P. Spätig

On the potentiality of using ferritic/martensitic steels as structural materials for fusion reactors

Reduced activation ferritic/martensitic (RAFM) steels are the reference structural materials for future fusion reactors. They have proven to be a good alternative to austenitic steels for their higher swelling resistance, lower damage accumulation and improved thermal properties. However, irradiated RAFM steels exhibit a low temperature hardening and an increase in the ductile-to-brittle transition temperature, which imposes a severe restriction on reactor applications at temperatures below about 350°C. Furthermore, a high density of small cavities (voids or helium bubbles) has been recently evidenced in specimens irradiated with a mixed spectrum of neutrons and protons at about 300°C at a dose of 10 dpa, which could affect their fracture properties at intermediate temperatures. The upper temperature for the use of RAFM steels is presently limited by a drop in mechanical strength at about 500°C. New variants that can withstand higher temperatures are currently being developed, mainly using a stable oxide dispersion. This paper reviews European activity in the development of RAFM steels.

Microstructure assessment of the low activation ferritic/martensitic steel F82H

The microstructure of the low activation F82H ferritic/martensitic steel has been investigated in the cases of the heat treated, irradiated and deformed material. The irradiation has been achieved with 590 MeV protons in the Proton IRradiation EXperiment (PIREX) facility to a dose of 0.5 dpa at a temperature of 523 K. The unirradiated material was deformed in tension to failure at room temperature. The dislocations character as well as the dislocation density are determined. The carbide chemical composition and the size distribution of the carbides are assessed.

Assessment of plastic flow and fracture properties with small specimen test techniques for IFMIF-designed specimens

The primary mission of the international fusion material irradiation facility (IFMIF) is to generate a material database to be used for the design of various components, for the licensing and for the assessment of the safe operation of a demonstration fusion reactor. IFMIF is an accelerator-based high-energy neutron source whose irradiation volume is quite limited (0.5 litre for the high fluence volume). This requires the use of small specimens to measure the irradiation-induced changes in the physical and mechanical properties of materials. In this paper, we have developed finite element models to better analyse the results obtained with two different small specimen test techniques applied to the tempered martensitic steel F82H-mod. First, one model has been used to reconstruct the load-deflection curves of small ball punch tests (SPTs), which are usually used to extract standard tensile parameters. It has been shown that a reasonable assessment of the overall plastic flow can be done with SPTs. Second, we have investigated the stress field sensitivity at a crack tip to the constitutive behaviour, for a crack modelled in plane strain, small-scale yielding and fracture mode I conditions. Based upon a local criterion for cleavage, which appears to be the basis to account for the size and geometry effects on fracture toughness, we have shown that the details of the constitutive properties play a key role in modelling the irradiation-induced fracture toughness changes. Consequently, we suggest that much more attention has to be paid and efforts made to investigate the post-yield behaviour of the irradiated specimens and, in order to reach this goal, we recommend the use of not only tensile specimens but also of compression ones in the IFMIF irradiation matrices.