Research of Low Activation Structural Material for Fusion Reactor in SWIP

Transcription

1 1 Research of Low Activation Structural Material for Fusion Reactor in SWIP Jiming Chen*, Pinghuai Wang, Haiying Fu and Zengyu Xu Southwestern Institute of Physics, P.O. Box 432, Chengdu , China contact of main author: Abstract: This paper briefly reviews the strategies for low activation structural materials development in Southwestern Institute of Physics (SWIP) and the current status of the materials under investigation. Two main kinds of low activation structural materials are under developing, one is reduced activation ferritic/matenstic steel CLF-1 for Chinese ITER test blanket modules (CN ITER TBM) and the other is vanadium alloy for the advanced blanket modules. Several 1-ton ingots of CLF-1 steel have been produced and the key technical issues for the fabrication of CN ITER TBM have been studied in the fabrication of small size TBM. Various properties are accumulating to form a database to support the design of a full size TBM. A 30kg V-4Cr-4Ti alloy made by electron beam and electric arc melting (SWIP-30) and various Ti3SiC2 PDS vanadium alloys made by mechanical alloying (MA-vanadium) were developed in SWIP. 1. Introduction Development of blanket structural materials with strong resistance to irradiation damage by fusion neutrons is a critical path to nuclear fusion power plant. For safety, environmental and economic attributes attentions have been paid on low activation structural materials, especially the reduced activation Ferritic/Martensitic (RAFM) steels and vanadium alloys. Southwestern Institute of Physics (SWIP) is one of the largest institutes in China, focusing on scientific researches in plasma physics, magnetically confined nuclear fusion and engineering researches for nuclear fusion energy. In SWIP, the development of low activation materials will follow the fusion reactor development strategy from ITER to a demonstration fusion reactor (DEMO) and nuclear fusion power plants finally. Both structural material development and blanket technology verification by operation of ITER test blanket module (ITER TBM) are key approaches towards the construction of tritium breeding blankets for fusion power demonstration plant (DEMO blanket) [1]. RAFM steel, which is chosen as the structural material of ITER TBM due to its advanced technological bases, will be fully qualified in ITER TBM and then used as the major candidate structural material of DEMO. Helium-cooled ceramic breeder (HCCB) with a pebble bed concept was selected as Chinese TBM [2]. The CLF-1 steel, a new grade of Chinese RAFM steel, has been developed in SWIP to provide material and technical database for the design and fabrication of the CN HCCB TBM [3]. Key issues for the RAFM steel will be studied, such as neutron irradiation effect, compatible with coolant and other materials, the effect of ferromagnetic characteristic on the strong magnetic field of fusion reactor, in order to meet the requirement for application as the DEMO blanket structural materials. Tritium breeding fusion blankets with vanadium alloys as structural materials and liquid lithium as breeding and cooling materials (self-cooled Li/V blankets) have been designed as advanced concepts for DEMO and commercial fusion reactors [4-5]. There are plenty of attractive features of Li/V blankets, such as much more simple structures and high thermal efficiency because of their low activation, excellent thermal stress factor, superior high temperature strength (operation temperature up to ~750 o C), good ductility at low temperature, good compatibility with liquid lithium, and high resistance to neutron irradiation [6-7]. In the world, lots of counties such as USA, Japan, Russia Federation, China, France and so on, are

2 2 devoted to develop the vanadium alloys especially V-4Cr-4Ti for application in the Li/V blankets [8-13]. To further enhance the thermal efficiency of the blanket for advanced reactor, not only vanadium alloys but also particle dispersion strengthened (PDS) vanadium alloys were developed in SWIP. 2. Reduced Activation Ferritic/Martensitic Steel CLF Technical Issues for Fabrication of CN HCCB TBM The design and performance analysis as well as the drafting of the design description document (DDD) of CN HCCB TBM have been completed, while the fabrication of a 1/3 size mock-up of the TBM has being started to assess the feasibility of fabrication and welding technologies for the full size TBM. One of the critical issues for manufacturing the small size TBM fabrication is the availability of structural material production in large scale, including various plates and tubes production. Reliability of welding technologies is another critical issue. As the structural materials for the CN HCCB TBM, the CLF-1 steel has been studied for several years, starting from small heats used to determine compositions and to improve mechanical properties by optimizing the smelting technologies and heat treatment. Processing routes like smelting, forging and rolling have been well developed for CLF-1 steel. These research activities have led to a better understanding of the correlation between alloy composition, processing behavior, heat treatment, microstructure and resulting mechanical properties. Based on the results obtained from the small heats, the chemical composition of CLF-1 steel was finalized as Fe-8.5Cr-1.5W-0.25V-0.5Mn-0.1Ta-0.1C (wt.