R.H. Jones

Status of silicon carbide composites for fusion

Abstract Silicon carbide composites are currently being investigated as potential fusion energy structural materials in the United States, the European Community and Japan. This paper presents an overview of some of the recent issues regarding the use of this low activation material including the radiation performance of present day materials and the direction towards the development of radiation hardened SiC composites.

Critical issues and current status of SiC/SiC composites for fusion

SiC fiber-reinforced SiC matrix composites (SiC/SiC) are being considered as a candidate structural material for fusion reactors because of their low induced radioactivity by 14 MeV neutron irradiation and high-temperature strength. Material development of SiC/SiC composites, including SiC fiber, matrix and interphase processing for fusion reactor applications, has been under investigation for several years. The purpose of this paper is to present an overview of current research in material development, properties and irradiation response of new SiC/SiC composites. Several technologically critical issues to be solved for fusion applications, such as irradiation resistance, thermal properties, environmental effects, hermeticity, joining technique and protective coating are also reviewed.

Current status of SiC/SiC composites R&D

Abstract Advantages of SiC-based ceramic matrix composites (CMCs) as structural materials in fusion applications rely on their high-temperature properties and stability, low density and reduced neutron activation. In recent years, experimental activities on industrial SiC CMCs have highlighted their main features under irradiation and provided important guidelines for further development of a radiation compliant material. Parallel efforts included design studies, development of advanced fibres and interfaces, alternative composite processing methods and joint development. In this paper, the current status of SiC/SiC R&D is reported and it is demonstrated that future activities require a strong collaboration with the industry as well as common efforts involving the different laboratories.

Effect of irradiation on phosphorus segregation

Abstract Phosphorus strongly segregated to the surface during irradiation of austenitic-type alloys in the temperature range 775–925 K. Much weaker but measurable radiation induced segregation of phosphorus occurred in a Ni + 0.03% P alloy. Irradiation under similar conditions produced no measurable phosphorus segregation in the ferritic HT-9 or Fe + 0.03% P alloy. The lack of segregation in the ferritic alloys was suggested to result from a weak point defect — impurity interaction in the bcc iron structure while a strong interaction was suggested for the fcc iron structure. The slow accumulation of radiation damage in bcc iron alloys is also consistent with a lack of observable segregation. The evidence strongly suggests a radiation induced mechanism but a radiation enhanced, thermally activated equilibrium segregation cannot be ruled out.

Status and prospects for SiCSiC composite materials development for fusion applications

Abstract Silicon carbide (SiC) composites are very attractive for fusion applications because of their low afterheat and low activation characteristics coupled with excellent high temperature properties. These composites are relatively new materials that will require material development as well as evaluation of hermiticity, thermal conductivity, radiation stability, high temperature strength, fatigue, thermal shock, and joining techniques. The radiation stability of SiCSiC composites is a critical aspect of their application as fusion components and recent results will be reported. Many of the non-fusion specific issues are under evaluation by other ceramic composite development programs, such as the US national continuous fiber ceramic composites. The current development status of various SiCSiC composites research and development efforts is given. Effect of neutron irradiation on the properties of SiCSiC composite between 500 and 1200 °C are reported. Novel high temperature properties specific to ceramic matrix composite (CMC) materials are discussed. The chemical stability of SiC is reviewed briefly. Ongoing research and development efforts for joining CMC materials including SiCSiC composites are described. In conclusion, ongoing research and development efforts show extremely promising properties and behavior for SiCSiC composites for fusion applications.

Radiation induced phosphorus segregation in austenitic and ferritic alloys

Abstract The radiation induced surface segregation (RIS) of phosphorus in stainless steel attained a maximum at a dose of 0.8 dpa then decreased continually with dose. This decrease in the surface segregation of phosphorus at high dose levels has been attributed to removal of the phosphorus layer by ion sputtering. Phosphorus is not replenished since essentially all of the phosphorus within the irradiation zone has been segregated to the surface. Sputter removal can explain the previously reported absence of phosphorus segregation in ferritic alloys irradiated at high doses 1,2 (>1 dpa) since irradiation of ferritic alloys to low doses has shown measurable RIS. This sputtering phenomenon places an inherent limitation to the heavy ion irradiation technique for the study of surface segregation of impurity elements. The magnitude of the segregation in ferritics is still much less than in stainless steel which can be related to the low damage accumulation in these alloys.

