Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (III)

Overpack, a high-level radioactive waste package for Japan's geological disposal program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, EBW (Electron Beam Welding), a typical application of high energy density beams widely used in various industries, was examined for its applicability to sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for single-pass welding in a narrow area for three different penetration depths. Weld joint tests including macro- and microstructure, and mechanical properties were conducted and assessed within tentatively applied criteria for weld joints. Measurement and numerical calculation for residual stress were also conducted and the tendency of residual stress distribution was discussed. These test results were compared with the basic requirements of the welding method for overpack, which were pointed out in our first report. The induced void and cold shut inside the weld joint and surface roughness were also discussed for their improvement and evaluation, which need to be established to assure the long-term integrity of overpack lid closure.     

Low activation steels welding with PWHT and coating for ITER test blanket modules and DEMO

EUROFER weldability is investigated in support of the European material properties database and TBM manufacturing. Electron Beam, Hybrid, laser and narrow gap TIG processes have been carried out on the EUROFER-97 steel (thickness up to 40 mm), a reduced activation ferritic-martensitic steel developed in Europe. These welding processes produce similar welding results with high joint coefficients and are well adapted for minimizing residual distortions. The fusion zones are typically composed of martensite laths, with small grain sizes. In the heat-affected zones, martensite grains contain carbide precipitates. High hardness values are measured in all these zones that if not tempered would degrade toughness and creep resistance. PWHT developments have driven to a one-step PWHT (750 °C/3 h), successfully applied to joints restoring good material performances. It will produce less distortion levels than a full austenitization PWHT process, not really applicable to a complex welded structure such as the TBM. Different tungsten coatings have been successfully processed on EUROFER material. It has shown no really effect on the EUROFER base material microstructure.     

Beryllium/ferritic martensitic steel joining for the fabrication of the ITER test blanket module first wall

The first wall of an international thermonuclear experimental reactor (ITER) test blanket module (TBM) is a multilayered component consisting of plasma facing armor and structural materials including the cooling channels. One of the main issues about the R&D on the TBM is to develop the joining technologies for a fabrication of the TBM first wall. The objectives of this study are to optimize the hot isostatic pressing (HIP) conditions and the interlayer combination for the fabrication of beryllium (Be)/ferritic martensitic steel (FMS) joints without a degradation of the mechanical properties of the FMS. Effects of HIP joining conditions including the temperature and interlayer types were investigated. The HIP temperature was selected for the anticipated tempering condition for FMS to avoid a grain coarsening which would deteriorate the mechanical properties of FMS. Several interlayer materials were applied in order to manufacture high strength joints. Be and FMS were joined successfully by the application of a Ti/Cu interlayer and it showed a relatively high bending strength, 257 MPa, among the interlayer types studied. The fracture was caused by a delamination of the reaction layer between FMS and the coated interlayer without a plastic deformation. This paper summarizes the results of a Be/FMS joints manufacturing and an investigation of their properties.     

A study on port plug distortion caused by narrow gap combined GTAW & SMAW and Electron Beam Welding

A study of port plug distortion resulting from narrow gap combined GTAW & SMAW and Electron Beam Welding was carried out. Thermomechanical finite element analysis of port plug becomes virtually impossible because of the requirement of huge number of nodes and elements. Hence an analysis method based on the concept of inherent strain was used in this work. The computational time required was about 40-50 min only in a Core (TM) 2 Duo, 2.66 GHz computer with 2 GB RAM, which otherwise was not possible with other conventional computation techniques. As was expected the overall distortion due to EB welding was found to be less compared to that of narrow gap GTAW & SMAW.     

Solid state bonding of CuCrZr to 316L stainless steel for ITER applications

Dissimilar metal bonds between CuCrZr and 316L stainless steel were prepared using two different solid state joining techniques. In the first instance, hot isostatic pressing, a high temperature diffusion bonding process was used to join the copper alloy to the stainless steel substrate at temperatures near 1000 °C. In the second instance, explosion bonding at ambient temperature was employed. These two techniques both yielded mechanically robust joints, where the strength of the interface exceeded that of the copper alloy, the weaker of the two substrates. However, the two bonding techniques produced near-joint microstructures that were very different. The microstructure and mechanical performance of CuCrZr/316L stainless steel joints prepared via both techniques are compared. Microstructural analysis of the joints included scanning electron microscopy, electron microprobe analysis and Auger spectroscopy techniques. The bulk mechanical properties of the substrate alloys were very different as well and are described. Particular emphasis is placed on the residual mechanical properties of the CuCrZr after thermal processing that simulate beryllium tile bonding since once the Be tiles are in place, the copper alloy cannot be solutionized and age-hardened to return it to full strength.     