%) and in addition of 0.025wt.% N to enhance the thermal stability of its microstructure and mechanical properties. With the cooperation of the domestic institutes and industrials, several 1000kg heats of CLF-1 steel were produced by vacuum induction melting and consumable electrode re-melting process, and were manufactured into different product forms including plates in various thickness, bars, tubes and wires. The content of oxygen was kept below 0.005wt.% and the radiological undesirable trace elements in the 1-ton CLF-1 steel ingots were controlled to low level. The materials will be used in normalized and tempered state, normalizing at 1253K for 45 min (actual time depends on product forms) followed by air cooling, and tempering at 1013K for 1.5 h and followed by air cooling. According to the structure design of CN HCCB TBM [2], the key process is the manufacturing of U-shape first wall, which has an overall thickness of 35mm including coolant channels and a 3mm thickness front wall. Hot iso-static pressing (HIP) is the most promising technologies in the process of the first wall joining and the manufacturing of sub-module. Electron beam (EB) welding will be applied for the fabrication of sub-modules and the joining of grids as well as the joining of the thick-plates and pipes of the back system, while the tungsten inert gas (TIG) welding mainly for the joining of sheets and pipes of the back system. By now the weldability of CLF-1 steel of different welding method has been investigated. HIP joining was performed on discs with a diameter of 120mm and a thickness of 35mm, which had a surface roughness of 1.8~3.2μm. HIPing was performed at 150MPa at temperatures ranging from 1000 o C to 1150 o C for 2h. After joining, all of the joints were treated by a post welding heat treatment (PWHT), which was the same as the heat treatment for the base metal that was normalized at 980 o C for 45min with following air cooling to RT and then tempering at 740 o C for 90min. Such treatment recovers the joint metal to the tempered martensitic microstructure. The experiment results showed that to get high quality

3 3 HIP joints, the joining surface finish shall be better than 3.2μm and the HIPing should be conducted at 150MPa for 2h around 1050 o C. TIG and EB welding were performed on 15mm thick plates. Macrostructure of the welds showed that three zones could be distinguished as molten zone (MZ), heat-affected zone (HAZ) and the matrix. The weld bead volume of TIG welding is much larger than that of EB welding, which could be preferable for the thick-plate welding. The tensile strength of the weld is rather high up to 720MPa in the TIG-welds as compared with that of 670MPa in the steel at as-received state. On the other hand, the total elongation and the impact properties of the TIG-welds were much worse. So a PWHT is required to improve the mechanical properties. The PWHT at 740 o C for 3h, which is the same as the tempering temperature of the base metal, led to the best mechanical properties in the welds. Although the tensile strength was about 40MPa lower than that of the base metal, the total elongation and the impact absorbed energy got closer to the base metal. The same PWHT process is also necessary for the EB welds. Both of the TIG and EB welding tests showed that preheating was not necessary because the steel was not sensitive to cracking based on the non-destructive testing results. Currently these joining technologies are being developed on various thick plates and on the joining between the plates and pipes to make it reliable for TBM manufacturing. 2.2 Property Database for Design of Full Size CN HCCB TBM The as-received normalized and tempered CLF-1 steel has a microstructure of fully martensitic with equiaxed prior austenitic grain size of ~13 μm, in which are thin martensitic laths of 300±100nm width, where M23C6 (M=Cr, W, Fe) carbides distribute mainly along lath and grain boundaries and MX (M=Ta, V; X=C, N) particles distribute in matrix. The mechanical properties of the CLF-1 steel from an 1-ton ingot, including tensile, impact, creep and fatigue, were evaluated. The tensile properties were measured with cylindrical specimens in a gauge dimension of 5mm diameter and 25mm length over a temperature range of RT-600 o C. The ultimate tensile strength (UTS) at room temperature and 500 o C was about 680MPa and 450MPa, respectively. The ductile-to-brittle transition temperature (DBTT) is derived from the impact property test using V-notch specimens in dimension of 10mm 