4.03.1 Introduction

The widespread acceptance of nuclear energy depends1 on the improved economics, better safety, sustainability, proliferation resistance, and waste man­agement. Innovative technological solutions are being arrived at, in order to achieve the above goals. The anticipated sustainability, rapid growth rate, and eco­nomic viability can be ensured by the judicious choice of fast reactor technology with a closed fuel cycle option. The fast reactor technology has attained (http://www. world-nuclear. org/info/inf98.html) a high level of maturity in the last three decades, with 390 years of successful operation. The emerging inter­national collaborative projects (http://www. iaea. org/ INPRO/; http://www. gen4.org/) have, therefore, cho­sen fast reactors as one of the important constituents of the nuclear energy in the twenty-first century.

The nuclear community has been constantly striv­ing for improving the economic prospects of the technology. The short-term strategies include the development of radiation-resistant materials and extension of the lifetime of the components. The achievement of materials scientists in this field is remarkable. Three generations of materials have been developed,2 increasing the burn-up of the fuel from 45 dpa for 316 austenitic stainless steel to above 180 dpa for ferritic steels. Presently, efforts are in progress to achieve a target burn-up of 250 dpa, using advanced ferritic steels. The attempts by nuclear technologists to enhance the thermal effi­ciency have posed the challenge of improving the high temperature capability of ferritic steels. Addi­tionally, there is an inherent disadvantage in ferritic steels, that is, their susceptibility to undergo embrit­tlement, which is more severe under irradiation. It is necessary to arrive at innovative solutions to overcome these problems in ferritic steels. In the long time horizon, advanced metallic fuels and cool­ants for fast reactors are being considered for increasing the sustainability and thermal efficiency respectively. Fusion technology, which is ushering (http://www. iter. org/proj) in a new era of opti­mism with construction of the International Ther­monuclear Experimental Reactor (ITER) in France, envisages the use of radiation-resistant advanced ferritic steels. Thus, the newly emerging scenario in nuclear energy imposes the necessity to reevalu­ate the materials technology of today for future applications.

The genesis of the development of ferritic steels is, indeed, in the thermal power industry. The develop­ment of creep-resistant, low alloy steels for boilers and steam generators has been one of the major activities in the last century. Today, the attempt to develop ultra super critical steels is at an advanced stage. Extensive research of the last century is responsible for identifying certain guidelines to address the concerns in the ferritic steels. The merit of ferritic steels for the fast reactor industry was established3 in the 1970s and since then, extensive R&D has been carried out4 on the application of ferritic steels for nuclear core component.

A series of commercial ferritic alloys have been developed, which show excellent void swelling resis­tance. The basic understanding of the superior resistance of the ferrite lattice to void swelling, the nature of dislocations and their interaction with point defects generated during irradiation have been well understood. The strengthening and deformation mechanisms of ferrite, influence of various alloying elements, microstructural stability, and response of the ferrite lattice to irradiation temperature and stress have been extensively investigated. The mechanism of irradiation hardening, embrittlement and methods to overcome the same are studied in detail. Of the dif­ferent steels evaluated, 9-12% Cr ferritic-martensitic steels are the immediate future solution for fast reac­tor core material, with best void swelling resistance and minimum propensity for embrittlement.

The high temperature capability of the ferritic steels has been improved from 773 to 973 K, by launching the next generation ferritic steels, which are currently under evaluation for nuclear applica­tions, namely the oxide dispersion strengthened (ODS) ferritic steels (see Chapter 4.08, Oxide Dis­persion Strengthened Steels). Conceptually, this series of steels combines the merits of swelling resis­tance of the ferrite matrix and the creep resistance offered by inert, nanometer sized, yttria dispersions to enhance the high temperature limit of the ODS steels to temperature beyond 823 K. The concerns of this family of materials include optimization of the chemistry of the host lattice, cost effective fabrication procedure, and stability of the dispersions under irra­diation, which will be discussed in this article.

The present review begins with a brief introduc­tion to the basic metallurgy of ferritic steels, summar­izing the influence of chemistry on stability of phases, decomposition modes of austenite, different types of steels and structure-property correlations. The main thrust is on the development of commercial ferritic steels for core components of fast reactors, based on their chemistry and microstructure. Hence, the next part of the review introduces the operating conditions and radiation damage mechanisms of core compo­nents in fast reactors. The irradiation response of ferritic steels with respect to swelling resistance, irra­diation hardening, and irradiation creep are high­lighted. The in-depth understanding of the damage mechanisms is explained. The main concerns of fer­ritic steels such as the inferior high temperature irra­diation creep and severe embrittlement are addressed. The current attempts to overcome the problems are discussed. Finally, the development of advanced creep-resistant ferritic steels like the ODS steels, for fission and fusion applications are presented. The application of ferritic steels for steam generator cir­cuits and the main concerns in the weldments of ferritic steels are discussed briefly. The future trends in the application of ferritic steels in fast reactor technology are finally summarized.