Development and characterisation of 9Cr ODS and reduced activation ferritic/martensitic steels for fast fission and fusion reactors

This paper presents the results of the indigenous efforts at IGCAR towards developing a 9Cr Oxide Dispersion Strengthened (ODS) and Reduced Activation Ferritic Martensitic (RAFM) steels for the Fast Breeder Reactors (FBR) and fusion program in India. The sodium cooled fast reactors require development of high temperature radiation resistant materials for achieving high fuel burn-up of 200GWd/t (∼160 dpa) or higher, which is one of the key factors for their efficient and economical operation. Ferritic/Martensitic steels (9-12% Cr) although exhibit higher void swelling resistance than austenitics, have poor high temperature creep strength, which limits the operating temperatures to ∼550 deg. C. Oxide dispersion strengthening (ODS) is a promising means of extending the creep resistance of F/M steels beyond 650 deg. C together with the advantages of high thermal conductivity and low swelling. Based on these well known principles, a developmental effort has been taken up to fabricate clad tubes using the yttria strengthened 9Cr ferritic steel. A small amount of Ti addition resulted in very fine mixed oxide particles of Y and Ti, thus improving creep rupture strength significantly. The process of clad tubes fabrication involved mechanical milling of alloy powders, consolidation by hot extrusion and tube formation by cold pilgering. Further, the particle size distribution studied using Analytical and High Resolution Electron Microscopy at intermediate stages and in the product showed a distribution of Y2O3 particles predominantly in the size range of 5-20nm. The process parameters have been optimized and tubes of outer diameter 6.6 mm, thickness 0.48 mm and length 1500mm have been produced. The RAFM steel for the test blanket module of International Thermonuclear Experimental Reactor (ITER) project has also been developed. The steel conforming to specifications has been achieved by replacement of Mo and Nb (elements that lead to high induced radioactivity) by W and Ta, in addition to strict control on Mo, Nb, B, Cu, Ni, Al, Co and Ti that induce high radioactivity and trace elements that promote embrittlement. The transformation temperatures (Ac1, Ac3 and Ms) for this steel have been determined and the transformation behavior of the high temperature austenite phase has been studied. The complete phase field diagram has been generated which is required for optimization of processing and fabrication parameters and schedules for the RAFM steel developed. (author)