10mm 55mm with a 2mm deep 45 angle V-notch and a 0.25mm root radius, which are aligned parallel to the hot-forged direction. Charpy tests were conducted in the range from room temperature to -196 o C, where the upper shelf energy (USE) and lower shelf energy (LSE) are about 280J and 25J, respectively. The DBTT was about -75 o C where the absorbed energy is the half of the sum of USE and LSE. It is comparable to the values of -70 o C for Eurofer97 in the as-received condition (980 o C 27min +760 o C 90min) [14]. Thermal ageing effect on the microstructure and mechanical properties was studied. No obvious degradation in tensile properties but a slightly DBTT shift was identified. The shift is about 5 o C and 10 o C after thermal ageing up to 6000h at 550 o C and 600 o C, respectively. Microstructure observation results show that thermal ageing strongly affect the precipitation behaviors. M23C6 carbides get coarsening and agglomeration along both the lath boundaries and the prior austenitic grain boundaries, but little change of the MX type precipitates were observed. The coarsening and agglomeration of M23C6 seems to be the reason of the impact properties deterioration after long term thermal ageing [15]. The high temperature performance of creep resistance and fatigue behavior were studied. Creep test was conducted at 550 o C at different applied stresses, and the results show that the minimum creep rate of CLF-1 steel was smaller and rupture time was longer than those of the commercial steels. The reason was considered to be the addition of nitrogen induced the increase of the number of MX (mainly vanadium-rich nitrides) precipitates. The material will

4 4 suffer combined mechanical and thermal cycling in blanket service condition, thus the low cycle fatigue life (LCF) was examined and total strain range versus number of cycles to failure curves was plotted at different temperature. The thermal-physical properties of the RAFM steel are required for the design of the TBM, considering the thermo-mechanical loading due to the exposure to a high heat flux and the required heat transfer function of the TBM. Currently data on thermal diffusivity, thermal conductivity and linear expansion coefficient of the 1-ton scale CLF-1 steel are available. The thermal conductivity, linear expansion coefficient and thermal Diffusivity of CLF-1 steel at 300 o C were evaluated to be 30.8 W/m oc, / o C and m 2 /s, respectively. RAFM steel, which is ferromagnetic and will be used under strong magnetic field, may induce a mechanical loading by an electro-magnetic force and magnetic ripple that is harmful for the plasma control. The electric conductivity and magnetic permeability at room temperature were thus evaluated, which are S/m and 2.9mH/m, respectively. Most of these physical properties are similar to those of other RAFM steels [14,16]. 3. Vanadium Alloys 3.1 Fabrication of a 30kg V-4Cr-4Ti Ingot and its Thermo-mechanical Treatment In SWIP, a 30kg V-4Cr-4Ti ingot (SWIP-30) was produced by EB melting combined with electric arc melting. It was canned with stainless steel to reduce air contamination during hot working process, including both hot forging and hot rolling. 5mm plates were produced after the final process of cold rolling. Chemical composition has been measured for the plates, as shown in table 1. For further development of vanadium alloys in advanced Li/V blanket systems, it is necessary to control the content of impurities, such as minimize Nb, Mo and Ag due to neutron activation; optimize Si ( wppm) to suppress neutron-induced swelling and limit other minor impurities of O (400wppm), N (200wppm), S (30wppm), P (30wppm) to avoid grain boundary segregation and brittle phases [8]. SWIP-30 almost satisfies the chemical composition requirements. Good control of impurities was achieved with the sum of C, N, and O less than 430wppm and other undesirable elements in low level. TABLE 1 CHEMICAL COMPOSITION OF MAIN ELEMENTS AND IMPURITIES (MASS %) Cr Ti C N O S Al Si K Fe Mg Ca < Ge Mo Na Ta Zr Ni < <0.005 <0.001 < Thermo-mechanical treatment (TMT) combining heat treatment with cold working (CW) was conducted in collaboration with NIFS, Japan. The as-received SWIP-30 alloys of 6mm hot rolled plates, which was vacuum annealed at 980 o C for 1.5 hours, was cold worked for different deformation for different TMT combinations. Table 2 shows the detail of the experiment. The deformation changes are listed in the table. All the heat treatment experiments have been done with vacuum less than 4.0x10-6 torr to avoid additional interstitial impurities contamination (such as oxygen) from atmosphere. Standard heat treatment (STD), solid annealing (SA) and aging (A) were conducted at 1000 o C for 2h, 1100 o C for 1h and 600 o C for 20h, respectively. Tensile tests were carried out in vacuum less than 4x10-6 torr using miniature specimens with gauge dimension of 0.25x1.2x5mm 3 at a strain rate of 6.7x10-4 s -1 over a temperature of RT to 800 o C.

