# Technological Processes for Steel Applications in Nuclear Fusion
## 1 Introduction  
In 2014 the EU started a Pre-Conceptual Design phase to explore and define the technical options for the European demonstration fusion power plant (DEMO). For the breeding blanket--a component within the vacuum vessel, in which neutron energy is converted to thermal energy and which also produces (\"breeds\") tritium from lithium--initially four concepts were considered. After a revision of the European Fusion R&D program and strategy, the focus is now on the two most promising concepts, namely, the Helium-Cooled Pebble Bed (HCPB) and Water-Cooled Lithium Lead (WCLL) blankets. These blankets form a torus shaped surface towards the plasma, the first wall (FW), which in turn covers about 1000 m\({}^{2}\). The number of single blanket boxes that form the blanket is in the order of 1000 pieces. The International Thermonuclear Experimental Reactor (ITER)--currently under construction in Southern France--will be equipped with testingports for such blanket boxes. Within a Test Blanket Modul (TBM) program, specific HCPB and WCLL prototype blanket boxes will be tested. A general overview, as well as details, can be found in [1; 2; 3; 4; 5; 6] and references therein. By progressing towards the Conceptual Design Phase, there is an increasing demand for industrial and technological feasible processes and a distinctive focus on economic considerations. A preliminary cost estimate on several manufacturing routes for the first wall and other breeding blanket subcomponents showed that Electro Discharge Machining (EDM) or \"wire cutting\" is a technically and economically sound option for the fabrication of cooling channels in a TBM. For the larger HCPB blanket boxes in a DEMO reactor, however, other solutions might be more appropriate and cost-effective as well [5; 7]. Cold Spraying, for example, in combination with subsequent Hot Isostatic Pressing (HIP) is a rather promising technology, not only for the first wall, but for every component that requires complex inner structures (details are given in [7; 8]). Additive manufacturing, in general, offers various options in component fabrication for nuclear fusion components, which is outlined in a separate paper within this special issue. Regardless of concept, design, or new fabrication routes, all approaches of blanket fabrication include similar processes, which play more or less important roles and that cannot be turned down. These are (1) machining, (2) joining, and (3) heat or thermo-mechanical treatments. Therefore, the main scope in the first part of this paper is a series of studies, in which the influence of such typical production processes on the mechanical behavior of the European reference low activation steel EUROFER97 are explored. In all cases, we applied industrially available and approved technologies only. Moreover, we present in a second part of this paper, a high heat flux test campaign with a prototype (mock-up) of a typical HCPB blanket first wall. The mock-up was produced by applying the most suitable fabrication processes that have been identified in the beforementioned studies. In this context it is important to know that the operating limit of the starter blanket for a European demonstration fusion reactor (DEMO) is predefined by a neutron dose of 20 dpa. In a second stage, the possible lifetime extension of a more advanced blanket will be tested up to doses of about 50 dpa. With today's available material technology, a dose of 50 dpa can only be survived by increasing the operating temperature in the steel parts to 350 \({}^{\circ}\)C or higher. Thus, most likely, the coolant for the second (advanced) blanket of the European DEMO will be helium gas [1; 2; 3; 4]. Regardless of the different breeding blanket concepts, the plasma-facing surface and a layer of several millimeters below is the highest loaded part with regard to thermal as well as neutron load.