Fabrication of First Wall Component of ITER Test Blanket Module by HIPping Reduced Activation Ferritic/Martensitic Steel

Takanori Hirose, Mikio Enoeda, Hiroyuki Ogiwara, Hiroyasu Tanigawa

Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011
Submitted: 12 April 2011, Accepted: 17 June 2011

Topics Covered

Abstract

Reduced-activation ferritic/martensitic (RAFM) steels are leading candidate structural material for the blanket system of fusion reactors. HIP process is the key technology to fabricate the first wall (FW) with built-in cooling channels for ITER test blanket module. This paper summarizes the R&D on optimization of HIP condition to obtain the excellent joints and evaluation of FW fabrication process. The preliminary HIP treatments, such as preliminary heating condition to degas, were optimized to decrease oxidation on the joint surfaces which cause degradation of impact properties. With optimized preliminary HIP treatment conditions, toughness of the joint was as well as that of the base metal. As for a component fabrication, a FW full-scale mockup has been developed using a RAFM. The deformation due to HIP process was within allowance level due to bracing. Moreover, fine-grained microstructure was obtained with optimized HIP condition and post HIP heat treatment.

Keywords

Ferritic/Martensitic Steel, Fusion Blanket, First Wall, Impact Property, Fracture Toughness

Introduction

The blanket is an in-vessel component to play rolls of heat-exchange and fuel breeding. The first wall (FW) of a blanket faces the fusion plasma, and it is subjected to 0.5 MW/m² of heat from plasma. Thus, the FW with built-in cooling channels have been adopted in most design of the blanket [1-3]. The FW is subjected to 0.78 MW/m² of neutron load. Therefore reduced activation Ferritic/Martensitic steels (RAFMs) are employed as the structural material for fusion blanket, because of its superior resistance to irradiation damage [4]. This exquisite combination of RAFMs and FW with built-in cooling channels has been extensively investigated all over the world. FW fabrication using Hot Isostatic Pressing (HIP) has been developed in Japan Atomic Energy Agency (JAEA) in cooperation with industries [5]. JAEA has conducted three developments simultaneously, validation of the fabrication process, performance tests on mock-ups and improvement of joint quality. Some FW mock-ups were developed using Japanese RAFMs, F82H over the past decades to validate the fabrication process in industrial scale [6]. Then the process was confirmed as feasible in industrial scale. As for the quality of the joint, it is reported that inclusion, which cannot be detected using conventional non-destructive inspection, significantly degrades impact properties of the interface [7]. Therefore, the recent interest is a mechanism of the degradation to improve the toughness. To improve the toughness, it is necessary to clarify the optimized preliminary treatment because pressure and temperature of HIP process were limited. The dimensions of FW for WCCB blanket are 232 mm width, 1700 mm height and 600 mm depth. At present, the available highest temperature and pressure for the FW are 2000 °C and 181 MPa, respectively [8]. F82H should be HIP’d at its austenizing temperature range, from 930 to 1150 °C [9]. In this temperature range, F82H shows below 100 MPa of yield stress. Thus the HIP temperature and pressure are within limit of the available HIP units. For improvement of the toughness, effects of degassing and surface-finishing were investigated in this work. This paper reviews the R&D status of fabrication of the FW and to clarify the key technical points for the high quality HIP joint.

Methods and Materials

The material used in this work is a RAFMs, F82H (Fe-0.1C-8Cr-2W-0.2V-0.04Ta). The samples for optimization study were cut out from 32 mm thick plate of F82H [10], and then the joint surface was mechanically polished. Some of the samples were followed by electrolytic polishing before canning. Canned samples were degassed at temperature range from 400 or 600 °C through the tube penetrating SUS304 canister. HIP was conducted at 1100 °C with 150 MPa for two hours using KOBELCO O₂-Dr. HIP with molybdenum heater or a commercial device in Metal Technology Co. Ltd. The HIP’d joint was heat treated at 960 and 750 °C to optimize the microstructure [9]. Sub-sized Charpy impact specimens (3.3 x 3.3 x 20 mm³) were fabricated from the joints, and V-notches were located on the joint interface. Microstructure over the joint interface was examined using a scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDX).

Figure 1. Configuration of the first wall mock-up.

Fabrication Procedure of the First Wall

Figure 1 shows the configuration of FW for water cooled ceramic breeder (WCCB) blanket. The FW consists of a 4 mm-thick surface plate, 10 mm-thick backing plate and square tubes with a rectangular cross-section 11 mm square (outer) and 1.5 mm thick, resulting in a total thickness of 25 mm. The fabrication process is briefly summarized as follows. The tubes and plates were bent to U-shape with 50 mm radius of curvature. The tubes were inserted between the plates and exterior HIP interface are closed using welding instead of canning. The HIP interface was degassed through a tube penetrating the outer surface-to-HIP interface. After these preliminary treatments, HIP and post-HIP-heat-treatment were conducted. F82H.In this procedure, the dimensional tolerance in tubes and plates are strictly controlled. The tube was precisely cold-drawn, and the dimensional error in the tube was controlled to less than 50 µm [11]. The outer radius in the square cross section was less than 1.4 mm.