Strength changes in vanadium and titanium irradiated with 14 MeV neutrons

The flow properties and microstructures of vanadium and titanium have been studied following irradiation at 300/sup 0/K with T(d,n) neutrons. The threshold fluence for observable hardening was about 1 x 10/sup 21/ m/sup -2/ for both materials while at 8 x 10/sup 21/ m/sup -2/ the yield strength in the vanadium was about 1.5 times higher than for the titanium. Neither material showed the low fluence hardening plateau that is commonly observed in bcc metals and attributed to interstitial impurity atoms. Small defect clusters were found in the vanadium irradiated to 8 x 10/sup 21/ m/sup -2/ while this fluence level appeared to be the threshold for observable defect clusters in titanium.

Fracture toughness of the F-82H steel-effect of loading modes, hydrogen, and temperature

The effects of loading mode, hydrogen, and temperature on fracture toughness and tearing modulus were examined for a ferritic/martensitic steel (F-82H). The introduction of a shear load component, mode III, significantly decreased the initiation and propagation resistance of cracks compared to the opening load, mode I, behavior. Mode I crack initiation and propagation exhibited the highest resistance. A minimum resistance occurred when the mode I and mode III loads were nearly equal. The presence of 4 wppm hydrogen decreased the cracking resistance compared to behavior without H regardless of the loading mode. The minimum mixed-mode fracture toughness with the presence of hydrogen was about 30% of the hydrogen-free mode I fracture toughness. The mixed-mode toughness exhibited a lesser sensitivity to temperature than the mode I toughness. The JIC value was 284 kJ/m2 at room temperature, but only 60 kJ/m2 at −55°C and 30 kJ/m2 at −90°C. The ductile to brittle transition temperature (DBTT) was apparently higher than −55°C.

High-temperature properties of SiC/SiC for fusion applications

Ceramic matrix composites (CMCs) such as SiC/SiC exhibit novel mechanical properties relative to their monolithic counterparts. The crack velocity (dadt) versus stress intensity (K) relationship for monolithic ceramics can be described by a simple power law relationship whereas SiC/SiC was found to exhibit a multistage dadt versus K relationship similar to that for stress corrosion of metals. A K-independent stage II was followed by a strongly K-dependent stage III similar to monolithic materials. Evidence also exists that the fracture resistance of these materials is greater if cracks are produced by subcritical growth processes relative to machined notches. Oxygen was found to increase dadt and decrease the K for the stage II to stage III transition while cyclic loads produced little damage at low K values but there was some evidence for increasing damage with increasing number of cycles and K.

Neutron irradiation experiments for fusion reactor materials through JUPITER program

Abstract A Japan–USA Program of irradiation experiments for fusion research, “JUPITER”, has been established as a 6 year program from 1995 to 2000. The goal is to study “the dynamic behavior of fusion reactor materials and their response to variable and complex irradiation environment”. This is phase-three of the collaborative program, which follows RTNS-II Program (Phase-1: 1982–1986) and FFTF/MOTA Program (Phase-2: 1987–1994). This program is to provide a scientific basis for application of materials performance data, generated by fission reactor experiments, to anticipated fusion environments. Following the systematic study on cumulative irradiation effects, done through FFTF/MOTA Program, JUPITER is emphasizing the importance of dynamic irradiation effects on materials performance in fusion systems. The irradiation experiments in this program include low activation structural materials, functional ceramics and other innovative materials. The experimental data are analyzed by theoretical modeling and computer simulation to integrate the above effects.