Status and Strategy of CLAM Steel for Fusion Application in China

A program for China Low Activation Martensitic (CLAM), since 2001, aims to satisfy the material requirements of test blanket module (TBM) for ITER, China fusion engineering test reactor (CFETR) and fusion demonstration reactor (C-DEMO) in China. It has been undertaking by INEST (Institute of Nuclear Energy Safety Technology, Chinese Academy of Science) under wide domestic and international collaborations. Recent years CLAM steel has been chosen as the primary structural material of CN helium cooled ceramic breeder (HCCB) TBM for ITER. To license a pressurized nuclear equipment e.g. the ITERTBM and the blanket for DEMO, it needs to present the design and safety analyses with sufficient data such as the consolidated materials data, design limits and qualified fabrication procedures specifications etc. to the Regulator (ESP/ESPN) and the Agreed Notified Body (ANB) of France or China’s Nuclear Safety Agency. A lot of work and efforts are being devoted on the R&D of CLAM steel to its final successful application in the fusion systems. Its properties database basically meets the requirement of the qualification for ITERTBM. The status and strategy of the CLAM steel project are reviewed in this paper. 1. A brief introduction to CLAM steel Reduced activation ferritic/martensitic (RAFM) steel is considered as one of the most promising candidates for structure materials of fusion reactors due to its high strength, good heat conductivity, resistance to irradiation and low activation features [1-4]. For example, RAFM steel is chosen as the structural materials for all the six International Thermonuclear Experimental Reactor (ITER) test blanket modules (TBM). The China low activation martensitic (CLAM) steel is developed leading by the Institute of Nuclear Energy Safety Technology (INEST), Chinese Academy of Sciences (CAS) with participation of more than thirty domestic and overseas research units [5-7]. It has been chosen as the structural material in the design of FDS series PbLi blankets for fusion reactors and China fusion engineering test reactor (CFETR) blanket in China [8-10]. Especially, it has been chosen as the primary candidate structural material for Chinese helium cooled ceramic breeder (HCCB) TBM for ITER. Since the TBMs will be tested in ITER, the properties of CLAM steel have to be evaluated and assessed based on the French regulations on pressure vessel equipment, probably in its nuclear extension, as well as high standards of quality assurance required for reliable and safe operation of ITER. A lot of efforts are being devoted on the development of CLAM steel in material fabrication processes [11], material property evaluation (including mechanical properties [12], corrosion [13], irradiation [14-15] and joining [16-17] etc.) and the establishment of a material database for its successful application in CN ITER-TBM. The status and strategy of these activities for CLAM steel are reviewed in this paper. 2. Newly prepared 6.4-ton CLAM steel and its properties 2.1 Chemical composition design & Fabrication process control Alloy chemistry refinement has been explored for RAFM steels, leading to the evolution of Eurofer97-1 to Eurofer97-2 and F82H-IEA to F82H-mod and F82H-BA. More dramatic alloy composition adjustment of RAFM steels has also been pursued. For instance, increasing the percentage of Ta (~ 0.2 wt %) exhibited some improvements in both the impact properties and creep resistance despite the negligible influence on yield strength [18]. According to this result, one industrial scale ingot of 6.4 tons, named HEAT 1506, was fabricated with vacuum induction melting (VIM) and vacuum arc remelting (VAR). Two types of plates, with the sizes of 7600 mm×1550 mm×15 mm and 2100 mm× 1500 mm×52 mm, were fabricated. The heat treatment is normalizing at 1000 °C for 40 min and then cooling in water and tempering at 740 °C for 90 min and then cooling in air. The chemical compositions of the product are listed in Table 1. According to the French code RCCMRx, the analysis on the structure and the grain size was carried out. The microstructure of as-received CLAM steel was tempered martensite. The delta ferrite content was less than 3%. The presence of primary carbides was not found. The austenitic grain size index was measured to be greater than 8. The non-metallic inclusion rate was less than 1 for type A, B, C and D inclusions. Table 1. Chemical compositions of 6.4-ton CLAM steel (wt %). Cr W Ta V C Si Mn P S O N Fe HEAT 1506 9.11 1.52 0.20 0.19 0.12 0.03 0.41 0.003 0.003 0.001 0.002 Bal. 2.2 Mechanical properties 2.2.1. Effect of refining processes on tensile and Charpy impact properties The influence of smelting processes on mechanical properties of CLAM steel was investigated. As shown in Table 2 [7], the combination process of VIM, electro-slag remelting (ESR) and VAR could further decrease the total amount of impurity elements including O, N, S and P etc. It can help to obtain more uniform distribution of Ta in the matrix, which leads to higher strength and lower ductile brittle transition temperature (DBTT) as shown in Figs. 1 and 2. Table 2. Four refining processes and impurity contents of CLAM steels [7]. Ingot No. Refining process Sum of impurity contents of O, N, S, P (ppm)