The surface roughness in Rz was as fine as 8 µm after acid wash [12]. The cover plates were hot-rolled and milled to flat surface. Working tolerance due to bending introduces assembly gap on the HIP interface. It was demonstrated that assembly gap less than 0.8 mm were closed by HIP in a full-scale mock-up, although wedge shaped gap was introduced at the HIP interface among tubes and plates at the elbow part. The bending deformed the cross section of the tubes to a trapezoidal shape with 10.6 mm outer edge and 11 mm inner edge. Figure 2 shows results of ultrasonic inspection on the bent part of a full-scale mock-up of FW. No defect larger than 0.1 mm was detected even at the elbow using a phased-alley ultrasonic with 10 to 50 MHz frequency. As shown in this figure, the inspection was conducted on the surface and the backing plates. Although the interface between plates and tubes can be inspected, it is necessary to develop a method to inspect the interface between the tubes. Microstructural observation was conducted on the samples cut out from both ends of the FW. In general, HIP interfaces were hardly detected with SEM even at triple points as shown in Figure 3(a). However, some inclusions in line indicated the interface as shown in Figure 3(b). The average size of the inclusions was less than 5 µm. The inclusions were identified as complex oxides contain tantalum and chromium.

Although the oxides could degrade the thermal and mechanical properties of the interface, the cooling ability were confirmed using the full scale mock-up with high heat flux testing under ITER equivalent thermal condition [13]. These tests demonstrated that the HIP’d FW satisfies thermal and hydraulic requirements for ITER-test blanket module. However, it is difficult to assure mechanical properties of HIP joint using conventional destructive test techniques due to volume limitation around the HIP joint in the FW. Therefore the fracture properties are evaluated using specimen from dummy joint made with the same HIP procedure. It is necessary to develop a destructive evaluation method using small specimen techniques, which enable to test the joint from FW itself [14].

With increasing scale, deformation due to its own weight requires attention. In a labo-scale mock-up, the deformation was successfully suppressed with 10 mm thick backing plate [5]. The deformation causes misalignment in butt weld joint between the FW and side walls. The full-scale mock-up had to stand upright in a HIP furnace. Thus grid-like bracing was welded on the backing plate. After post HIP heat treatment, the largest deformation was less than 7 mm in the middle of plasma facing plane. It was small enough to be compensated before the following welding process [15]. From these results, the technical issues are not in the fabrication process, but in quality assurance including destructive and non- destructive inspection method.

Optimization of HIP Condition for First Wall Fabrication

Previous works demonstrated tensile properties are not sensitive to quality of HIP joint at all. The strength and ductility of the HIP joint were very similar to that of base metal. On the other hand, toughness is very sensitive to the quality of the joint [7]. Therefore, Charpy impact tests were employed to explore the condition of preliminary-treatments. Surface finishing and degassing are important for high quality joint. The former is to remove surface layer including oxides and to improve contact condition. The latter suppress oxidation in canned material. Figure 4 shows upper shelf energy of HIP joints with various surface finishing. As shown in this figure, toughness of surface after electrolytic polishing are very similar to that of base metal regardless of mechanical finishing. On the other hand, mirror like finishing does not improve the toughness without electrolytic polishing. Thus it is necessary to remove a work affected layer to achieve excellent toughness rather than surface roughness.

Figure 5 shows effects of degassing temperature on the impact properties of F82H HIP joint. The surface of HIP joint was mechanically polished to 0.1 µm of Ra with alumina powder. It is notable there is no significant difference in ductile to brittle transition temperature. However, fracture surface of HIP joint showed tiny dimple including oxide. The high density oxides arrested the growth of ductile dimple and it allowed crack to propagate easily along the interface, and it results in less toughness.

Figure 2. Results of ultrasonic inspection on bent part of a full-scale mock-up

Figure 3. HIP interface in a sample cut out from the mock-up. (a) Triple point of HIP interface. (b) HIP interface between surface plate and tube in a longitudinal cross section

Figure 4. Effects of preliminary treatment on Charpy impact properties of F82H HIP joint.

Figure 5. Effect of degassing temperature on Charpy impact properties of F82H HIP joint.

According to thermal desorption spectrometry, desorption peak of oxidizing gas are below 400 °C [16]. However, the joint should be degassed above 600 °C to suppress the oxides, and the degassing condition should be strictly controlled. From these results, key points for excellent HIP joint are removal of work affected zone and control of surface oxidation. Extremely to say, electrolytic polish and assembly process should be conducted in a reducing atmosphere after degassing.

Conclusions

The R&D status of HIP was reviewed to identify the key technical issues for the FW fabrication suggested from the recent achievements in JAEA. Major conclusions derived are as follows:

1. A Full-scale mock-up has been successfully developed in an industrial scale. It satisfies thermal and hydraulic requirements for ITER-test blanket module.
2. It is necessary to establish quality assurance procedure including destructive and non-destructive inspection method.
3. Removal of a work affected layer and suppression of oxidation are key factor for robust joint.

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Contact Details

Takanori Hirose¹, Mikio Enoeda¹, Hiroyuki Ogiwara², Hiroyasu Tanigawa^3
1 Japan Atomic Energy Agency, 801-1 Mukouyama, Naka, Ibaraki 3110193, Japan
² Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
³ Japan Atomic Energy Agency, 2-166 Oaza-Obuchi-Aza-Omotedate, Rokkasho, Kamikita, Aomori 0393212, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[1] (2011) 34-38.