Jens Reiser

Materials for DEMO and reactor applications-boundary conditions and new concepts

DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.

Tungsten as a Structural Divertor Material

Refractory materials, in particular tungsten base materials are considered as primary candidates for structural high heat load applications in future nuclear fusion power plants. Promising helium-cooled divertor design outlines make use of their high heat conductivity and strength. The upper operating temperature limit is mainly defined by the onset of recrystallization but also by loss of creep strength. The lower operating temperature range is restricted by the use of steel parts for the in- and outlets as well as for the back-bone. Therefore, the most critical issue of tungsten materials in connection with structural divertor applications is the ductile-to-brittle transition. Another problem consists in the fact that especially refractory alloys show a strong correlation between microstructure and their manufacturing history. Since physical and mechanical properties are influenced by the underlying microstructure, refractory alloys can behave quite different, even if their chemical composition is the same. Therefore, creep and thermal conductivity have been investigated using typical commercial tungsten materials. Moreover, the fracture behavior of different tungsten based semi-finished products was characterized by standard Charpy tests which have been performed up to 1100 °C in vacuum. Due to their fabrication history (powder mixing, pressing, sintering, rolling, forging, or swaging) these materials have specific microstructures which lead different fracture modes. The influence of the microstructure characteristics like grain size, anisotropy, texture, or chemical composition has been studied.

HE-COOLED DIVERTOR DEVELOPMENT TOWARDS DEMO

Abstract A He-cooled divertor concept for DEMO has been pursued at Forschungszentrum Karlsruhe within the framework of the EU power plant conceptual study. The design goal is to achieve a DEMO-relevant heat flux of at least 10 MW/m2. The HEMJ (He-cooled modular divertor with multiple-jet cooling) was chosen as the reference concept. It employs small tiles made of tungsten, which are brazed to a thimble made of tungsten alloy W-1%La2O3. The W finger units are connected to the main structure of ODS Eurofer steel by means of a transition piece. The divertor modules are cooled by helium jets (10 MPa, 600°C) impinging onto the heated surface of the thimble. In cooperation with the Efremov Institute a combined helium loop & electron beam facility (60 kW, 27 keV) was built in St. Petersburg, Russia, for experimental verification of the design. Technological studies were performed on manufacturing of the W finger mock-ups. The results of high heat flux (HHF) tests till now confirm the divertor performance required. The knowledge gained from these experiments and some aspects on the design improvement are discussed in this contribution.

Failure study of helium-cooled tungsten divertor plasma-facing units tested at DEMO relevant steady-state heat loads

Tungsten was selected as armor material for the helium-cooled divertor in future DEMO-type fusion reactors and fusion power plants. After realizing the design and testing of them under cyclic thermal loads of up to ~14 MW m−2, the tungsten divertor plasma-facing units were examined by metallography; they revealed failures such as cracks at the thermal loaded and as-machined surfaces, as well as degradation of the brazing layers. Furthermore, in order to optimize the machining processes, the quality of tungsten surfaces prepared by turning, milling and using a diamond cutting wheel were examined. This paper presents a metallographic examination of the tungsten plasma-facing units as well as technical studies and the characterization on machining of tungsten and alternative brazing joints.

Helium-Cooled Divertor Development: Thermo Hydraulic and Thermo Mechanic Analysis Studies of a Single Jet Design

Abstract Within the EU power plant conceptual study (PPCS), a modular He-cooled divertor concept (Ref. 1) has been investigated at the Forschungszentrum Karlsruhe to achieve a heat flux of at least 10 MW/m2. The divertor conceptual design is based on the use of a tile made of tungsten, a structural element made of tungsten alloy, and a steel cartridge. The cooling of the divertor module is realized by an impingement of helium jets (10 MPa, 600 °C) flowing through an array of small jet holes located at the top of the cartridge, able to remove the high heat flux incident on the top surface of the tiles. In this paper a modular design of a helium cooled divertor is introduced. A method of design examination regarding the cooling capability and the component stresses are pointed out. The method is based on the use of a combined system of modern computer tools. For the 3D design construction, the CAD program CATIA V5 was utilized. The simulation calculations were performed in two steps: thermo-hydraulic CFD calculations using the ANSYS CFX tool and thermo-mechanical FEM calculations with the ANSYS code. The CFD computations were done taking into account the design geometry with an appropriate meshing and the boundary conditions, i.e. the defined heat flux, the helium pressure and temperature at the inlet. Among other things, the heat-transfer-coefficients received from the CFD runs were then used for the following FEM analyses. The simulation results and a potential of design improvement will be discussed.

3D Structural Analysis of Selected High-Temperature Materials

Abstract Research and development projects on materials for applications exposed to high temperatures and radiation include comprehensive mechanical and metallographic examinations as well as analysis methods based on electron microscopy investigations using SEM (scanning electron microscope) and TEM (transmission electron microscope). In addition to 2D EDS and EBSD analyses in the μm range, the imaging 3D analysis method has become more and more important in the past few years. Particularly, the combination of SEM images with simultaneously obtained EDS and EBSD data during 3D analyses is a test method which complements previous material testing and produces clear results since much larger data sets are used in order to determine the composition of a material and the phases present in its microstructure. Since 2014, our institute has been working with a workstation (FIB/SEM) from the company Zeiss, a crossbeam AURIGA 40 with 3D analysis. This paper will present unparalleled results of the investigations carried out on the structural and high-temperature material 12Cr0.36Ta steel, nanocrystalline tungsten and intermetallic NiAl-Cr alloys using the above-mentioned 3D technique.

The interface in molybdenum-copper-composites used for thermal management applications

Molybdenum-copper-composites are interesting materials in the field of thermal management of gallium nitride based electronic devices. Depending on the application and packaging requirements, the coefficient of thermal expansion and thermal conductivity can be tailored for these composites by varying structure and composition. In this work, the interface between molybdenum and copper is studied. Transmission electron microscopy shows a sharp interface between the molybdenum and copper layers without interdiffusion zone. The low thermal contact resistance between the layers also suggests that there is sharp interface between molybdenum and copper. The electrical resistivity was measured and compared to estimations based on the Wiedemann-Franz law.