D.S. Gelles

Low-activation ferritic and martensitic steels for fusion application

Abstract This paper reviews the history and the present status of the development of low-activation ferritic/martensitic steels for fusion applications, followed by a summary of the status of the International Energy Agency fusion materials working group activities, where an international collaborative test program on low-activation ferritic/martensitic steels for fusion is in progress. The objective of the test program is to verify the feasibility of using ferritic/martensitic steels for fusion by an extensive test program covering the most relevant technical issues for the qualification of a material for nuclear application. The development of a comprehensive data base on the representative industrially processed reduced-activation steels of type 89Cr2WVTa will provide designers a preliminary set of material data within about 3 years for the mechanical design of components, e.g., for demo relevant blanket modules to be tested in ITER. Knowledge on the current limitations of low-activation ferritic steels for application in advanced fusion systems is reviewed and future prospects are defined.

Ferritic/martensitic steels - overview of recent results

Considerable research work has been conducted on the ferritic/martensitic steels since the last International Conference on Fusion Reactor Materials in 1999. Since only a limited amount of that work can be reviewed in this paper, four areas will be emphasized: (1) the international collaboration under the auspices of the International Energy Agency (IEA) to address potential problems with ferri tic/marten si tic steels and to prove their feasibility for fusion, (2) the major uncertainty that remains concerning the effect of transmutation helium on mechanical properties of the steels when irradiated in a fusion neutron environment, (3) development of new reduced-activation steels beyond the F82H and JLF-1 steels studied in the IEA collaboration, and (4) work directed at developing oxide dispersion-strengthened steels for operation above 650degreesC

Current status and future R&D for reduced-activation ferritic/martensitic steels

Abstract International research and development programs on reduced-activation ferritic/martensitic steels, the primary candidate-alloys for a DEMO fusion reactor and beyond, are briefly summarized, along with some information on conventional steels. An International Energy Agency (IEA) collaborative test program to determine the feasibility of reduced-activation ferritic/martensitic steels for fusion is in progress and will be completed within this century. Baseline properties including typical irradiation behavior for Fe–(7–9)%Cr reduced-activation ferritic steels are shown. Most of the data are for a heat of modified F82H steel, purchased for the IEA program. Experimental plans to explore possible problems and solutions for fusion devices using ferromagnetic materials are introduced. The preliminary results show that it should be possible to use a ferromagnetic vacuum vessel in tokamak devices.

Progress and critical issues of reduced activation ferritic/martensitic steel development

The inherent properties of reduced activation ferritic/martensitic (RAFM) steels include reduced swelling and high recycling potential, which make them likely candidates for application in commercial fusion power plants. The International Energy Agency (IEA) agreement has been an effective framework for international co-operation in developing RAFM steels. The progress and critical issues observed in this co-operation are reported. The production of RAFM steels on an industrial scale has been demonstrated. Various methods of fusion welding and solid hot isostatic pressing (HIP) are feasible for joining the steels. Manufacturing of complex shapes with the powder HIP method works well for RAFM steels. Major critical issues addressed concern the effects of simultaneous introduction of helium and displacement damage. The availability of a 14 MeV neutron source is identified as an essential tool to determine this effect. Finally, the potential of oxide dispersion strengthening to increase the operating temperature of RAFM steels is considered as an issue that has to be resolved to enlarge the application temperature window of RAFM steels.

Development of oxide dispersion strengthened ferritic steels for fusion

An oxide dispersion strengthened (ODS) ferritic steel with high temperature strength has been developed in line with low activation criteria for application in fusion power systems. The composition Fe–13.5Cr–2W–0.5Ti–0.25Y2O3 was chosen to provide a minimum chromium content to insure fully delta-ferrite stability. High temperature strength has been demonstrated by measuring creep response of the ODS alloy in uniaxial tension at 650°C and 900°C in an inert atmosphere chamber. Results of tests at 900°C demonstrate that this alloy has creep properties similar to other alloys of similar design and can be considered for use in high temperature fusion power system designs. The alloy selection process, materials production, microstructural evaluation and creep testing are described.