5 5 TABLE 2 DETAILS OF TMT EXPERIMENT Abbreviation Deformation (reduction in thickness) Tensile test at STD 95% RT SAA 95% RT,700 o C,800 o C AACW 94%, and then 17% RT,700 o C SACWA 94%, and then 17% RT,700 o C,800 o C SACWACWA 90% to 40%, and then to 17% RT,700 o C,800 o C Fig. 1 shows tensile results of SWIP-30 at different process conditions. Whether tested at RT or high temperature (HT), STD has the lowest strength (RT ultimate strength is 398 MPa), but the best ductility (RT total elongation is 26.4%). It may be possible that, while treated at STD condition (1000 o C /2h), most of the interstitials such as C, N and O were dissolved into the bcc matrix of vanadium alloy, insufficient precipitations for strengthening. While treated at TMT condition combined with CW and aging, the strengthening is very prominent, especially when tensile tested at RT and HT, SAACW has the highest strength, but lowest elongation regretfully, its RT uniform elongation is only 1.7%; SACWACWA has second high strength and relatively satisfactory ductility with RT uniform elongation 8%; SACWA also has preferable strength and elongation results whether tested at RT or HT. In short, SACWA and SACWACWA increased the alloy s high temperature strength with satisfying ductility both. Compared with SACWA, repeated CWA increased the strength further, its RT ultimate strength is 692MPa, and RT total elongation is 12.2%. TMT has prominent strengthening effect on the alloy. That is maybe because there will be a portion of the interstitials reacts with Ti solutes to form globular-shaped Ti-oxycarbonitride (Ti-CON) during aging, and these precipitations intricately react with dislocations created during CW. SAACW-RT SACWACWA-RT SAACW-700 SACWACWA-700 SACWACWA-800 SACWA-RT SAA-RT SACWA-700 SACWA-800 STD-RT SAA-700 SAA-800 Fig. 1 Tensile test results at RT and HT of SWIP-30 in different condition 3.2 Preparation of Particle Dispersion Strengthened Vanadium Alloys Mechanical alloying (MA) is a good method to produce high strength vanadium alloys for application of Li/V blankets structural material. In SWIP, V-4Cr-4Ti alloys with different Ti 3 SiC 2 addition from 0 to 1.2 mass% have been prepared by MA, the following consolidation of the MA powder by spark plasma sintering (SPS) and the final hot isostatic pressing (HIP). The particle size of starting powders of V, Cr, Ti, Ti 3 SiC 2 and YH 2 are 200 mesh, and their purity are more than 99.5%. The aim of addition of YH 2 is to absorb O and N impurities in the raw materials after annealing to form Y 2 O 3 and YN, and also to avoid the powders adhering on the inner wall of the milling vessel due to the brittle characteristic of YH 2. MA treatments were conducted by planetary ball mill with balls and vessels made by yittria-stabilized zirconia. For the designed chemical composition of V-4Cr-4Ti-1.8YH 2, XRD profiles and crystallite size distribution of mixture powders of V, Cr, Ti and YH 2 before and after MA is shown in

6 6 Fig.2 (XRD profile of MA treatment for 0h is for pure vanadium powders). Diffraction peaks of YH 2 disappeared after 10h MA, and the peaks of Ti disappeared after 30h MA. While treated for 50-70h, the mechanical alloying has almost completed with just little Cr left. While treated after 90h, ZrO 2 was detected which is collided from the ceramic balls and vessels. After 50-70h ball milling, the crystallite size distribution of the mixed powders has almost the same, no remarkable changes occurred as the milling time extended. Fig. 2 XRD profiles and crystallite size distribution for the powders