Elsevier

Fusion Engineering and Design

Volume 87, Issue 9, September 2012, Pages 1639-1646
Fusion Engineering and Design

An overview of the welding technologies of CLAM steels for fusion application

https://doi.org/10.1016/j.fusengdes.2012.06.009Get rights and content

Abstract

China Low Activation Martensitic steel (CLAMs), a kind of RAFM steel with Chinese intellectual property rights, is considered as the primary structural material for the China-designed ITER test blanket module (TBM). As one of the key issues in the fabrication of the fusion reactor, the welding technologies of CLAMs are reviewed. Emphasis is placed on the weldability of CLAMs by different welding methods, and on the properties of as-welded and post-weld heat-treated joints. Recent highlights in research and development for the welding of CLAMs show that proper welding procedure could provide welds with adequate tensile strength but the welds exhibit lower impact toughness compared with the base metal. Post-weld heat treatment (PWHT) and the application of ultrasonic energy during TIG welding could dramatically improve impact toughness. Research also shows that welds in CLAMs have sufficient resistance to swelling under irradiation as well as suitable compatibility with liquid LiPb. The microstructure, mechanical and other physical properties of welds are significantly different from those of the base metal due to the complicated welding thermal cycle. The weld joint is the area most likely to fail one or more of the design requirements within the fusion reactor. Therefore significant additional research is necessary to ensure safe application of welded CLAM steel for fusion reactor construction.

Highlights

► Welding technologies of China Low Activation Martensitic steel is overviewed. ► Most welding technologies in use are discussed and suggestions are given. ► Proper welding technologies could ensure weld properties but more detailed work are necessary.

Introduction

Energy shortage and environment stewardship are two grand challenges that human beings must face in 21st century [1]. The developing industrialization and rapidly growing human population make the energy issue more serious today than ever before. Traditional energy resources such as coal, petroleum and natural gas are non-renewable and produce byproducts harmful to environment, so the exploration of new efficient and clean energy resources is absolutely necessary and of paramount importance around the world in this century. As a kind of low-carbon or even zero-carbon energy, nuclear power continues to attract extensive worldwide attention [2]. Compared with the fossil fuels, nuclear power is much more environment-friendly and can be produced safely and efficiently. Furthermore, the amount of nuclear energy contained in the earth is quite huge.

Fission and fusion reactions are the two types of nuclear energy. Fusion is the energy source of all the stars in the universe. Among the fusion reactions, the reaction between deuterium and tritium (both are isotopes of hydrogen) can be easily realized through artificial ways [3]. Deuterium is abundant in the sea. The energy contained in deuterium extracted from 1 L of seawater is equivalent to that of 300–400 L of gasoline. Fusion power embodies an ideal resource with high efficiency and low pollution. Development and application of fusion energy is regarded as the one of the top strategic objectives of nuclear power development planning in China [4].

Development of appropriate materials which are suitable for use in fusion reactors is mandatory for realizing practical fusion energy. Activation refers to the capacity of a material itself becoming radioactive when subjected to a source of radiation. A low or reduced activation material is less likely to become radioactive and when it becomes radioactive can shed radioactivity quickly for easier disposal. Reduced/low activation materials used in fusion reactor are required during the entire reactor life-cycle and provide: neutron-induced radioactivity resistance during operation, reduced maintenance during a temporary shutdown, and simpler waste management and disposal at decommissioning [5]. Low activation is only one of the requirement for fusion reactor blanket material but it is the most important to achieve a safe and environmentally attractive fusion reactor. To date, RAFM steel would likely be among the top choices for structural applications in fusion systems [6].

RAFM steel is a promising candidate material because of its excellent thermo-physical and mechanical properties. In a high radiation flux environment, RAFM steel exhibits high geometric stability, low irradiation swelling, low thermal expansion and high thermal conductivity [7]. In addition, RAFM steels are appropriate for commercial production and require no large-scale industrial investment in new steel-making processes. Research of RAFM steel has been carried out worldwide since 1992 when the RAFM study group was founded by International Energy Agency (IEA) and typical RAFM steels are well studied, such as Japanese F82H and JLF-1, European EUROFER97 and American 9Cr2WVTa. To keep pace with the research and development of RAFM in other countries and to adapt to the tendency of constructing the DEMO fusion plant and the first fusion power reactors, the FDS team in Plasma Physical Institute of the Chinese Academy of Science started the research of CLAM steel in cooperation with other organizations in 2001 [8]. Maturation of welding technologies and processes is crucially important for the practical application of CLAM steel to fusion reactors. To date, some aspects of CLAM steel have been preferentially studied, such as optimized design of composition, melting and processing, physical and mechanical properties, and irradiation properties [9]. Less attention has been paid to the welding technologies though some research has been done. In fact, weld joints represent one of the potentially weakest structural locations because of changes in microstructure, mechanical and physical properties resulting from the welding thermal cycle. This paper reports on the R&D progress of welding CLAM steel.

Section snippets

Properties of CLAM steel

The concept of low activation materials was firstly proposed by Bloom and colleagues in 1983 [10]. The idea is that the neutron hardening ability of a material should be enhanced within the matrix and at the grain-boundaries when designing the composition and structure of these materials. Neutron hardening can be achieved by adding elements which have a negative effect on activation. At the present time, RAFM steels have been widely studied all around the world [11]. Table 1 shows the general

Development of welding technologies

Due to the recent introduction of CLAM steel, many significant tests and research efforts still need to be carried out. As is known, maturation of welding technologies and processes is vitally important for the practical application of CLAM steel. Because of the chemical composition and metallurgy characteristics of CLAM steel, proper welding procedures must be developed and tested. The properties, microstructure and composition in and near the weld metal differ significantly from the base

Compatibility of CLAM steel weldments in liquid LiPb

Liquid LiPb is regarded as a preferred tritium breeding material to be used in a fusion reactor. The compatibility of RAFM steels in liquid LiPb has been one of the hot issues in TBM study [44]. The first liquid metal LiPb loop in China, which named as DRAGON-I, was built by the FDS team in 2005 and a schematic diagram of its structure is shown in Fig. 8. Corrosion tests of CLAM steel and 316L stainless steel have been successfully conducted in this loop [45]. The results indicate that CLAM

Summary

A series of R&D activities on the structural material CLAM steel for fusion reactor construction are being carried out under world-wide collaboration with many institutes and universities in China and overseas. This paper provides a brief overview of the current development of the welding technologies of CLAMs steel as well as a review of composition development, mechanical properties, corrosion resistance and material processing simulation. The weld properties produced by a variety of welding

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant no. 50905079), China Postdoctoral Science Foundation (Grant no. 2011M501175), Postdoctoral Project of Jiangsu University (no. 1143002045) and Innovative Research Team of Jiangsu University.

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