Mechanical properties and microstructural investigations of TIG welded 40 mm and 60 mm thick SS 316L samples for fusion reactor vacuum vessel applications

The development of advanced fusion reactors like DEMO will have various challenges in materials and fabrication. The vacuum vessel is important part of the fusion reactor. The double walled design for vacuum vessel with thicker stainless steel material (40–60 mm) has been proposed in the advanced fusion reactors like ITER. Different welding techniques will have to be used for such vacuum vessel development. The required mechanical, structural and other properties of stainless steels have to be maintained in these joining processes of components of various shapes and sizes in the form of plates, ribs, shells, etc. The present paper reports characterization of welding joints of SS316L plates with higher thicknesses like 40 mm and 60 mm, prepared using multi-pass Tungsten Inert Gas (TIG) welding process. The weld quality has been evaluated with non-destructive tests by X-ray radiography and ultrasonic methods. The mechanical properties like tensile, bend tests, Vickers hardness and impact fracture tests have been carried out for the weld samples. Tensile property test results indicate sound weld joints with efficiencies over 100%. Hardening was observed in the weld zone in non-uniform manner. Macro and microstructure studies have been carried out for Base Metal (BM), Heat Affected Zone (HAZ) and Weld Zone (WZ). Scanning Electron Microscopy (SEM) analysis carried out for the impact fractured specimens show ductile fracture. The microstructural study and ferrite number data indicate the presence of high content of delta ferrite in the weld zone as compared to the delta ferrite in base metal.     

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

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.     

HTR-10反应堆压力容器筒体纵缝成形性能试验

介绍了10MW高温气冷实验堆反应堆压力容器的结构特征和主要技术参数。阐述了压力容器筒体纵缝的焊接工艺和筒体成形工艺，针对筒体纵缝成形进行了焊接和成形试验．对成形后焊缝的主要力学性能进行了试验。结果表明：纵缝成形并热处理后，较低的成形应变率对力学性能的影响程度较小。     

A study on influence of heat input variation on microstructure of reduced activation ferritic martensitic steel weld metal produced by GTAW process

Reduced activation ferritic martensitic (RAFM) steel is a major structural material for test blanket module (TBM) to be incorporated in International Thermonuclear Experimental Reactor (ITER) programme to study the breeding of tritium in fusion reactors. This material has been mainly developed to achieve significant reduction in the induced radioactivity from the structural material used. Fabrication of TBM involves extensive welding, and gas tungsten arc welding (GTAW) process is one of the welding processes being considered for this purpose. In the present work, the effect of heat input on microstructure of indigenously developed RAFM steel weld metal produced by GTAW process has been studied. Autogenous bead-on-plate welding, autogenous butt-welding, butt-welding with filler wire addition, and pulsed welding on RAFMS have been carried out using GTAW process respectively. The weld metal is found to contain δ-ferrite and its volume fraction increased with increase in heat input. This fact suggests that δ-ferrite content in the weld metal is influenced by the cooling rate during welding. It was also observed that the hardness of the weld metal decreased with increase in δ-ferrite content. This paper highlights the effect of heat input and PWHT duration on microstructure and hardness of welds.     

The neutron irradiation effect on mechanical properties of HIP joint material

Dispersion strengthened copper (DSCu) and stainless steel are the candidate material for the heat sink and the structural material of the ITER shielding blanket and these materials are joined by hot isostatic pressing (HIP). In this study, the neutron irradiation effect on mechanical properties of HIP joint material was examined by tensile and impact tests using specimens with irradiation damage of about 1.5 dpa. The results of tensile tests show that tensile strength of HIP joint material was about the same as that of DSCu base material, and this trend did not change after neutron irradiation. On the other hand, the impact value of HIP joint material was smaller than that of DSCu base material because of the diffusion of main elements at joint boundary. It was shown that embrittlement by the neutron irradiation effect is smaller than that of the effect by HIP joint.     