after MA treatments. Thus, considering fully MA process without ZrO 2 detected, 50-70h is proper for MA. For all the V-4Cr-4Ti alloys with designed Ti 3 SiC 2 from 0 to 1.2 mass%, 60h is chosen for MA conservatively. After 60h MA, the powders were consolidated by spark plasma sintering (SPS) at 1350 o C, and then hot isostatic pressing (HIP) at 1150 o C /120MPa for 2h. Finally, annealing at o C for 1h was conducted to study the effect of heat treatment temperature on RT tensile properties. The RT tensile tests also utilize miniature samples with gauge dimension of 0.25x1.2x5mm 3 and the strain rate is s -1. By evaluating RT tensile results synthetically, it was found that, 1100 o C annealing is a proper temperature to get good mechanical properties for all the V-4Cr-4Ti alloys with designed Ti 3 SiC 2, as shown in Fig. 3. For the V-4Cr-4Ti alloy with 0.4 wt. % Ti 3 SiC 2, it has the highest strength. Its RT ultimate strength is 1108MPa, yield strength 855MPa, and total elongation is 16.8%. As the addition of Ti 3 SiC 2 increases, the strength of the alloys decreases, but elongation improves. For the 1.2 wt. % Ti 3 SiC 2 vanadium, it has the best ductility with total elongation 20 %, ultimate strength 965MPa, and yield strength 730MPa. Fig. 3 RT tensile curves of MA vanadium alloys with different content of Ti 3 SiC 2 addition from 0 to 1.2 wt. % vacuum annealed at 1100 o C. TEM observation has been taken for the V-4Cr-4Ti alloys with Ti 3 SiC 2 additions. They all have typical microstructure of m grains with a few nanometer particles in the grains

7 7 or on the boundaries, as shown in Fig.4 (a). In Fig.4 (b), dot 1 is an Y 2 O 3 particle less than 50nm analyzed by electron diffraction pattern, whereas dot 2 is V-4Cr-4Ti matrix. These ultra fine grain and particles attributed to the high strength of the alloy. a b Fig.4 (a) TEM bright field image of V-4Cr-4Ti-1.8YH 2, (b) electron diffraction pattern of Y 2 O The Efforts Towards the Neutron Irradiation Behaviors Study Although the irradiation conditions in the fission reactor are not representative of the fusion service conditions and the neutron spectrum is not as high energetic as in fusion, the irradiation experiments in the fission reactor are still considered to be important steps in the characterization of the fusion structural materials. A neutron irradiation campaign has been planned by SWIP. The irradiation experiments will be performed in the high flux engineering test reactor (HFETR) at the reactor operation research center in China, which has a power of 125MW and a maximum fast neutron flux of n/cm 2.s [17]. The irradiated materials are the CLF-1 steel and vanadium alloys with different size samples for mechanical properties test and microstructural observation. In 2013, the target dose level for the irradiation is up to 1dpa and the nominal irradiation temperature is 300 o C, which can be controlled within ±15 o C of the actual measured specimen irradiation temperature. The irradiation experiments are characterized to provide the basic irradiation database required for fusion blanket design, including the effects of neutron irradiation on the mechanical properties, dimensional stabilities, microstructures and so on. After irradiation, the mechanical properties including tensile, Charpy impact and fracture toughness will be tested to obtain an experimental assessment of the main effects of neutron irradiation on the irradiation hardening and embrittlement. In addition, the ion irradiation experiment will be performed in the materials research terminal of the Heavy Ion Research Facility in Lanzhou using 20Ne ion beam for the irradiation hardening mechanism and helium effects studies. 