nicalon) was studied using the thermal desorption method with annealing temperatures between 500°C and 1600°C in 100°C increments. Helium release from SiC/SiC composites began below 500°C, while from monolithic β-SiC, helium release was observed above 1000°C. The magnitude of the helium released from β-SiC was 1/10 to 1/50 of the helium released from the composites. Helium release from the fiber became apparent above 1300°C. Helium release from composites at low temperature might be attributed to helium release from the carbon interphase. Behavior of transmuted helium in the SiC/SiC composites under fusion reactor conditions is discussed.

Effect of thickness and loading mode on the fracture properties of V–4Cr–4Ti at room temperature

Abstract The effect of thickness on the room temperature (RT) mode I fracture behavior of V–4Cr–4Ti has been investigated. Mode I fracture properties were measured from J -integral tests of compact tension (CT) specimens ranging in thickness from 6.4 to 25.4 mm. All specimens were machined in the T–L orientation and vacuum annealed following final machining. Two heats of V–4Cr–4Ti were tested. Specimens 6.4 and 12.7 mm thick were taken from Wah Chang Heat No. 832665. The 25.4 mm thick specimens were obtained from Wah Chang Heat No. 832864. The effect of loading mode on fracture of V–4Cr–4Ti at RT was also studied using material from Heat No. 832665. Mode I fracture behavior was compared to mixed-mode (I/III) fracture properties obtained from modified CT specimens. Crack angles of 0° and 25° were used to vary the ratio of mode III to mode I loading. J – R curves were generated as the basis for determining the affect of loading mode. The specimen loaded in mixed-mode exhibited lower resistance to crack initiation and propagation than pure mode I specimens.

Helium implantation effects on mechanical properties of SiCf/SiC composites

Abstract Helium effects on mechanical properties of SiC/SiC composites were studied to apply the composites on structural materials of a fusion reactor. Two types of 2D-SiC/SiC composites were examined in this work. Helium was implanted using 36 MeVα particles with an energy degrader system to obtain a uniform helium depth distribution in the specimen. The implantation temperature was 400–800°C and the helium concentration after implantation was about 150–170 appm. Three-point bending test was carried out at room temperature. Results of bending test showed that a small decrease of bend strength was observed in the composite which had the higher bend strength before implantation. Helium implantation effect was not clearly observed in the lower bend strength composite. SEM observation results on the fracture surface gave no evidence for fiber coating failure. The decrease of bend strength after helium implantation may be attributed to a degradation of mechanical properties of the fibers by implanted helium.

Failure mechanisms in continuous-fiber ceramic composites in fusion energy environments

Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These composites are relatively new materials with a limited database; however, there is sufficient understanding of their performance to identify key issues in their application. To date, dimensional changes of the constituents, microstructural evolution, radiation-enhanced creep, and slow crack growth have been identified as potential lifetime limiting mechanisms. Experimental evidence of these mechanisms, the factors that control them, and their implications on component lifetime will be discussed.

The performance of Chinese 316L and 316Ti stainless steel irradiated at 300, 400, 500 and 600 °C in HFIR JP-23 test capsule

Shear punch tests, density measurements and microstructure observations are reported for Chinese 316 austenitic stainless steel specimens which were isothermally irradiated at temperatures ranging from 300 to 600 °C to doses between 7 and 9 dpa.