Microstructural examination of commercial ferritic alloys at 200 dpa

Abstract Microstructures and density change measurements are reported for martensitic commercial steels HT-9 and modified 9Cr1Mo (T91) and oxide dispersion strengthened ferritic alloys MA956 and MA957 following irradiation in the FFTF/MOTA at 420°C to 200 dpa. Swelling as determined by density change remains below 2% for all conditions. Microstructures are found to be stable except in recrystallized grains of MA957, which are fabrication artifacts, with only minor swelling in the martensitic steels and α′ precipitation in alloys with 12% or more chromium. These results further demonstrate the high swelling resistance and microstructural stability of the ferritic alloy class.

Irradiation creep mechanisms: An experimental perspective☆

Abstract An extensive review was conducted of a variety of radiation-induced microstructural data, searching for microstructural records of various irradiation creep mechanisms. It was found that the stress-affected evolution of dislocation microstructure during irradiation is considerably more complex than envisioned in most theoretical modelling studies, particularly in the types of interactive feedback mechanisms operating. Reasonably conclusive evidence was found for a SIPA-type mechanism (stress-induced preferential absorption) operating on both Frank loops and network dislocations. Stress-induced preferential loop nucleation (SIPN) processes may also participate but are thought to be overshadowed by the stronger action of SIPA-type processes operating on Frank interstitial loops. It was not possible to discern from microstructural evidence between second-order SIPA and first-order SIPA mechanisms, the latter arising from anisotropic diffusion. Evidence is presented, however, that validates the operation of stress-induced preferential unfaulting of Frank loops and stress-induced growth of previously stressed material following removal of applied stress. Dislocation glide mechanisms are also participating but the rate appears to be controlled by SIPA-type climb processes. Applied stresses are shown to generate very anisotropic distributions of Burgers vector in the irradiation-induced microstructure.

Void swelling in binary FeCr alloys at 200 dpa

Abstract Microstructural examinations have been performed on a series of binary Fe Cr alloys irradiated in the FFTF/MOTA at 425°C to 200 dpa. The data represent the highest swelling levels reported to date in neutron-irradiated ferritic alloys. The alloy compositions ranged from 3 to 18% Cr in 3% Cr increments and the irradiation temperature corresponded to the peak swelling condition for this alloy class. Density measurements showed swelling levels as high as 7.4%, with the highest swelling found in the Fe 9Cr and 6Cr alloys. Microstructural examinations revealed that the highest swelling conditions contained well-developed voids, often as large as 100 nm, and a dislocation network comprised of both a /2〈111〉 and a 〈100〉 Burgers vectors. Swelling was lower in the other alloys, and the swelling reduction could be correlated with increased precipitation. These results are considered in light of the current theories for low swelling in ferritic alloys, but no theory is found to completely explain the results.

Neutron irradiation of FeMn, FeCrMn and FeCrNi alloys and an explanation of their differences in swelling behavior☆

Abstract The swelling and phase stability of neutron-irradiated FeCrMn and FeCrNi alloys are compared in the range 420–600°C. While the behavior of the two alloy systems exhibits many similarities, that of the FeCrMn system is more complex, involving a higher level of phase instabilities. However, the sensitivity of radiation-induced density changes to composition is less in the FeCrMn system. In the FeCrMn system there are three major components of density change, namely void growth, ferrite formation and lattice parameter changes of the retained austenite; whereas void swelling is the only major component of density change in the FeCrNi system.

Swelling and dislocation evolution in simple ferritic alloys irradiated to high fluence in FFTF/MOTA

Abstract Microstructures of a series of Fe Cr binary ferritic alloys were examined following neutron irradiation to 140 dpa at 698 K in FFTF/MOTA. The chromium concentration ranged from 3 to 18% in 3% increments and the irradiation temperature corresponded to the peak swelling condition for this alloy class. The swelling varied from 0.4 to 2.9% depending on chromium concentration, and the highest swelling was found in the Fe 9Cr alloy. The cavity microstructures corresponded to transient to early steady-state swelling regime. Dislocations were composed of networks with botha〈100〉 and (a/2)〈111〉 Burgers vector anda〈100〉 type interstitial loops. The dislocation density was negatively correlated with swelling. Explanation for the observed chromium concentration dependence of microstructural development and low swelling in the ferritic alloys will be studied in connection with the dislocation bias efficiency and the theory of sink strength ratio.