Interfacial properties of HIP joints between beryllium and reduced activation ferritic/martensitic steel

ITER test blanket modules are the most important components to validate energy production and fuel breeding for future fusion demonstration reactors. Reduced activation ferritic/martensitic steel is recognized as one of the promising structural materials for the breeding blanket systems. Beryllium is a primary candidate plasma facing materials for ITER blanket. In this work, the interfacial properties of Be/reduced activation ferritic/martensitic steel (RAF/Ms) joints were investigated for the first wall of an ITER test blanket module (TBM). The joints were produced by the solid-state hot isostatic pressing (HIP) method. Chromium (Cr) was used as a diffusion barrier with a thickness of 1 μm or 10 μm, formed by plasma vapor deposition on the Be surface. The HIPping was conducted at 1023 K and 1233 K with 160 MPa of static pressure. The temperatures are standard normalizing and tempering temperatures of F82H. EPMA showed the Cr layer effectively worked as a diffusion barrier at 1023 K. However, for the F82H/Be interface which underwent HIP at 1233 K followed by tempering a Be rich layer was formed. Bend tests revealed that a thin Cr layer and low temperature HIP is preferable. The joint with a thick Cr layer suffer from brittleness of Cr itself.     

The Laser Welding with Hot Wire of 316LN Thick Plate Applied on ITER Correction Coil Case

ITER correction coil (CC) cases have characteristics of small cross section, large dimensions, and complex structure. The cases are made of heavy thick (20 mm), high strength and high toughness austenitic stainless steel 316LN. The multi-pass laser welding with hot wire technology is used for the case closure welding, due to its low heat input and deformation. In order to evaluate the reliability of this welding technology, 20 mm welding samples with the same groove structure and welding depth as the cases were welded. High purity argon was used as the shielding gas to prevent oxidation because of the narrowness and depth of the weld. In this paper investigation of, microstructure characteristics and mechanical properties of welded joints using optimized welding parameters are presented. The results show that the base metal, fusion metal, and heat affected zone (HAZ) are all have fully austenitic microstructure, and that the grain size of fusion metal was finer than that of the base metal. The welding resulted in an increase of hardness in the fusion metal and HAZ. It was confirmed that the tensile strength of fusion metal was higher than that of base metal and the impact toughness value is higher than industry standard requirement. Thus, this welding process was determined to be reliable for manufacture of the ITER CC cases manufacture.     

Study on corrosion of welded steel for LBE-cooled fast reactors

Lead-bismuth eutectic (LBE) is a candidate coolant for fast reactors. Its physical, chemical and neutronic properties make it a prospect coolant for the reactors. However, corrosion of structure is the main problem of utilizing LBE as a coolant. Compatibility of welded structure with LBE at high temperature should be considered clearly for feasibility of lead-bismuth-cooled fast reactors. This study was preformed to investigate the mechanical properties and corrosion characteristics of the welded ferritic-martensitic (FM) steel, HCM12A, in LBE at 650 °C for 500 h. The welding methods were TIG welding (137 mm/min; 480 W), YAG laser welding (780 mm/min; 287 W) and electron beam welding (1000 mm/min; 60 kW). The oxygen concentration of test environment was maintained at 7 x 10⁻⁷ wt% by injecting Ar-H2-steam gas mixture. Vickers hardness test and SEM/EDX analysis were conducted on the cross section of welded HCM12A. It was found that oxide layer was larger in the weld zones than base metal (BM). However, outer layer was detached on some areas.     

Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (V) Applicability of MAG Welding Method to Overpack Final Closure

Overpack, a high-level radioactive waste package for Japan's geological disposal program, is required for preventing the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, metal active gas (MAG) welding, a typical arc welding method, was evaluated for its applicability in sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and parameters were examined with multilayer welding for three different groove depths. Welded joint tests including the observation of macro- and microstructures were conducted, and mechanical properties were within tentatively applied criteria. Measurements and numerical calculations for residual stress were also conducted, and residual stress distribution tendencies were discussed. These test results were compared with those previously reported for tungsten inert gas welding (TIG) and electron beam welding (EBW). MAG welding possesses the potential to complete overpack lid closure with a maximum groove depth of 190 mm, but the applicability of MAG welding to overpack final closure should be discussed from the viewpoint of the presence of slag inclusions possibly induced in the weld metal.     

