A brief summary of tungsten technology development in Korea

Tungsten https://doi.org/10.1007/s42864-020-00036-8 Tungsten www.springer.com/42864 REVIEW PAPER A brief summary of tungsten technology development in Korea Suk‑Ho Hong1 Received: 25 April 2019 / Revised: 4 June 2019 / Accepted: 4 June 2019 © The Nonferrous Metals Society of China 2020 Abstract A review of tungsten technology development in Korea is briefly given. According to the upgrade plan of Korea superconducting tokamak advanced research (KSTAR) tokamak associated with Korean fusion energy R&D program, graphite plasma-facing components will be replaced with tungsten-based ones to handle the high-peak heat load caused by an increase of heating power up to 26 MW. Brazing technique to bond tungsten was developed and tungsten blocks were manufactured. Blocks were installed at the central divertor in KSTAR and exposed to high heat flux. Under high heat flux and long-pulse discharge, tungsten blocks were severely damaged. Molten tungsten materials show movements towards the high field side, which is j×B direction. The COMSOL® modeling described the melting event quantitatively well. The failure of a water cooling system with a metal wall environment during a long-pulse plasma operation is very critical. Keywords Demonstration reactor · Divertor · Tungsten 1 Introduction The use of fusion energy in the sun can be realized and utilized on earth by constructing a device called “tokamak”. In the fusion reaction, a so-called “hydrogen burning process”, six protons fuse together resulting in a helium (He) atom and two protons, is the most effective energy production process in nature. Furthermore, fuels—deuterium (D) and tritium (T)—are easily obtained from the ocean (D) and lithium (T). Tokamaks create plasmas confined inside very strong magnetic fields. The plasma is hot enough to fuse D atoms into T, 3He, and 4He through the sequential fusion reaction. The components inside the vacuum vessel, the so-called plasma-facing components (PFCs), are the final destination of particles, heat flux, and radiation: PFCs are exposed to harsh environments, and consequently, the lifetime of them is directly connected to the interactions between plasma and the surrounding surfaces of PFCs, which is called “plasma–surface interaction” (PSI). Recently, as the research interests on PSI move from low atomic number (Z) to high Z metal PFCs, issues such as (1) erosion/recrystallization/ * Suk‑Ho Hong 1 DEMO Technology Division Advanced Technology Research Center, National Fusion Research Institute, Daejeon 34133, Korea melting of PFCs due to high heat flux; (2) high Z impurity accumulation in core plasma will be critical for the longpulse tokamak operation. Tungsten (W) is considered as one of the favorite PFC materials because of its excellent physical and chemical properties such as the high thermal conductivity, high melting point, high sputtering resistance, and low deuterium/ tritium retention. As international thermonuclear experimental reactor (ITER) has chosen full tungsten divertors from the beginning of its first campaign [1], Joint European Torus (JET) transformed the machine configuration for ITER-relevant experiments, which is the so-called ITER-like wall (ILW) consisting of beryllium and a full tungsten divertor. The axially symmetric divertor experiment (ASDEX) upgrade (AUG) has changed the inner wall to tungsten coated graphite tiles [2, 3]. The experimental advanced superconducting tokamak (EAST) has a full tungsten divertor since 2018 and tungsten environment in steady-state tokamak (WEST) started with several tungsten monoblock fingers and tungsten coated graphite blocks [4, 5]. Initial design limits of PFCs in Korea superconducting tokamak advanced research (KSTAR) were 20 s of operation time with a 16 MW input power (KBSI internal report: KSTAR physics validation review documents, 1997). Since carbon-based graphite tiles cannot tolerate heat flux more than 3.5 MW·m−2 in such a condition, KSTAR has a PFC upgrade plan to employ the tungsten divertor, and thus, 13 Vol.:(0123456789) S.-H. Hong National Fusion Research Institute started the development activities of tungsten first wall components [6, 7]. For longterm technology development, the pre-conceptual study of the Korean demonstration reactor (K-DEMO) divertor was also performed [8]. This paper reviews the tungsten technology development associated with Korean fusion energy R&D program performed since 2012. In Sect. 2, the status of the tungsten technology development in Korea is summarized. In Sect. 3, the research activities related to the tungsten PFCs, especially the damage and melting of tungsten PFCs are reviewed. In Sect. 4, a summary is given. 2 Status of the tungsten technology development in Korea 2.1 Development of flat‑type tungsten bonding technology To develop required tungsten bonding technology, we started with the bonding between tungsten plate and CuCrZr plate since 2013: In this section, we briefly review the main results reported in our previous publications [9]. A tungsten plate [ASTM B760-86 (1999)] was chosen as a plasma-facing material (PFM) and CuCrZr (ASTM C18150) was selected as a PFC material and cooling/heat sink material [10]. Direct bonding between tungsten and CuCrZr is not easy due to the large residual stress coming from the mismatch of their expansion coefficients. A thin interlayer of oxygen-free-copper (ASTM C10200) was employed for low yield strength and elastic modulus [11, 12]. Table 1 summarizes the properties of the materials used for the brazing process. The filler alloy with a thickness of 0.05 mm was used. The size of the brazing sample was 50 mm in length, 40 mm in width, and 27 mm in height [10 mm W, 2 mm oxygen-free high conductive (OFHC), and 15 mm CuCrZr], respectively. The brazing process consists of five steps: (1) a nickel plate of a thickness of 5 µm was plated on tungsten prior to the brazing process for the improvement of wettability and bonding strength of the brazing filler, and a heat treatment at a temperature of 600 °C for an hour was followed, which enhanced the quality of the nickel plating; (2) W was brazed onto CuCrZr at 980 °C for 30 min in vacuum; (3) fast cooling from 980 to 400 °C was followed; (4) the aging treatment for CuCrZr was carried out for ~ 180 min at 470 °C; (5) cool down to the room temperature [14]. Figure 1 shows the waveform of the W/Cu/CuCrZr brazing process. The ultrasonic test (UT) was used for the detection of any defects between the brazed boundaries. The probe was a flat type with a size of 6.35 mm in diameter with the operating frequency of 10 MHz. Defects of a size larger than 2 mm in diameter could be detected. Figure 2 shows UT images of the samples brazed with different loadings, namely 5 kPa (sample A), 10 kPa (sample B), and 20 kPa (sample C). Only small defects in red circles were detected at the edge part of the W/Cu interface, while many defects were identified in the whole area of Cu/CuCrZr interface. (...truncated)