5. Summary Based on Chinese development strategy of fusion reactor, China is implementing the fusion blanket design and R&D plan. In SWIP, presently efforts on the development blanket structural materials are focus on the reduced activation Ferritic/Martensitic (RAFM) steels CLF-1 and vanadium alloys. For CLF-1 steel, different sizes of plates and tubes have been produced and various welding techniques have been invested to fulfill the need for the fabrication of a small size CN HCCB TBM. Materials development, welding techniques optimization and properties databases accumulating will be continued for the design and fabrication of full size ITER TBM. For SWIP-30, effect of TMT Combined aging with cold working on mechanical property was investigated, which increases the vanadium alloy s tensile strength significantly. For MA-vanadium alloys, with the addition of Ti 3 SiC 2 particle

8 8 and Y, after vacuum annealing at o C, these types of particle vanadium alloys demonstrate better mechanical property. In the near future, a neutron irradiation campaign will be started in SWIP to provide the basic irradiation database required for fusion blanket design, both for CLF-1 steel and vanadium alloys. Acknowledgement: This work was carried out with the supports of China Nuclear Energy Development Program (No.H ). References: [1] Akihiko Kimura, Current Status of Reduced-Activation Ferritic/Martensitic Steels R&D for Fusion Energy, Materials Transactions, Vol. 46, No. 3 (2005): [2] K.M. Feng, G.S. Zhang, T.Y. Luo, et al., Progress on solid breeder TBM at SWIP, Fusion Engineering and Design, Vol. 85, (2010): [3] P. H. Wang, J. M. Chen, Z. Y. Xu, et al., 15th Intenational Conference on Fusion Reactor Materials, , Charleston, South Carolina. [4]Takeo Muroga, Vanadium Alloys for Fusion Blanket Applications, Materials Transactions, Vol. 46, 3(2005) [5] Mikio Enoeda, Masato Akiba, et al., Overview of design and R&D of test blankets in Japan, Fusion Enginnering and Design, Vol. 81, 1-7(2006) [6] H. Matsui, K. Fukumoto, D.L. Smith, Hee. M. Chung, W. van Witzenburg, S. N. Votinor, J. Nucl. Mater (1996) [7] R. J. Kurtz, K. Abe, V.M. Chernov, V. A. Kazakov, G. E. Lucas, H. Matsui, T. Muroga, G. R. Odette, D. L. Smith, S. J. Zinkle, J. Nucl. Mater (2000) [8] W. R. Johnson, J. P. Smith, J. Nucl. Mater (1998) [9] T. Muroga, T. Nagasaka, et al. J. Nucl. Mater (2000) [10]T. Muroga, T. Nagasaka, et al. J. Nucl. Mater (2002) [11] T. Nagasaka, J. M. Chen, et al., Overview of the development of vanadium alloys, 15th International Conference on Fusion Reactor Materials, Oct , 2011, Charleston, USA. [12] M. M. Potapenko, et al. Proc. IEA/JUPITER-ⅡWorkshop on Critical Issues of Vanadium Alloy Development for Fusion Reactor Applications, Dec 15-16, 2003, NIFS, Japan. [13] Haiying Fu, Jiming Chen, Pengfei Zheng, et al., Fabrication using electron beam melting of a V-4Cr-4Ti alloy and its thermo-mechanical strengthening study, 15th International Conference on Fusion Reactor Materials, Oct , 2011, Charleston, USA. [14] R. Lindau, A Moslang, M.Schirra, Fusion Engineering and Design (2002) [15] P. H. Wang, J. M. Chen, Z. Y. Xu, et al., 15th Intenational Conference on Fusion Reactor Materials, , Charleston, South Carolina. [16] S.Jitsukawa, M. Tamura, B. van der Schaaf, et al., Journal of Nuclear Materials, 2002 ( ) [17] Peng Feng, Irradiation Ability of Research and Test Reactors, Nuclear Power Engineering, 2004, 25 (1): (in Chinese)