First wall fabrication of 1/3 scale china dual functional lithium lead blanket

The dual functional lithium lead blanket is chosen as one of the candidate blankets for China fusion reactor, for its advantages of tritium breeding and good heat exchange performance. As one of the most important components of the blanket, the first wall (FW) is assembled with China low activation martensitic (CLAM) rectangular tubes and plates by hot isostatic pressing (HIP)–diffusion bonding (DB). In this work, the rectangular tube fabrication and FW assembly were carried out in order to verify the feasibility of the FW fabrication scheme. The mechanical property and dimensional accuracy of CLAM rectangular tubes were tested, the microstructure observation and non-destructive detection revealed the sound of the FW mock-up, and the reliability of the FW mock-ups is under evaluation.     

主管道材料高温力学性能研究

提供核电厂破前漏（LBB）设计所需材料性能参数需要测量主管道母材和焊接材料在高温下的力学性能（包括材料在地震环境下的高温动态力学性能）。基于万能伺服材料试验机和高速材料试验机测量了核电厂主管道母材控氮00Cr17Ni12Mo2不锈钢及焊接材料OK Tigrod 316L在高温（350℃）下的静动态拉伸力学性能、裂纹扩展率和静动态断裂韧性。与主管道母材和焊接材料的常温力学性能相比,2种材料在350℃下的静动态拉伸力学性能以及OK Tigrod 316L在350℃下的静动态断裂韧性都较常温有较大幅度的降低,2种材料在350℃下的抗裂纹扩展性能较常温略有下降。研究成果可为核电厂管道的LBB设计提供试验技术和材料参数支持。     

Properties of heavy components of steel grade 91 and their welds

The use of the modified 9% Cr1% Mo steel grade (grade 91) for the manufacturing of heavy products (forgings, plates) for pressure vessels is evaluated on the following aspects: (i) possibility to produce heavy components (base material) with the required properties, (ii) conditions for welding without cracking, and (iii) availability of welding products and determination of their mechanical properties. Heavy plates with thickness ranging from 20 to 300 mm have been manufactured and tested. All mechanical properties (tensile, Charpy V notch impact, creep…) have been obtained in conformity with the requirements of ASTM specifications and literature data on thinner materials. Some laboratory tests have been carried out to determine the sensitivity of this material to cracking phenomena during welding (cold cracking) or during the postweld heat treatment (reheat cracking). They show that this material does not present major welding difficulties in comparison with more conventional materials used for the manufacture of pressure vessels. To obtain high toughness properties of the weld metal and low residual stresses, the postweld heat treatment temperature was found to be high (750 to 770°C). A significant modification of the weld metal deposit chemistry was found to be necessary to achieve the convenient level of impact properties, but correlatively, a decrease of its creep resistance may be expected for this very low Nb weld material.     

焊接接头非均匀力学性能对J积分和失效评定曲线的影响

由于焊缝填充金属的不同和焊接热循环的作用,焊接接头不同区域有着不同的力学性能。本文将焊接接头模化为由两种材料组成,即母材金属和焊缝金属,用弹塑性有限元分析法计算了焊接接头强度组配对Ｊ积分和失效评定曲线的影响。结果表明,焊接接头不均匀强度组配对裂纹驱动力－Ｊ积分和失效评定曲线的形状与位置有很大影响。对于一个特定的焊接接头,如果接头为低组配或等组配,则以母材金属的极限载荷作为无因次载荷而建立的安全评定     

Fabrication of W/FMS joint mock-ups using a hot isostatic pressing

Blocks of tungsten and ferritic–martensitic steel (FMS) were joined without any interfacial defects or cracks. For the joining, two times of a hot isostatic pressing (HIP) were performed. The first HIP (900 °C, 100 MPa, 1.5 h) facilitates the diffusion bonding between W and FMS. The second HIP (750 °C, 70 MPa, 2 h) corresponds to a tempering process to retain the mechanical properties of the FMS. As an interlayer material, titanium foil that can mitigate the thermal expansion difference between W and FMS was used. In addition, a molybdenum foil was inserted to prevent an unwanted bonding of W to a canning material. The lateral cracks in W plates, which were usually observed in the case of a conventional HIP process, were not observed when the molybdenum separator was used. W/FMS joint mock-ups with a dimension of 50 mm × 50 mm × 32 mm (T) were successfully fabricated. The shear strength of the joints was 89 MPa on average.     

锆合金与不锈钢过渡管接头的爆炸焊接

研究了Ｚｒ－２－不锈钢和Ｚｒ２．５Ｎｂ－不锈钢两种过渡管接头的爆炸焊接工艺,检验了焊接结构区组织,测试了力学性能,指出了这种管接头在核工程中的良好应用前景。